US3004194A - Temperature responsive control - Google Patents

Description

Oct. 10, 1961 J. L. BOHNERT 3,

TEMPERATURE RESPONSIVE CONTROL Filed Jan. 29, 1958 2 Sheets-Sheet 1 A /9 THERMAL CONTROLLER TO All.

POWER SUPPLY 14 15 14 AC 46 m4 m6 DOWER INPUT P 05 INVENTOR. JACKSON L. BOHNERT BY WMM Oct. 10, 1961 .1. L. BOHNERT TEMPERATURE RESPONSIVE CONTROL 2 Sheets-Sheet 2 Filed Jan. 29, 1958 INVENTOR.

mm Wh\ JACKSON L. BOHNERT United States Patent 3,004,194 TEMPERATURE RESPONSIVE CONTROL Jackson L. Bohnert, Cleveland, Ohio, assignor to Addressograph-Multlgraph Corporation, Cleveland, Ohio,

a corporation of Delaware FiledJan. 29, 1958, Ser. No. 712,044 7 Claims. (Cl. 311-449) g This invention relates to a new and improved thermal controller. The thermal controller of the invention is particularly advantageous when applied to a printing machine of the kind in which images are transferred by heat and pressure from a master to a print-receiving sheet and will therefore be described in that connection; it should be understood, however, that the controller may be applied toother devices and apparatus in which control of temperature and of electrical or mechanical operations is required. I

This application is a continuationin-part of application Serial No. 660,956, filed May 22, 1957 and now abandoned. I

In some printing machines, an image is formed upon a master strip or sheet and is subsequently transferred to a copy sheet .at a printing station by pressing the master against the copy sheet and simultaneously applying heat thereto. Printing machines of this kind are frequently used in the preparation of mailing strips or in other analogous applications. A transfer printing machine of this kind is described and claimed in a ,co-pending application of John H. Grnver, Ser. No. 480,032, filed January 5, 19.55, and now Patent No. 2,844,094. The machine described in the Gruver application utilizes a heated platen in conjunction with a master strip comprising a relatively thin porous carrier bearing an image formed from a thermal-softening ink.

In printing machines of the kind described in the aforesaid Gruver application, it is essential that the temperature of the heated platen be maintained within relatively narrow limits during operation in order to achieve clear, legible transfer images. The operating temperature is not fixed, however, but may vary over a substantial range, depending upon anumber of difierent factors. For example, variations inthe composition of the transfer ink may require relatively large changes in the platen temperature. Similarly, changes in the composition of the carrier employed for the master and of the copy sheets may require substantial variations in the operating temperature of the printing machine. It is essential that the machine be shut down whenever the platen temperature is not confined within the requisite limits. For example, if the platen becomes overheated the master strip and copy sheets may he ignited or at least may become charred or otherwise damaged by the excessive heat. On the other hand, an insufficiently heated platen may result in incomplete or illegible images on the copy sheets, which might not be discovered until after the machine is shut down. Under either circumstance, a critical printing operation may easily be disrupted.

It is thus vseen that a thermal controller for a printing machine of the kind .set forth hereinabove should aiford an overall operating temperature range of substantial magnitude; for example, in a typical machine, the cverall operating temperature range maybe required to extend from 150 F. *to 500 The controller must be capable of preventing machine operation when the platen is below the desired control temperature and must be equally capable of interrupting machine operation whenever the platen exceeds the desired control temperature. Moreover, variations in temperature during a given run of the machine must be restricted to a very limited range of values; for example, .in a typical machine, it may be essential that the platen temperature be held to within 7.5 F. of a preselected normal temperature. The thermal controller should also be capable of interrupting machine operation in the event of the failure, electrical or mechanical, of at least some of its components.

There are other applications which present substantially similar control requirements. For example, in a continuous heat-treating process, it may be desirable to interrupt loading of a furnace conveyor whenever the heat-treating furnace or other equipment falls below or exceeds a preselected normal temperature. Where the processing apparatus is employed to treat a variety of different objects or materials, the operating control temperatures for different applications of the same equipment may vary over a substantial range. 'It is thus seen that the control requirements for an apparatus of this kind may be substantially similar to those of the printing machine set forth hereinabove.

A principal object of the invention, therefore, is a new and improved thermal controller which affords relatively precise temperature control over a wide range of operating temperatures.

7 Another object of the invention is a new and improved thermal controller which aifords effective protection against both excessive and inadequate operating temperatures.

A further object of the invention is anew and improved thermal controller in which operation is effectively independent of fluctuations in supply voltage.

An additional object of the invention is a new and improved thermal controller which affords a relatively high degree of control sensitivity.

A further object of the invention is a new and improved thermal controller which eifectively protects the controlled apparatus in the event of power failure or like external failure.

Another object of the invention is'a new and improved thermal controller which is inherently simple and economical in construction and which is relatively small 1n size.

A thermal controller constructed in accordance with the invention comprises an error signal generator, including a thermal-sensing element, for generating an error signal having an amplitude representative of variations in magnitude of the temperature of the sensing element from a given normal temperature and further having a phase relation, with respect to a reference signal, representative of the direction of variation inthat temperature. Means are provided for applying a reference signal, preferably .a conventional sixty cycle alternating current, to the error signal generator. The control further includes a first control device actuatable between a first and I a second operating condition in response to one or more Patented Oct. 10, 1961 and the reference signal are effective conjointly to actuate the second control device from its first to its second operating condition whenever the error signal exceeds a given amplitude and is in a given phase relationship with respect to the reference signal.

Other and further objects of the present invention will be apparent from the following description and claims and are illustrated in the accompanying drawings which, by way of illustration, show preferred embodiments of the present invention and the principles thereof and what is now considered to be the best mode for applying those principles. Other embodiments of the invention embodying the same or equivalent principles may be used and structural changes may be made as desired by those skilled in the art without departing from the present invention.

In the drawings:

FIG. 1 is a block diagram of a printing machine in which the invention may be employed;

FIG. 2 is a schematic wiring diagram of a thermal controller constructed in accordance with one embodiment of the invention; and

FIG. 3 is a schematic wiring diagram of a thermal controller constructed in accordance with another embodiment of the invention.

The printing machine shown in FIG. 1 illustrates a typical application in which a thermal controller constructed in accordance with the invention may be emselected value within a wide temperature range. The thermal controller 40 is also electrically coupled to the operating mechanism 20 and exercises an over-riding control function with respect to actuation of the mechanical movement of the platen 17 The thermal controller 40 is further electrically coupled to the machine drive control unit to afiord a means for stopping that portion of the machine in response to certain thermal conditions.

FIG. 2 affords a detailed schematic wiring diagram of one embodiment of the thermal controller 40 constructed in accordance with the invention. As indicated therein, the thermal controller includes an input transformer 41 having a primary winding 42 and a secondary winding 43. The transformer 41 may comprise a standard filament transformer of the kind employed as a power supply for the tube filaments of vacuum tube control apparatus and preferably has a secondary voltage of the order of 6.3 volts. The primary winding 42 of the transformer is connected to a conventional 115 volt 60 cycle AC. power supply, a switch 44 and a suitable fuse 45 being connected in one of the power supply leads. A capacitor 46 may I be connected across the primary winding 42.

ployed. The printing machine 10 includes a supply reel 11 from which a master image strip 12 is fed, over a pair of rollers 13 and 14 and a supply spindle 15, into the operating head 16 of the printing machine. The operating head 16 includes an electrically heated platen 17 and an impression plate or anvil 18. The master strip 12 is fed between the platen 17 and the anvil 18 and above a copy sheet 19 which is fed through the operating head in the direction indicated by arrow A. The operating head 16 also includes an operating mechanism 20 which is utilized to raise and to lower the platen 17 in synchronism with movements of the master strip 12 and the copy sheet 19. From the operating head 16, the master strip 12 is fed over a second feed spindle 21 and one or. more additional guide rollers 22 to a takeup or rewind reel 23.

The printing machine 10 further includes a suitable machine drive control unit 25 which is utilized to drive the several spindles and reels of the master strip feed arrangement as indicated by the drive shafts26, 27, 28, 29 and 30. The machine drive control unit 25 may also be employed to control the movementof the copy sheet or strip ,19.

Operation of the printing machine 10, as thus far described, is essentially similar to that of the printingapparatus described in the aforementioned Gruver application Ser. No. 480,032. Thus, the stripof copy sheets 19 to be The temperature controller further includes an error signal generator 47 constructed in the form of a Wheatstone bridge having four terminals 48, 49, 50 and 51. Two arms of the bridge 47 are formed by a pair of fixed resistors 52 and 53; resistor 52 is connected between terminals 48 and 49 whereas resistor 53 interconnects terminals 49 and 51. A third arm of the bridge comprises a thermal sensing element 54 which is connected between terminals 48 and 50 of the bridge in parallel with a resistor 55. The thermal sensing element 54 is of the resistive type and preferably comprises a thermally-variable resistance element having a negative temperature coefiicient of resistivity of the kind conventionally known as a thermistor. The resistor 55 partially offsets the non-linear thermal operating characteristic of the thermistor S4. The

fourth armof the bridge comprises a resistor 56 and a potentiometer 57 connected in series between the terminals 50 and 51.

The two terminals 48 and 51 of the error signal generator 47 are connected to the ends of the secondary winding 43 of the input transformer 41. The remaining pair of terminals 49 and are connected to the primary winding 60 of an output transformer 61 having a secondary winding 62. One terminal of the secondary winding 62 is coupled to the control electrode 63 of a vacuum triode imprinted with image data from the master strip 12 is advanced stepwise through the machine in the direction indicated bythe arrowA to locate successive portions of the copy sheet in printing position above the anvil 18 and below the platen 17. At the same time, the master strip 12 is advanced stepwise through the printing position between the platen and anvil. The master strip feeding arrangement is maintained inv registry with the copy sheet feeding apparatus so that, as each copy sheet is positioned on the impression plate 18, the image carried by the masterstrip 12 next subsequent to the last one printed is positioned over the new, segment of the copy sheet. A synchronized dual-feed arrangement suitable foruse in the printing machine 10 is described in detail in Patent No. 2,740,354 to John H. Gruver, issued April 3, 1956 section 64, the other terminal of the secondary winding being returned to a conductor 65 which is connected to the bottom of the primary 42 of input transformer 41.

The thermal controller 40 also includes a power supply of conventional type comprising a half wave rectifier and filter circuit arrangement. Thus, the power supply includes a resistor 66, a selenium diode 67, and a capacitor 68 connected in series with each other across the primary winding 42 of the input transformer 41.

The vacuum triode section 64- is incorporated in an error signal amplifier and is coupled to a pair of control devices comprising a second vacuum triode section 70 and a grid-controlled ionic rectifier 71. The cathode 72 of tube 64 is connected to the conductor through a self-biasing circuit comprising a resistor 73- and a shunt on an application filed July 22,- 1950; inasmuch as the drive arrangement employed in. the printing machine is not critical to the present invention, it has not been illustrated in the drawing.

The printing machine 10 further includes a thermal controller40; The thermal controller '40 is utilized to maintain the platen 17 at a predetermined normal temperature, which temperature may be established at any capacitor 74.

The anode 75 of the tube is connected to the rectifier 67 through a load resistor 76. The anode 75 is also coupled to the control electrode 77 of the triode 70 by means of a coupling circuit comprising a cou pling capacitor 7 8, a coupling resistance 79, and a second coupling capacitor 80. The input circuit for the tri-. ode 70 further includes a parallel R-C circuit 81, 82 connecting the conductor 65 to the common junction of the capacitor 78 and the resistor 79. A crystal diode 83 is connected between the control electrode 77 and the return lead 65 and an input resistor 84 .isconnectedin parallel with the diode. The cathode 85 of the tube 70 is returned to the conductor 65 through a self-bias:

ing circuit comprising a resistor 86 and a shunt capacitor 87. Preferably, a potentiometer 116 is connected in series with the resistor 86. The anode -88 of the tube is connected to the rectifier 67 through a current limiting resistor 89 and a relay operating coil 91, a capacitor 90 being connected in parallel with the coil.

The anode of the amplifier tube 64 is also coupled to the control electrode 92 of the thyratron 71 by means of a series input resistor 93. The cathode 94 and the shield electrode 95 of the thyratron are connected to each other and are returned to the conductor 65, a capacitor 96 being connected between the control electrode 92 and the cathode 94. The anode 97 of the thyratron is returned to the top of the primary winding 42 of the input transformer through a relay operating coil 98 and a current-limiting resistor 110. Preferably, a capacitor 99 is connected in shunt with the relay operating coil 98.

The thermal controller 40 further includes the relay contacts 98A and 988 which are connected in parallel with each other and which are controlled by the relay operating coil 98. The normally open contacts 98A and 98B are connected in series with the power supply circuit for the heater incorporated in platen 17, the platen heater being generally indicated by the resistor 100. A thermal fuse 101 may be incorporated in the platen heater circuit.

Suitable relay contacts controlled by the operating coil 91 are also included in the thermal controller; these may include both normally open contacts such as the contacts 91A and normally closed contacts 91B. The relay contacts 91A and 91B are suitably connected in the machine drive control circuit 25 (see FIG. 1). In addition, a further set of relay contacts 91C is provided; the normally closed contacts 91C are connected in series with an indicator lamp 103 across the terminals '48 and 51 of the error signal generator 47. A second indicator lamp 104 is connected across the secondary winding 43 of the input transformer 41; the filaments 105 and 106 for the tubes of the thermal controller may also be energized from the secondary winding 43.

When the thermal controller 40 is placed in operation, the main switch 44 is closed, energizing the primary winding 42 of the input transformer 41. Accordingly, a 60 cycle alternating current signal is applied across terminals 48 and 51 of the error signal generator, bridge 47. Assuming that the bridge is balanced, terminals 49 and 50 of the bridge are at the same potential, and no signal voltage is applied to the interstage transformer 61. Under these conditions, only a low-amplitude current flows through the triode 64 and no effective output signal is applied therefrom to the two control devices comprising the tubes 70 and 71 and their associated relay coils. Consequently, the relay contacts 98A and 98B remain open, leaving the heater 100 dc-energized. In addition, the relay contacts 91A, 91B and 91C remain in their normal condition as shown in FIG. 2, with the result that the machine control unit 25 is maintained in its first or active operating condition and the indicator light 103 is lit, indicating that the machine is in operation.

Whenv the switch 44 is first closed, however, the balanced bridge conditions described hereinabove does not normally prevail, since the platen 17 (FIG. 1) is not usually at the desired operating temperature. Rather, when the machine is first placed in operation, the bridge circuit 47 is usually unbalanced, the impedance between terminals 48 and 50 being substantially greater than the impedance across the terminals 50 and 51. Under these circumstances, an error signal of substantial amplitude is developed across the terminals 49 and 50 of the bridge and is applied to the control electrode 63 of the amplifier tube 64 through the interstage transformer 61. This error signal, which is approximately 180" out of phase with respect to the reference signal applied to the primary 42 of the input transformer, is amplified and inverted in phase in the error signal am- "6 pli fier 64. The amplified'eiror signal is applied to the control electrodes 7? and 92 of tubes 70 and 71 respectively.

The error signal applied to the machine control amplifier tube 70 causes that tube to conduct an A.C. output signal of substantial amplitude and energizes the relay 91. Consequently, the contact 91A are closed and the contacts 91B are opened, thereby de-energizing the control circuits of the machine control unit 25 and interrupting operation of the machine drive. At the same time, contacts 91C are opened, extinguishing indicator light 103 to inform the machine operator that the thermal controller 40 is preventing machine operation.

The control signal, as applied to the thyratron control electrode 92, is in phase with the reference signal supplied to the thyratron anode 97. The thyratron 71 tires on positive half cycles of the two signals and energizes the second control device comprising relay 98. Accordingly, the relay contacts 98A and 98B are closed, energizing the heater element 100 to heat the platen 17.

As the platen 17 increases in temperature, the resistance of the thermistor 54 decreases. Consequently, the total resistance between the two terminals 48 and 50 of the bridge gradually approaches the resistance between terminals 50 and 51 thereof. As the bridge thus approaches a balanced condition, the error signal developed across terminals 49 and 50 decreases in amplitude. The signal output from the error signal amplifier 64 is correspondingly reduced and eventually is insufiicient to cause the two control device amplifiers 70 and 71 to conduct. Accordingly, the two relays 91 and 98 are de-energized, restoring the relay contacts to their initial condition. The machine control unit 25 is again energized and the heater 100 is de-energized, permitting operation of the printing machine.

Under some circumstances, the platen 17 may be heated to a temperature exceeding the desired normal operating temperature. When this occurs, the thermistor 54 may be reduced in resistance to an extent such that the total resistance between terminals 48 and 50 of the bridge is substantially less than that across terminals 50 and 51. Consequently, the bridge is again in an unbalanced condition and an error signal is developed across bridge terminals 49 and 50 and applied to the control electrode 63. In this instance, however, the error signal is in phase with the reference or supply voltage signal; consequently, the output signal from the error signal amplifier tube 64 is out of phase with respect to the reference signal applied to the anode 97 of thyratron 71. This error signal cannot trigger the thyratron into conduction and the heater 100 therefore remains de-energized whenever the phase of the error signal indicates that the platen is overheated. The machine control amplifier 70, however,'is not controlled in its operation by the relative phase of the error signal. .Consequently, the phase-inverted error signal indicative of overheating of the platen renders the tube 70 conductive, energizes relay 91, and interrupts the machine operation. As before, the indicator light 103 is extinguished by the operation of the relay 91 and the operator is informed that it is the thermal controller which has arrested the printing operation.

Adjustments in the normal operating temperature of the thermal controller unit are achieved by adjusting the effective resistance of the bridge arm 50-51 by varying the setting of potentiometer 57. It is of course necessary to afford some range of deviation from the normal temperature within which the controller may operate without interrupting machine operation; this may be achieved in part by proper selection of the kind of relay employed as relay '91. The relay may be adjusted to permit actuation thereof only when a given current flows through the relay coil. Determination of the thermal operating limits is primarily a matter of design selection and is controlled to a substantial extent by the criticality of the temperature for the printing or other operation being controlled by the unit 40. In the particular printing machine illustrated in FIG. 1, thermal operating limits of the order of plus and minus 7.5-. F. are permissible. The same design considerations maybe applied to the second control device comprising the relay 98 and the tube 71. The potentiometer 116 afiords a means for adjusting the operating range of the temperature control device, comprising the relay 98 and the tube 71, independently of-the first or machine control device including the tube 70 and the relay 91.

The thermal controller 40 thus utilizes a single error signal generator to afiord two distinct types of control for the printing machine 10. The first control device of the controller 40, comprising the triode 70 and the relay 91, is actuated from one operating condition to another by an error signal of predetermined amplitude, regardless of the phase relationship between that error signal and the reference A.C. input signal. The second control device, comprising the thyratron 71 and the relay 98, is actuated from one operating condition to another whenever the error signal exceeds a predetermined amplitude and is in a given phase relationship with respect to the reference signal supplied to the anode 97 thereof; this second control device is not actuated by the error signal, regardless of magnitude, however, when the error signal is not in the desired phase relationship with respect to the reference signal. Machine operation is interrupted, therefore, whenever the platen 17 is below or above the desired operating temperature and the platen heater is energized only when the platen is at a reduced temperature as compared with the desired temperature.

In many cases of failure of the controller components, the heater control relay 98 is de-energized or the machine control relay 91 is energized. In either instance, the printing machine is interrupted in its operation. There may be some time delay in this control action if the component failure or other failure of the controller results in continuous action of the heater control relay 98, since interruption of the machine operation may occur only after the platen exceeds its desired normal temperature. The thermal fuse 101 in the controller circuit effectively protects the machine against continuous operation of the platen heater.

In order to afford a clear and complete illustration of the thermal controller of the invention, impedance values, tube types, and other specific data for a typical embodiment of the circuit of FIG. 2 are set forth hereinafter. It should be understood that this material is supplied solely by way of illustration and in no sense a limitation on the invention.

ELECTRIC DISCHARGE DEVICES CAPACITORS Microfarads OPERATING POTENTIALS Across Terminals 6.3 volts.

48 and 51 A.C. 60 cycle.

Anodes 75 and 88 +145 volts D.C. (approximately 0.5 volt A.C. ripple).

Anode 97 volts A.C. 60 cycle.

The thermal controller is essentially immune to ordinary fluctuations in power supply voltage. Thus, in the embodiment described in detail hereinabove, fluctuations in the supply voltage between 105 and volts has no appreciable effect on operation of the control unit. The temperature range of the controller is of course limited only by the particular thermal sensing element 54 selected for use therein and by the overall operating resistance range for the potentiometer 57. a

The thermal controller 40 described in connection with FIGS; 1 and 2 is highly satisfactory in operation and, as pointed out hereinabove, offers substantial advantages in the control of heat-and-pressure image transfer machines and like apparatus. There is some imbalance in the control range of the controller, however, with respect to overheated and underheated platen operating conditions. That is, the change in sensed temperature below the control point required to de-energize the machine control relay 91 may be somewhat greater than the increase above the control point which is effective to actuate the relay. This imbalance is effectively compensated in the circuit of FIG. 3, which is a detailed schematic wiring diagram of a preferred embodiment of a thermal controller constructed in accordance with the invention. In many respects, the thermal controller illustrated therein-is essentially similar to the previously described control de-. vice 40 of FIG. 2. Consequently, in those portions of the circuit in which similar components are utilized in essentially the same manner as in the embodiment of FIG. 2, they are identified by similar reference numerals.

As in the previously described embodiment, the thermal controller 140 includes an input transformer 41 having a primary winding 42 and a secondary winding 43. The primary winding 42 is connected to a suitable A.C. power supply through the switch 44 and the protective device 45; as before, a capacitor 46 may be connected across the primary winding 42. The error signal generator 47 included in the temperature controller 140 is essentially similar to that described in connection with FIG. 2 and includes a Wheatstone bridge having four terminals 48, 49, 50 and 51. The two fixed arms of the bridge comprise the fixed resistors 52 and 53; the sensing arm of the bridge includes the thermal sensing element 54, preferably a thermistor, which is connected in parallel with the resistor 55. As before, the adjustment arm of the bridge includes the resistor 56 and the potentiometer 57.

The terminals 48 and 51 of the error signal generator 47 are again connected to the secondary winding 43 01 the input transformer 41. The terminals 49 and 50 art connected to the primary winding 60 of the output trans former 61. One terminal of the secondary winding 61 of the output transformer is connected to the contro electrode 63 of the amplifier tube 64, the other termina of the winding 62 being connected to the return con ductor 65. In this embodiment of the invention, a ca pacitor 141 is preferably connected across the secondary winding .62. The power supply 'in this embodiment of the invention isessentially similar to that described hereinabove and'includes the resistor 66, the rectifier 67, and the capacitor68 which are connected in series with each other across the primary winding of the input transformer 41.

The error signal amplifier 64 is again coupled to a pair of control devices comprising the vacuum triode 70 and the thyratron 71. The anode 75 of the tube 64 is coupled to the rectifier through a load resistor'76. The anode 75 is also coupled to the control electrode 77 of the triode 70 by means of a coupling circuit which in this instance includes the coupling capacitor 78 and a coupling resistance 142. A parallel 'R-C circuit comprising a resistor 1'46 and a capacitor 147 connects the conductor 65to the common junction of the capacitor 78and'the resistor 142. The input circuit for the triode70 further includes the diode 143 connected between the control electrode 77 and the return conductor 65 and a parallel R-C circuit comprising a capacitor144 and a resistor 145 connected in shunt with the diode. The cathode '72 of the error amplifier tube 64 is again returned to the conductor 65 through a self-biasing circuit comprising the resistor 73 and the capacitor 74. The cathode 85 of the control tube 70 is returned to the conductor 65 through the series-connected resistors 148 and 149, a variable resistor being used for the impedance 148. The cathode '85 is alsoc'onnected, through an input resistor 155, to the top of the primary winding 42 of the input transformer. The anode 88 of the tube 70 is again connected to the rectifier 67 through the current-limiting resistor 89 and the relay operating coil 91, a capacitor 90 being connected in parallel with the coil.

The input circuit to the thyratron 71 comprises the capacitor 78 and the series input resistor 93. The cathode 94 and the shield electrode 95 of the thyratron are interconnected and are returned to the conductor 65, a capacitor 150 being connected between the control electrode 92 and the cathode 94. The thyratron anode 97 is returned to the top of the primary winding 42 of the input transformer through the relay-operating coil 98 and the series connected resistor 151. As before, acapacitor 99 is preferably connected in parallel with the coil 98. The relay contacts 98A and 98B are again connected in parallel with each other and in series with the power supply circuit for the platen heater, generally indicated by the resistor 100. A fuse 101 may be employed to protect the platen heater circuit.

The relay contact arrangement for the relay 91 may include both normally open contacts 91A and normally closed contacts 913. As before, the contacts 91A and 91B are connected into the control circuit 25 (FIG. 1). In this embodiment of the invention, however, a set of normally open relay contacts 9113 are utilized instead of the normally closed contacts 91C of thefirst-described embodiment. The normally open contacts 91D are connected series with the indicator lamp 103 across the bridge terminals 48 and 51. The additional indicator lamp 104 and the filaments 105 and 106 may be connected as in the first-described embodiment.

When the thermal controller 140 is first placed. in operation by closing of the switch 44, the platen 17 (-FIG. l) is usually well below the desired operating temperature. Consequently, the bridge circuit 47 is unbalanced and an error signal of substantial amplitude is developed across the bridge terminals 49 and 50. As before, this error signal is applied to the control electrode 63 of the amplifier tube 64 and develops, in the output circuit of the error signal amplifier, a signal which is in phase with the reference signal from the transformer primary 42. The amplified in-phase error signal is applied to the control electrode 92 of tube 71; the signal is also rectified and applied to the control electrode 77 of the triode 70.

In this cmbodimcnt the tube 70 is normally maintained i0 conductive by application of the reference signal to the cathode 185 of the tube through the input resistor 1'55. Consequently, in the absence of an error signal, the relay 91 is normally energized. At the time the controller is placed in operation, however, with the platen below the control point temperature, an error signal of substantial amplitude is developed, rectified by diode I43, and applied to the control electrode 77 to bias the tube 10 toward out 01f and reduces conduction in the tube to an extent sufficient to effectively de-energize the relay 91'. It should be noted that the negative-polarity rectified error signal, as applied to the control electrode 77, is approximately in phase with the reference signal applied to the cathode '85, and thus eifectively reduces conduction in the tube 70 whenever the error signal indicative of reduced temperature attains even a relatively low amplitude. Consequently, the contacts 91A and 91D remain open and the contacts 91B remain closed. The contacts 91A and 91B are connected :to the machine control circuit in a manner such that, under these circumstances, the .machine control unit 25 prevents operation of the machine drive. Because the contacts 91D remain open, the indicator light 103 is extinguished, thereby informing the machine operator that the thermal controller 140 is preventing opcration of the machine. Because the error signal is in phase with the reference signal, as applied to the thyratron 71, the thyratron is rendered conductive on positive half cycles of the two signals, actuating the relay 98 and energizing the heater element of the machine platen.

As the platen 17 heats up, the resistance of the thermistor 54 decreases and the bridge gradually becomes balanced. As a consequence, the error signal output from the bridge decreases in amplitude, as does the output signal from the error signal amplifier 64. Eventually, the error signal amplitude is reduced to an extent such that the tube 71 can no longer conduct, with the result that the relay 98 is effectively de-energized, opening the heater circuit. Moreover, the reduction in amplitude of the error signal elfectively reduces the negative control potential on the grid 77 of tube 70, permitting a relatively high level of conduction in that tube and elfectively energizing the relay 91. Accordingly, the machine control unit 25 is actuated to its operating condition, permitting operation of the printing machine. f I

As before, it is possible that the printing machine platen 17 may be overheated under some circumstances, producing an output signal from the error amplifier '64 which is 180 out of phase with respect to the reference signal supplied to the tubes 70 and 71. This error signal cannot trigger the thyratron '71 into conduction and thus does not energize the platen heater 100. The error signal is rectified by the diode 143 to develop a half-wave pulsating negative control potential on the control electrode 77 of the machine control amplifier 70, however, with the result that the current drawn by the tube 70 is reduced in amplitude. Accordingly, the relay 91 is effectively deenergized and interrupts the machine operation. As before, the indicator light 103 is extinguished when the relay 91 is de-energized so that the operator knows that it .is the thermal control unit which has interrupted the printing operation. Because the error signal in this instance is substantially out of phase with respect to the AC. bias afforded by the referencesignal applied to the cathode 85 through the resistor 155, a somewhat greater error signal amplitude is required to cut off the tube 70, thus extending the control range for overheating. By proper selection of theresistor 155 and the cathode resistors 148 and 149, the overheat and underheat control ranges can be effectively balanced, thus making the controller equally effective for both conditions of temperature deviation. Moreover, this embodiment of the invention alfords :some safety advantages as compared with the circuit of FIG. 2, since, in the event of a failure of circuit components in the machine control amplifier circuit, the relay 91 remains de-energized and prevents operation of the controlled machine, whereas in the firstdescribed embodiment positive actuation of the relay is necessary to interrupt machine operation. In virtually all other respects, the thermal controller 140 operates in essentially the same manner as the controller 40.

In order to afford a more complete illustration of the preferred embodiment of the invention, the following specific data for a typical embodiment of the circuit of FIG. 3 are set forth hereinafter. It should be understood that these data are supplied solely by way of illustration and in no sense as a limitation on the invention. Only the components that are difierent from the embodiment of FIG. 2 are listed hereinafter, since the other components may be as set forth hereinabove in connection with the exemplary data supplied with regard to the thermal controller 40.

As before, the thermal controller 140 is essentially immune to power supply voltage fluctuations and is limited in its range only by the characteristics of the thermal sensing element 54 and the impedance range of the potentiometer 57.

.While preferred embodiments of the invention have been described and illustrated, it is to be understood that these are capable of variation and modification. Accordingly, the aim in the appended claims is to cover all such variations and modifications as may fall within the true spirit and scope of the invention.

I claim:

1. A thermal controller comprising: an error signal generator, comprising a bridge circuit including a thermal sensing element in one leg thereof, for generating an error signal having an amplitude representative of variations in magnitude of the temperature of said element from a given normal temperature and further having a phase relation, with respect to a reference signal, representative of the direction of variation of the temperature of said element; a first control device actuatable between a first and a second operating condition in response to an applied signal; a second control device actuatable between a first and second operating condition in response to two applied signals, each of said control devices including an electric discharge device and a control relay connected to said discharge device; means for applying the reference signal to the discharge device in said second control device; and means for applying the error signal to the discharge de vices in said first and second control devices to actuate the first device from its first to its second operating condition whenever the error signal exceeds a given amplitude, independently of phase relation between said reference and error signals, and to actuate the second control device from its first to its second operating condition only when the error signal exceeds a predetermined amplitude and is in a given phase relationship with respect to the reference signal.

2. A thermal controller comprising: an error signal generator, including a thermal sensing element, for generating an error signal having an amplitude representative of variations in magnitude of the temperature of said element from a given normal temperature and further having a phaserelation .with respectto a reference signal representative of the direction of variation of the temperature of said element; a first control device actuatable between a first and a second operating condition in response to an applied signal and including an amplifier and a first control relay; a second control device comprising a normally non-conductive thyratron amplifier and a second control relay connected in the anode-cathode circuit thereof; means for applying the error signal to the first control device to actuate the first device from its first to its second operating condition whenever the error signal exceeds a given amplitude; and means for applying the reference signal and the error signal to said thyratron amplifier to render said amplifier conductive and actuate the second relay from a normal to an actuated operating condition whenever the error signal exceeds a predetermined amplitude and is in a given phase relationship with respect to the reference signal.

3. A thermal controller comprising: an error signal generator, including a thermal sensing element, for generating an error signal having an amplitude representative of variations in magnitude of the temperature of said element from a given normal temperature and further having a phase relation, with respect to a reference signal, representative of the direction of variation of the temperature of said element; a first control device, including a relay, actuatable between a first and a second operating condition in response to an applied signal; circuit means, including a rectifier, for developing a control potential having an amplitude proportional to said error signal and for applying said control potential to said first control device to actuate said device from its first to its second operating condition Whenever the error signal exceeds a given amplitude; a second control device, including a relay, actuatable between a first and a second operating condition in response to two applied signals; and means for applying the error signal and the reference signal to said second control device to actuate said device from its first to its second operating condition whenever the error signal exceeds a given amplitude and is in a given phase relationship with respect to the reference signal.

4. A thermal controller comprising: an error signal generator, including a thermal sensing element, for generating an error signal having an amplitude representative of variations in magnitude of the temperature of said element from a given normal temperature and further having a phase relation, with respect to a reference signal, representative of the direction of variation of the temperature of said element; a first control device, including a relay, actuatable between a first and a second operating wndition in response to two applied signals; a second control device, including a relay, actuatable between a first and second operating condition in response to two applied signals; means for applying the reference signal to said first and second control devices; and means for applying the error signal to the first and second control devices to actuate the first device from its first to its second operating condition in response to an increase in the error signal above a first threshold amplitude and when said error signal is in a first phase relation relative to said reference signal, to actuate said first device from its first to its second operating condition in response to an increase in the error signal above a second threshold amplitude when said error signal is in a second phase relation relative to said reference signal, and to actuate the second control device from its first to its second operating condition whenever the error signal exceeds a predetermined amplitude and is in said first phase relationship with respectto the refer ence signal.

5. A thermal controller comprising: an error signal generator, comprising a Wheatstone bridge circuit having a thermal sensing element connected in one arm thereof and a variable impedance connected in an adjacent arm thereof, for generating an errorsignal having an amplitude representative of variations in magnitude of the temperature of said element from a given normal temperature determined by the setting of the variable impedance and further having a phase relation,'with respect to a reference signal applied across said two arms of the bridge, representative of the direction of variation of the temperature of said element; a first control device, including a relay, actuatable between a first and a second operating condition in response to two applied signals; a second control device, including a relay, actuatable between a first and a second operating condition in response to two applied signals; means for applying the reference signal to said first and second control devices; and means'for applying the error signal to the first and second control devices to actuate the first device from its first to its second operating condition Whenever the error signal exceeds a first threshold amplitude and when said error sig nal is in a first phase relation relative to said reference signal, to actuate said first device from its first to its second operating condition in response to an increase in the error signal above a second threshold amplitude when said error signal is in a second phase relation relative to said reference signal, and to actuate the second control device from its first to its second operating condition whenever the error signal exceeds a predetermined amplitude and is in said first phase relationship with respect to the reference signal.

6. A thermal controller comprising: an error signal generator, incl-udinga thermal sensing element, for generating an error signal having an amplitude representative of variations in magnitude of the temperature of said element from a given normal temperature and further having a phase relation with respect to a reference signal representative of variation of the temperature of said element; a first control device comprising a normally conductive vacuum-tube amplifier having an anode, a cathode, and a control electrode and a first control relay connected in the anode-cathode circuit thereof; a second control device including a second control relay actuatable between a first and second operating condition in response to two applied signals; means for applying said reference signal to the cathode of said first control device; means for applying the reference signal to the second control device; and means for applying the error signal to the control electrode of said first control device to render said amplifier non-conductive and actuate the first relay from a normal energized condition to a de-energized operating condition whenever the error signal exceeds a first threshold amplitude and is in a first phase relation relative to said reference signal and to actuate said first relay from its normal to its de-energized operating condition whenever the error signal exceeds a second threshold amplitude and is in a second phase relation relative to said reference signal; and means for applying the error signal to the second control device to actuate the second control relay from its first to its second operating condition whenever the error signal exceeds a predetermined amplitude and is in a given phase relationship with respect to the reference signal.

7. A thermal controller comprising: an error signal generator, including a thermal sensing element, for generating an error signal having an amplitude representative of variations in magnitude of the temperature of said element from a given normal temperature and further having a phase relation with respect to a reference signal representative of the direction of variation of the temperature of said element; a first control device comprising an amplifier and a control relay'actuatable between a first and a second operating condition in response to an applied signal; a second control device comprising a normally nonconductive thyratron amplifier and a control relay connected in the anode-cathode circuit thereof; means for applying the error signal to the first control device to actuate the first device from its first to its second operating condition whenever the error signal exceeds a given amplitude; a biasing circuit, connected to said first control device amplifier, for applying the reference signal to said first control device to balance the action of said device with respect to thermal variations above and below said normal temperature; and means for applying the reference signal and the error signal to said thwatron amplifier to render said amplifier conductive and actuate the relay of the second control device from a normal to an actuated operating condition Whenever the error signal exceeds a predetermined amplitude and is in a given phase relationship with respect to the reference signal.

References Cited in the file of this patent UNITED STATES PATENTS

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