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CHAPTER 3
INDUSTRIAL ELECTRONICS & ACTUATOR
3.1 Industrial Electronics
 The signal conditioning discussed this far has referred mainly to measurement signal
modification.
 It is often nescessary to perform a type of SC on the controller’s output to activate the
final control element.
o For example, the 4 – 20 mA controller output may be required to adjust heat input to
a large, heavy-duty oven for baking crackers. Such heat may be provided by a 2 kW
electrical heater.
 Some sort of conditioning is required to :
o Allow such a high power system to be controlled by a low-energy current signal.
3.1.1 Silicon-Controlled Rectifier (SCR)
 SCR’s work principle is similar with diode’s. It will conduct current in only one
direction (like diode), if it is turned on or fired.
o Diode :
o SCR :
Anode
Cathode
Anode
Cathode
Gate
 If the voltage (trigger voltage) is placed on the gate with respect to cathode, the SCR
will behave like normal diode. Even if the gate voltage is taken away, it will continue to
conduct like normal diode.
 The only way to turn the SCR back “off” is :
o The voltage must drop below the forward voltage drop of the SCR so that the current
drops below a minimum value, called the holding current.
o The polarity form anode to cathode must actually reverse.
 The fact that the SCR cannot be turned off easily limits its use in DC applications.
 In ac circuits, the SCR will automatically turn off in every half cycle when the ac voltage
applied to the SCR reverses polarity.
 Characteristics and specification of SCRs are as follows:
1. Maximum forward current. There is a maximum current that the SCR can carry in the
forward direction without damage.
2. Peak reverse voltage. Like a diode, there is a peak reverse bias voltage that can be
applied to the SCR without damage.
3. Trigger voltage. The minimum gate voltage to drive the SCR into conduction.
4. Trigger current. There is a minimum current that the source of trigger voltage must
be able to provide before the SCR can be fired.
5. Holding current. This refers to the minimum anode to cathode current necessary to
keep the SCR conducting in the forward conducting state.
AC operation
 Operation of an SCR in varying the rms dc voltage in half wave operation (Fig.7-8).
o SCR turns on in repetitive way.
o SCR is turned back off in each half cycle when the as polarity reverses.
o By changing the part of the positive half cycle when the trigger is applied, the
effective (rms) value of dc voltage applied to the load cabn be increased.
 If more power is required, use the SCR with a full wave bridge circuit (Fig. 7-9).
 In control application, the controller output signal would be used to drive a circuit that
changed the time at which the pulses were applied to the gates, and thus changed the
power applied to the load.
 The voltage applied to the load is pulsating dc. So, this configuration could not be used
with a load that required ac voltage for operation.
Trigger control
 To use the SCR in control application, special circuitry to converts control signal into
suitable trigger signal is required. Look at figure 7-10.
 The control signal voltage is used to provide base drive to a transistor via an LED that
ensures isolation of the control circuit from the power circuit.
o At low base drive the capacitor is charged slowly, and will not reach the SCR trigger
voltage until late in the cycle (hence low load power).
o A large control signal will provide high base drive, and the capacitor will charge
much more quickly. Then the SCR will turn on much earlier in the cycle and more
power will be delivered to the load.
3.1.2 TRIAC
 An extension of the SCR discussed previously is a device that conduct in either
direction. The TRIAC can be thought of as two SCRs connected in parallel and reversed,
but with the gates connected.
 A positive trigger will cause it to conduct in one direction, and negative trigger will
cause it to conduct in the other direction. The TRIAC thus can be used in ac application.
 Figure 7-13 shows the symbol of a TRIAC and a circuit for typical application.
 The voltage across the load, as shown, remains ac. The effective ac rms current value of
voltage applied to the load can be changed by changing the time in the phase of the
cycles when the TRIAC gate is pulsed.
 The trigger voltage must be bipolar, one pulse in one polarity and the next of the
opposite polarity.
 Specifications of TRIACs are similar to those of SCRs: maximum rms current, peak
reverse voltage, trigger voltage, and trigger current.
DIAC
 A DIAC is a special kind of two terminal semiconductor switch that is often used in
conjunction with TRIACs for triggering.
 This device, with a schematic symbol shown in Figure 7-14, is nonconducting (off) in
either direction as long as the applied voltage is below a certain critical value, V L. If that
value is exceed in either polarity, the device will begin to conduct (on).
Trigger Control
 As with the SCR, for the TRIAC to be useful in control applications it is necessary to
link the control signal to the application of trigger signal. This involves adjusting the
time (or phase) on a cycle of ac voltage when the TRIAC is triggered.
 The circuit (Figure 7-15) uses a DIAC between the capacitor and the TRIAC trigger
terminal.
 The principle of operation is the same as the SCR system, but the load is impressed with
an ac voltage and the bridge rectifier is not needed. The capacitor charges in either
polarity until the DIAC conducts and the TRIAC turns on.
3.2 ACTUATORS
 The actuator is a translation of the (converted) control signal into action on the control
element
o For example, if a valve is to be operated, then the actuator is a device that converts
the control signal into action in the control element.
3.2.1 Electrical Actuator
3.2.1.1 Solenoid
 Solenoid is an elementary device that converts an electrical signal into mechanical
motion, usually rectilnear (in a straight line).
 As shown in Figure 7.18, the solenoid consists of a coil and plunger.
o The plunger may be free standing or spring loaded.
o The coil will have some voltage or current rating and may be dc or ac.
 Solenoid specifications include the electrical rating and the plunger pull or push force
(expressed in Newton or kilogram) when excited by the specified voltage.
 Some solenoids are rated only for intermittent duty because of thermal constraints. In
this case, the maximum duty cycle (percentage on time to total time) will be specified.
 Solenoids are used when a large, sudden force must be applied to perform some job.
 In figure 7.19, a solenoid is used to change the gears of a two-position transmission. An
SCR is used to activate the solenoid coil.
3.2.1.2 Permagnent Magnet DC Motor
 DC motor uses a permanent magnet to produce a static magnetic field across two poles.
o Between the poles is connected a coil of wire that is free to rotate (the armature) and
which is connected to a source of DC current through a switch mounted in the shaft (a
commutator).
 For the condition shown in Figure 7.20a, the current in coil will produce a magnetic field
with a north/souith orientation like Figure 7.20b. The repulsion of the PM south and the
coil south (and the norths) will cause a torque that will rotate the coil as shown.
 If the commutator were not split (without commutator), the coil would simply rotate until
the PM and coil north were lined up and then stop.
 The effective voltage is the difference between the applied voltage and the counter-emf
produced by the rotation.
3.2.1.3 Electromagnet DC Motor
 Many DC motors use an electromagnet instad of a permanent magnet to provide the
static field. The coil used to produce this field is called the field coil.
 The current for this field coil can be provided by placing the coil in series with the
armature or parallel (shunt). In some cases the field is composed of two windings, one of
each type. This is a compound motor.
 Characteristics of DC motors with a field coil are as follows.
1. Series field. This motor has large string torque but is difficult to speed control.
2. Shunt field. This motor has a smaller starting torque, but good control characteristics
produced by varying armature excitation current.
3. Compound field. This motor attempts to obtain the best features of both of them.
Generally, starting torque and speed control capability fall predictably between the
two pure cases.
3.2.1.4 AC Motor
 A synchronous AC motor’s speed of rotation is determined by the frequency of the ac
voltage that drives it.
 Its primary application is in timing, because of the high stability of the power line
frequency.
 The rotor is a PM, and the field is provided by coils driven from the ac line. Because of
the inertia of the PM, the starting torque is not very high, but once rotation is started the
PM will rotate in phase with the field reversals caused by the oscillations of the line ac
voltage.
 An induction motor replaces the PM with a heavy wire coil, into which is induced a
current from the changing field of the ac excited field coils.
o As before, once rotation is started the rotor will continue rotation in phase with the
line frequency-induced changes of field coil excitation.
o The difficulty with these motors is that they are not self-starting (starting torque is
very low), and there is no convenient method of its speed control.
3.2.1.5 Stepping Motor
 The stepping motor is a rotating machine that actually completes a full rotation by
sequencing through a series of discete rotational steps.
 It’s easy to be interfaced with digital circuits or computer.
 The simple model shown in Figure 7.24.
o In this motor, the rotor is a PM that is driven by a particular set of electromagnets.
o The switches are transistor, SCRs, or TRIACs.
o The switch sequencer will direct the switches through a sequence of positions as the
pulses as received.
 Look at figure 7.25 to understand the simple operation of a stepping motor.
3.2.2 Pneumatic Actuator
3.2.2.1 Pneumatic Signals
Principles
 In a pneumatic system, information is carried by the pressure of dry air as the gas in a
pipe. The signal information adjusted to lie within the range 3-15 psi (20-100 kPa).
 The pressure signal speed : 330 m/s. It is slower than electrical signal.
 Pneumatics is still employed because of reliability and safety of electrical technology.
Amplification
 A pneumatic amplifier, also called a booster or relay, raises the pressure and/or air flow
volume by some linearly proportional amount from the input signal.
 A schematic diagram is shown in figure 7.4.
 As the signal pressure varies, the diaphragm motion will move the plug in the body block
of the booster. If motion is down, the gas leak is reduced and pressure in the output line
is increased.
Nozzle/Flapper System
 An important signal conversion is from pressure to mechanical motion. This conversion
can be provided by a nozzle/flapper system.
 A diagram device is shown in Figure 7.5a.
Current to Pressure Converters
 The I/P converter gives us linear way of translating the 4-20 mA current into a 3-15 psig
signal.
 A diagram device is shown in Figure 7.6.
 It needs adjustment of the springs and the position relative to the pivot. So that 4 mA
corresponds to 3 psig and 20 mA corresponds to 15 psig.
3.2.2.2 Pneumatic Actuator
 The Pneumatic actuator is most useful for translation of a control signal into a large force
or torque as required to manipulate some control element.
 The principle is based in this linear equation :
o
F  ( p1  p 2 ) A
p1 – p2 = pressure difference (Pa)
A = diaphragm area (m2)
F = force (N)
 The most common are those associated with control valves.
 The action of direct pneumatic actuator is shown in Figure 7.27.
o Figure 7.27a shows the condition in low signal-pressure state, where the spring S
maintains the diapraghm and the connected control shaft in position as shown.
o The pressure on the opposite (spring) side of the diaphragm is maintained at
atmospheric pressure by the open hole H.
o Increasing the control pressure (gauge pressure), applies a force on the diaphragm,
forcing the diaphragm and connected shaft down against the spring force.
 The shaft position is linearly related to the applied control pressure
o x 
A
.p
k
x = shaft travel (m)
p = applied gauge pressure (Pa)
A = diaphragm area (m2)
k
= spring constant (N/m)
 The reverse actuator is shown in Figure 7.28
 The inherent compressibility of gases causes an upper limit to the usefulness of gas for
propagating force.
3.2.3 Hydraulic Actuator
 To overcome the upper limit in pneumatic actuator, we can use hydraulic actuator.
 The basic idea is the same as for pneumatic actuators, except that an incompressible fluid
is used to provide the pressure. The hydraulic pressure is given by :
o
p H  F1 / A1
pH = hydraulic pressure (Pa)
F1 = applied piston force (N)
A1 = forcing piston area (m2)
 The resulting force in the working piston is
o FW  p H .A2
FW = force of working piston (N)
A2 = working piston area (m2)

Thus, the working force is given in the terms of the applied force by:
o FW 
A2
F1
A1
Hydraulic servos
 To control the position of large loads as part of the control system.
 This can be done by using the low energy controller output as the setpoint input to a
hydraulic control system. Look at Figure 7.30.
 On this system, high pressure hidraulic fluid can be directed to either side of a force
piston, which causes motion in either direction. The direction is determined by the
position of a control valve piston in the hydraulic servo valve.
 This position is controlled by a linear motor driven by the controller’s output
Source : “Process Control Instrumentation Technology”, Curtis D. Johnson