Page 130 - Programmable Logic Controllers, Fifth Edition - Mobile version

an RTD is used as part of a temperature control system. A
controller uses the signal from the RTD sensor to moni- 1450 rpm
tor the temperature of the liquid in the tank and thereby CONTROLLER Load
control heating and cooling lines.
Separate Motor
Flow Measurement tachometer
generator
Many industrial processes depend on accurate measure-
ment of fluid flow. Although there are several ways to Tach
measure fluid flow, the usual approach is to convert
the kinetic energy that the fluid has into some other
measurable form.
Turbine-type flowmeters are a popular means of mea-
surement and control of liquid products in industrial, Figure 6-38 Tachometer generator feedback.
chemical, and petroleum operations. Turbine flowme- Source: Courtesy ATC Digitec.
ters, like windmills, utilize their angular velocity (rota-
tion speed) to indicate the flow velocity. The operation a constant AC supply. In either case, the rotor of the
of a turbine flowmeter is illustrated in Figure 6-37. Its tachometer is mechanically connected, directly or indi-
basic construction consists of a bladed turbine rotor in- rectly, to the load.
stalled in a flow tube. The bladed rotor rotates on its axis Figure 6-38 illustrates motor speed control applica-
in proportion to the rate of the liquid flow through the tions in which a tachometer generator is used to pro-
tube. A magnetic pickup sensor is positioned as close vide a feedback voltage to the motor controller that is
to the rotor as practical. Fluid passing through the flow proportional to motor speed. The control motor and ta-
tube causes the rotor to rotate, which generates pulses chometer generator may be contained in the same or
in the pickup coil. The frequency of the pulses is then separate housings.
transmitted to readout electronics and displayed as gal- An encoder is used to convert linear or rotary motion
lons per minute. into a binary digital signal. Encoders are used in appli-
Velocity and Position Sensors cations where positions have to be precisely determined.
Tachometer generators provide a convenient means of The optical encoder illustrated in Figure 6-39 uses a light
converting rotational speed into an analog voltage sig- source shining on an optical disk with lines or slots that
nal that can be used for motor speed indication and con- interrupt the beam of light to an optical sensor. An elec-
trol applications. A tachometer generator is a small AC tronic circuit counts the interruptions of the beam and
or DC generator that develops an output voltage (pro- generates the encoder’s digital output pulses.
portional to its rpm) whose phase or polarity depends
on the rotor’s direction of rotation. The DC tachometer Optical
generator usually has permanent magnetic field excita- sensor
tion. The AC tachometer generator field is excited by Light
source

Readout
electronics Gallons/minute
Optical
Magnetic pickup Generated pulses encoder

Optical
disk

Flow
Lines

Rotating
turbine
Figure 6-39 Optical encoder.
Figure 6-37 Turbine type flowmeter. Source: Photo courtesy Avtron, www.avtron.com.

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 111

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