Page 400 - Mechanical Engineers' Handbook (Volume 2)

3 Transducers and Error Detectors 391

velocity can be measured by converting it to angular velocity with gears, for example. Ta-
chometers using ac signals are also available.
Other velocity transducers include a magnetic pickup that generates a pulse every time
a gear tooth passes. If the number of gear teeth is known, a pulse counter and timer can be
used to compute the angular velocity. This principle is also employed in turbine flowmeters.
A similar principle is employed by optical encoders, which are especially suitable for
digital control purposes. These devices use a rotating disk with alternating transparent and
opaque elements whose passage is sensed by light beams and a photosensor array, which
generates a binary (on–off) train of pulses. There are two basic types: the absolute encoder
and the incremental encoder. By counting the number of pulses in a given time interval, the
incremental encoder can measure the rotational speed of the disk. By using multiple tracks
of elements, the absolute encoder can produce a binary digit that indicates the amount of
rotation. Hence, it can be used as a position sensor.
Most encoders generate a train of transistor–transistor logic (TTL) voltage level pulses
for each channel. The incremental encoder output contains two channels that each produce
N pulses every revolution. The encoder is mechanically constructed so that pulses from one
channel are shifted relative to the other channel by a quarter of a pulse width. Thus, each
pulse pair can be divided into four segments called quadratures. The encoder output consists
of 4N quadrature counts per revolution. The pulse shift also allows the direction of rotation
to be determined by detecting which channel leads the other. The encoder might contain a
third channel, known as the zero, index, or marker channel, that produces a pulse once per
revolution. This is used for initialization.
The gain of such an incremental encoder is 4N/2 . Thus, an encoder with 1000 pulses
per channel per revolution has a gain of 636 counts per radian. If an absolute encoder
produces a binary signal with n bits, the maximum number of positions it can represent is
16
n
2n, and its gain is 2 /2 . Thus, a 16-bit absolute encoder has a gain of 2 /2 10,435
counts per radian.

3.2 Temperature Transducers
When two wires of dissimilar metals are joined together, a voltage is generated if the junc-
tions are at different temperatures. If the reference junction is kept at a fixed, known tem-
perature, the thermocouple can be calibrated to indicate the temperature at the other junction
in terms of the voltage v. Electrical resistance changes with temperature. Platinum gives a
linear relation between resistance and temperature, while nickel is less expensive and gives
a large resistance change for a given temperature change. Seminconductors designed with
this property are called thermistors. Different metals expand at different rates when the
temperature is increased. This fact is used in the bimetallic strip transducer found in most
home thermostats. Two dissimilar metals are bonded together to form the strip. As the
temperature rises, the strip curls, breaking contact and shutting off the furnace. The temper-
ature gap can be adjusted by changing the distance between the contacts. The motion also
moves a pointer on the temperature scale of the thermostat. Finally, the pressure of a fluid
inside a bulb will change as its temperature changes. If the bulb fluid is air, the device is
suitable for use in pneumatic temperature controllers.

3.3 Flow Transducers
A flow rate q can be measured by introducing a flow restriction, such as an orifice plate,
2
and measuring the pressure drop p across the restriction. The relation is p Rq , where