US3015477A - Coal-rock sensing device - Google Patents

Description

Jan. 2, 1962 s. l. PERSSON ET AL 3,015,477

COAL-ROCK SENSING DEVICE Filed Aug. 20, 1958 4 Sheets-Sheet 1 RESISTANCE lN OHMS COAL I04 I ROCIVK" (SANITSTONE) o 5 IO I5 20 V 0 U S INVENTORS STEN 1. PERSSON FIG. 2 BY CHARLES H. REYNOLDS Jan. 2, 1962 s. l. PERSSON ET AL 3,015,477

COAL-ROCK SENSING DEVICE Filed Aug. 20 1958 4 Sheets-Sheet 2 R R R C R s 8 7 6 5 4 3 2 I O .r m moz km wmm J Ou xOOI H w VOLTS FIG.

INVENTORS PERSSON By CHARLES H. REYNOLDS STEN FIG. 4

Jan. 2, 1962 s'. l. PERSSON ET AL 3,015,477

COAL-ROCK SENSING DEVICE Filed Aug. 20, 1958 4 Sheets-Sheet 5 +4OV 250 V i- Y 54 i g 58 57 eo ROCK COAL l 2 FIG. 6

INVENTORS STEN I. PERSSON BY CHARLES H. REYNOLDS AT ORNEY Jan. 2, 1962 Filed Aug. 20 1958 S. l. PERSSON ETAL COAL-ROCK SENSING DEVICE 4 Sheets-Sheet 4 "I 70 AVERAGE 0c LEVEL e '7l' AVERAGE DC LEVEL Fl G 7 INVENTORS STEN l. PERSSON BY CHARLES H. REYNOLDS United States Patent Ollice 3,015,477 Patented Jan. 2, 1962 3,015,477 COAL-ROCK SENSENG EEVICE Sten l. Persson and Charles H. Reynolds, Rochester, N.Y., assignors to General DynamicsCorporation, Rochester, N.Y., a corporation of Delaware Filed Aug. 2t), 1958, er. No. 756,183 Qlaims. (Cl. 262-47) This invention relates to the art of coal mining and is particularly directed to means and methods of detecting rock at the boundaries of a coal'seam.

Coal is usually found in generally flat horizontal layers or seams of various vertical depths. The seam is entered from one edge with cutting tools or drills to break, loosen, and remove the coal. The floor and roof of the seam is usually of minerals much harder to out than coal and seri- "ously shortens the life of the cutting tools when the roof or floor rock is accidentally. entered by the tools.

Attempts have meen made in the past to measure the electrical resistance of the strata in which the cutting tools are working. Electrical resistance, however, has proven heretofore to be a poor criteria for identifying bore-hole materials because of the many uncontrolled variables which affect resistance. The presence and amount of moisture near the test probes changes resistance widely, for example. Quantitative resistance values cannot, as a practical matter, be obtained because reference values of resistance cannot be assigned. The problem of identifying minerals by resistance measurements is generally not feasible because in practice the resistance varies from near zero to infinite values.

An object of this invention is to provide improved methed and means for finding the boundary between two dissimilar mineral formations in their native state.

A more specific object of this invention is to provide improved methods and means for sensing rock formations at the boundaries of coal seams, particularly in connection with the operation of continuous mining machines.

The objects of this invention are attained by mounting one or two test probes adjacent the cutting bits of a coal mining machine in such a fashion that the probes follow the bits and alternately contacts the coal and the rock during cutting operation. The probes are electrically connected in a bistable circuit of such a character that the resistance of the minerals effectively connected between the probes causes the bistable circuit to assume either of two characteristic stable states. The potential applied between the probes is chosen at an optimum, relatively high, value at which the ratio of resistances of the two minerals is at a maximum, so that as the probes pass alternately through the two minerals two distinct signals are generated by the bistable circuit.

Other objects and features of this invention will become apparent to those skilled in the art by referring to the specific embodiments of this invention described in the following specification and shown in the accompanying drawing in which:

FIG. 1 is a fragmentary view of a mining machine including a sensing probe or cutter of this invention;

FIG. 2 is a voltage-resistance graph of typical minerals;

FIG. 3 is a graph showing values of the ratio of resistances of coal and rock, plotted against voltage;

FIG. 4 is a circuit diagram of a rock sensing device embodying tube-type amplifiers;

FIG. 5 is a circuit diagram of a rock sensing device embodying transistor amplifiers;

FIG. 6 is a circuit diagram of a sensing device embodying transistors in a multivibrator sensing type circuit; a d.

FIG. 7 is a pulse chart of the output of the multivibrator of FIG. 6.

In FIG. 1 is shown a portion of a coal boring machine having a wheel or arm 10 rotating on a center, not shown, for carrying cutting tools horizontally forward in a coal vein. The vein contemplated here is of the type generally found throughout the central United States at various levels below ground. The vertical depth of the vein will typically vary between three to six feet. The floor and ceiling of the vein comprises an underlay and overlay of rock generally much harder than the coal. The rock may be classified-as slate; shale, boney, fire clay, or sandstone. lron pyrites sometimes appear with the coal vein. Rock is generally Stratified as indicated at 11 in FIG. 1, while the coal is shown at 12. Obviously the cutting tool siwinging on a center within the vein will dig into the rock in the floor or ceiling if the mining machine drifts up or down from its approximate center position. The objects of this invention are attained by sensing the electrical resistance of the coal and comparing that resistance with the resistance of the rock so that the cutting tool or sensing probe adjacent the cutting tool passes alternately from coal to rock to coal.

On the end of arm 10 is mounted the sensing tool or probe 13 with a hardened lead edge and with a shank l4 clamped in a mechanically strong bushing 15 of insulating material. The sensing tool and its insulating support must be rigidly mechanically attached to the mining machine, as with the pillow block 16. It is important that the sensing tools 13 make intimate contact with the strata being cut and although a cuttin type bit is shown a spring-pressed probe with a rounded nose could, if desired, be brought in contact with the strata immediately behind the standard cutting tools of the machine.

Further, although a single sensing tool is shown in FIG. 1, it is contemplated that two tools a distance apart may be used. If a single tool is used, the electrical circuit to the tool is completed through the frame of the machine. Where the sensing tool is mounted on a rotating head, slip rings must be usedto complete electrical connections.

Contrary to expectations, the electrical resistance in ohms of most bituminous coal is many times higher than the resistance of most rock formations. too, it was found that the electrical resistance of both rock and coal is a function of the voltage employed to make the resistance measurement. No ready explanation can be offered for this unusual phenomenon. In FIG. 2, for example, the resistances of a sandstone type rock and of a common bituminous coal are plotted for various voltages. That is, the resistance of rock and coal is plotted for various values of direct current voltage from zero to 20 volts. It can be seen that in the specific example considered, the resistance of rock and coal is the same at about 2 volts. As the voltage increased up to about 7 volts the resistance of the rock dropped sharply,

and at voltages above 7 volts the resistance remains substantially constant. This finding suggests that the resistance measurements must be made at relatively high voltages and in the region where the resistances. are comparatively stable. In the case of the standstone and coal samples of FIG. 2, the ratio of coal and rock resistance is largest in the 7 to 20 volt region. Fortunately, the

high ratio of coal resistance to rock resistance is effectively Surprisingly,

concerned because of the high impedance of the circuitry. Specifically different coal and rock minerals would, of course, yield specifically different resistances and resistance ratios but the order of resistances and their ratios would be about the same and there would be one optimum test voltage.

The lead 17, FIG. 1, from the sensing tool 13 is connected, as shown in FIG. 4, to the input of a bistable circuit which, according to this invention, produces two distinct signals as the sensing tool 13 moves from contact with coal to contact with rock and back again. Lead 18 is connected either to the frame of the machine or to a second sensing tool either of which is considered the reference ground for the system. The particular bistable device shown in FIG. 4 comprises a boot strap-type circuit having triode amplifiers 20 and 21. Both amplifiers are connected across the bus bars 22 and 23, the latter being grounded when the lead 18 is connected thereto. Windings of the relay 24 are connected in series with the anode and the cathode resistor 25 is connected in series with the cathode. The grid of amplifier 20 is connected to an intermediate point on the potentiometer comprising resistances 26 and 27 which are so proportioned that tube 20 is cut off in one stable state, the cutoff gridcathode voltage being made amply high by the normally high current flowing in amplifier 21 and its series-com nected resistance 25. That is, normal high current through 21 and 25 will hold the cathode of 20 at a relatively high positive potential, thus assuring cutoff. If now the sensing tool resistance in shunt with 27 is suddenly increased, as when the sensing probe enters coal, the grid of 20 momentarily rises unblocking amplifier 20, permitting the cathode end of resistance 25 to rise and the anode end of winding 24 to drop. These two effects applied respectively to the cathode and grid of amplifier 21 reduces the current flow through 21 which in turn reduces the bias voltage generated across 25, the regenerative action producing a runaway effect that positively and abruptly reverses the normally nonconductiverconductive condition of amplifiers 20 and 21. With amplifier 20 now conducting, winding 24 is energized and armature 28 is moved to illuminate lamp 22 turning off lamp 341. The two lamps may be colored distinctively, say, red and green so that the operator can determine momentarily whether his cutting bit is in rock or coal.

When the resistance across 27 momentarily decreases, as when the sensing tools enter rock, the grid bias of 20 momentarily changes to a cutoff value permitting amplifier 21 to momentarily conduct and to trigger the action which results in regenerative amplification and sudden and positive restoration of the normal nonconducting condition of amplifier 20 and the conducting condition of amplifier 21.

Resistance 27 is made variable so as to easily control the threshold resistance in the grid circuit which will cause the shift from conductive to nonconductive state, or vice versa.

The circuit of FIG. is the equivalent of the circuit of FIG. 4 insofar as the philosophy of operation is concerned but with transistors substituted for the vacuum tube type amplifiers. The transistors found particularly useful for this bistable device is of the P-N-P type connected between ground and a positive 20 volt bus connected to the emitters. The collector of transistor 40 is coupled to the base of transistor 41, while the emitter of 40 is coupled to the emitter of 41. Transistor 40 is normally cut off by virtue of the normally high resistance between the sensing tools, labeled 13 and 13a in FIG. 5. The sensing tools are connected in the potentiometer circuit to which the base of 40 is tapped, and one leg of which comprises adjustable resistance 42. Transistor 41 is normally conducting and supplies the bias across resistance 45 for holding transistor 40 nonconducting. When the resistance at 13-13a suddenly drops as when rock is entered, the base of 40 drops, the collector of 40 and base of 41 rises, and the current through 41 momentarily drops. Regenerative amplification is the same as in FIG. 4 and full stable conducting current abruptly flows actuating relay 44. Operation of armature 48 switches the lamps 49 and 50 and the operator instantly is warned of the entry of his cutting bit into the new strata. The two lamps will alternately illuminate as the cutting bit enters rock and coal, and the operator can promptly respond by appropriately steering the mining machine. Resistance 42 is adjusted so that the base bias will shift across the required threshold value for the particular rock-coal strata being cut.

Alternate cutting in rock and coal can likewise be displayed to the operator by the multivibrator type circuit of FIG. 6. The characteristic of the circuit of FIG. 6 is that two stable states of the amplifiers are normally switched back and forth at one predetermined frequency and that this frequency abruptly changes to a new and distinct frequency as the sensing tool passes into a different mineral formation. That is, frequencies and f of the bistable multivibrator appear at the output of the circuit immediately upon the cutting bit entering rock and coal, respectively. The output circuit comprises the relay winding 54, operating armature 58 and lamps 59 and 60. In this case, energization of relay 54 is obtained by integrating the higher frequency pulses of the multivibrator. Only frequencies above a certain value will contain sufficient energy and current to operate the armature. At this certain frequency, the relay is preferably designed to operate positively and unambiguously. This particular mode of operation can best be evaluated by referring to FIG. 7. If the frequency 1; produces an average direct current in the winding represented by level 70, obtained by integrating areas under the current loops, the spring or gravity bias of the armature 58 can be made sufficient to prevent operation. When the frequency of the multivibrator increases to or above f however, the average direct current level rises to 71 which can easily be adjusted to operate the relay armature and cause the desired response in the lamps.

The specific multivibrator shown comprises transistors 72 and 73, the collector of one being capacitively coupled, as shown, to the base of the other. The RC time constant circuits of each transistor is determined respectively by resistance 74 and condenser 75 on the one hand, and resistance 76 and condenser 77 on the other hand. The resistance between the sensing tools are, in FIG. 6, connected in parallel with resistance 76 so that minerals of different resistance across the probes unbalances one RC time constant with respect to the other. The pulse repetition rate when the sensing tools are in rock may be, for example, 100, while the pulse repetition rate for sandstone could be made 500, or any frequency distinctly different than the other. Since one time constant remains fixed and the other variable, pulse widths remain fixed while the frequency changes. However, either or both time constants are preferably variable, as by variable resistances 74 and 76 to increase the sensitivity of the device and accommodate the device to specifically different rock-coal formations.

The rock-coal sensing device of this invention comprises a bistable amplifier device which instantly indicates to the operator the presence of his cutting bit in rock or coal. Advantage is taken of the fact that at a certain relatively high test voltage, a large ratio of resistances of dissimilar minerals can be observed and employed to reliably trigger a bistable multivibrator. Many modifications may be made in the specific circuit details without departing from this central thought of the invention.

What is claimed is:

1. In combination in coal mining machinery, at powerdriven cutting bit for cutting coal in a natural coal seam having a rock boundary; and means for sensing rock encountered by said bit, said means comprising a sensing tool disposed adjacent said bit for intimately and alternately contacting the face of the coal and rock being cut, a resistance sensitive bistable device having a trigger circuit, said trigger circuit being connected directly to said sensing tool and being responsive to the electrical resistance of the coal and rock being cut for abruptly changing the bistable device from one state to another as said electrical resistance changes from one value to another, respectively, above and below a predetermined resistance value, and means for manually changing the level of said predetermined resistance value.

2. A system for continuously indicating the strata being cut by the cutting tool of a coal bore mining machine having a rotary cutting head, said system comprising a strata cutting tooth mounted on said head, means for alternately and instantaneously measuring and indicating the electrical resistance of the two principal strata being cut, said means comprising a test probe mechanically mounted on and electrically insulated from said rotary cutting head and including a source of voltage connected to said probe and of a value for producing a maximum resistance ratio between the two principal strata being cut, a bistable circuit connected to said probe and responsive to the two resistances of said ratio for producing, respectively, the two stable states of said bistable circuit, and means for distinctively indicating each stable state.

3. In combination, a bore mining machine having a rotary cutting head adapted to be advanced into desired strata, means for indicating a change in the strata being cut comprising a cutting tooth mounted on said rotary head in cutting relation to the strata being cut and means for sensing successive electrical resistances in the difierent strata as the cutting tooth moves alternately through the strata; the sensing means comprising a bistable amplifier device for producing current of two distinct levels, said amplifier device being connected to said cutting tooth and being responsive to said electrical resistances in the different strata to shift from one stable state to the other, and means for adjusting said amplifier device to change the threshold value of electrical resistance across which the strata resistance must change to shift said stable states.

4. in combination, a bore-type coal mining machine with a rotary cutting tool, a rock-coal sensing device for operation with said coal mining machine, said device comprising sensing means for continuously and instantaneously sampling the electrical resistance of mineral strata being out immediately adjacent the cutting region; a bistable amplifier means for generating two distinct signals, and for shifting from one stable state to the other, in response to two electrical resistance values, a resistance sensitive shift controlling circuit in said amplifier means, means for connecting the sensing means into said shift controlling circuit for causing a shift each time said sensing means passes through the boundary between two different mineral strata of different electrical resistance.

5. In combination, a bore mining machine having a cutting bit for tunnel boring along a seam of natural coal lying between a floor and ceiling of rock, and including test probes for engaging the open face of the tunnel being bored, and a rock-coal sensing system comprising a flipflop type bistable amplifier, a potentiometer with a plurality of resistances connected across a voltage source, the trigger circuit of said bistable amplifier being connected to an intermediate point on said potentiometer, one resistance of said potentiometer being manually adjustable for controlling the threshold triggering voltage of said bistable amplifier, and means for connecting said test probes in parallel with saidone resistance for dynamically shifting the potential of said intermediate point on the potentiometer to either side of said threshold triggering voltage.

References Cited in the file of this patent UNITED STATES PATENTS

Images