Lec 3 Displacement Measurment
Lec 3 Displacement Measurment
Lec 3 Displacement Measurment
Chapter 3
Measurement of Displacement
Transducers and sensor
• Transducer is a device that convert/transforms one
form of energy into another
• Sensor is a component, device or equipment that
detects variation of physical phenomenon and
responds (or provides) equivalent electrical or non-
electrical quantity corresponding to it.
Classification of sensors
1. The physical effect employed as resistive, inductive,
capacitive sensors
2. The source of energy the used to provide an
output as active and passive sensors
3. The physical quantity they convert as displacement
sensor, temperature sensor, speed sensor, pressure
sensor e.t.c
2
Displacement, position & proximity
Sensors
Alternative use of a strain gauge for measuring the force applied to a cantiliver beam…Top
view and Side view
• Strain gauges attached to
flexible cantilever.
• During deformation
tensile and compressive
forces on the element
change the resistance i.e.
the strain thus it can be a
measurement of
displacement or
deformation of the
flexible element.
• Strain gauges are
designed for maximum
sensitivity by arranging
the strain gauge
elements in opposing
directions.
Capacitive element
• The capacitance C of a parallel plate capacitor is
given by
Ɛ� Ɛ� �
�=
�
• where er is the relative permittivity of the
dielectric between the plates, e0 a constant
called the permittivity of free space, A the area of
overlap between the two plates and d the plate
separation. Capacitive sensors for the monitoring
of linear displacements might thus take the forms.
• In (a) one of the plates is moved by the displacement
so that the plate separation changes; in (b) the
displacement causes the area of overlap to change; in
(c) the displacement causes the dielectric between
the plates to change.
• The linear patterns and associated timing diagrams are what the
photo-detectors sense as the code disk circular tracks rotate with the
shaft.
• The gray code is designed so that only one track (one bit) will change
state for each count transition, unlike the binary code where multiple
tracks (bits) change at certain count transitions.
• For the gray code, the uncertainty during a transition is only one count,
unlike with the binary code, where the uncertainty could be multiple
counts.
• Since the gray code provides data with the least uncertainty but the
natural binary code is the preferred choice for direct interface to
computers and other digital devices, a circuit to convert from gray to
binary code is desirable.
Cont…
Cont…
Cont…
Decimal code Rotation range (deg.) Binary code Gray code
• Light sources: laser diodes and light-emitting diodes (LEDs). These emit light in a
narrow frequency band in the infrared region and are not affected by sunlight.
• Light detector: photocells (cadmium sulphide or cadmium selenide being the most
common type of photocell), phototransistors and photodiodes. Resistance is reduced
according to the intensity of light to which they are exposed.
Scan Techniques
• Scan technique: is a method used by photoelectric
sensors to detect an object (target).
• In part, the best technique to use depends on the target.
– targets are opaque and others are highly reflective.
– Scanning distance is also a factor in selecting a scan technique.
Some techniques work well at greater distances while others
work better when the target is closer to the sensor.
• Thru-Beam: Separate emitter and receiver units are
required for a thru-beam sensor.
– An object (target) placed in the path of the light beam blocks
the light to the receiver, causing the receiver’s output to
change state.
– When the target no longer blocks the light path the receiver’s
output returns to its normal state.
• Suitable for detection of opaque or reflective objects.
• It cannot be used to detect transparent objects.
examples