Hardware Reference
In-Depth Information
Figure 9.15
Interface circuit for a resistive position transducer (either linear
or rotary type)
Shaft encoders
Shaft encoders can be used for sensing both rotary position and shaft speed. A
typical shaft encoder produces 100 pulses per revolution and can thus provide a
resolution of better than 1
◦
. Such a device generally produces two phase-shifted
outputs (to enable detection of direction of rotation) plus a third synchronizing
pulse output (one pulse per revolution).
Shaft encoders are generally supplied in kit form comprising an encoder
module, slotted disc, and hub. The encoder module usually contains three
infra-red emitting LEDs and three matching photodetectors. The slotted disc is
bonded to the hub ring which is, in turn, fitted to the rotating shaft. The encoder
module is then mounted so that the disc is interposed between the LEDs and
photodetectors.
The outputs of the encoder module are sinusoidal (as shown in Figure 9.16)
and these must be converted to TTL-compatible input pulses in order to interface
with a standard digital input port. For simple speed-sensing applications, a
typical input stage based on an operational comparator and low-pass filter is
shown in Figure 9.17.
Unfortunately, the simple circuit of Figure 9.17 is ineffective at very low
frequencies and for stationary position indication. In such cases, the circuit
shown in Figure 9.18 may be employed. Here, the potentiometer (RV1) must be
adjusted so that the potential at the inverting input of the comparator is equal
to that present at the non-inverting input. In this condition, the comparator
produces a near 50% duty cycle.
A further refinement is that of providing an output which indicates the sense
of rotation (i.e. clockwise or anticlockwise). This may be achieved with the aid
of some additional logic and a single JK bistable element as shown in Figure
9.19. The Q output of the bistable goes high (logic 1) for clockwise rotation and
low (logic 0) for anticlockwise rotation. Figure 9.20 shows typical waveforms
for the logic shown in Figure 9.19.
Fluid sensors
A number of specialized sensors are available for use with fluids. These sensors
include float switches (both horizontal and vertical types) and flow sensors.
