Chapter 5
INDUSTRIAL CONTROLS
2 What is the difference between a continuous variable and a discrete variable?
Answer: A continuous variable (or parameter) is one that is uninterrupted as time proceeds, and it is generally considered to be analog, which means it can take on any value within a certain range. A discrete variable (or parameter) is one that can take on only certain values within a given range, such as on or off. 3 Name and briefly define each of the three different types of discrete variables. Answer: The three different types of discrete variables are (1) binary, (2) discrete other than binary, and (3) pulse data. Binary means the variable can take on either of two possible values, ON or OFF, open or closed, and so on.
Discrete variables other than binary are variables that can take on more than two possible values but less than an infinite number. Pulse data consist of a train of pulses and each pulse can be counted.
5 What is feedforward control?
Answer: Feedforward control is a means of control that anticipates the effect of disturbances that will upset the process by sensing them and compensating for them before they can affect the process. The feedforward control elements sense the presence of a disturbance and take corrective action by adjusting a process parameter that compensates for any effect the disturbance will have on the process.
6 What is adaptive control?
Answer: Adaptive control combines feedback control and optimal control by measuring the relevant process variables during operation (as in feedback control) and using a control algorithm that attempts to optimize some index of performance (as in optimal control). Adaptive control is distinguished from feedback control and steady-state optimal control by its unique capability to cope with a time-varying environment.
11 What is an interlock? What are the two types of interlocks in industrial control? Answer: An interlock is a capability by which the controller is able to sequence the activities in a work cell, ensuring that the actions of one piece of equipment are completed before the next piece of equipment begins its activity. The two types of interlocks are (1) input interlocks, which are
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signals originating from external devices and (2) output interlocks, which are signals sent by the controller to external devices.
15 Are programmable logic controllers (PLCs) more closely associated with the process industries or the discrete manufacturing industries?
Answer: The discrete manufacturing industries. They replaced the electromechanical relays previously used to control on-off type control actions.
Chapter 6

HARDWARE COMPONENTS FOR AUTOMATION AND INDUSTRIAL
CONTROL
REVIEW QUESTIONS

1 What is a sensor?
Answer: As defined in the text, a sensor is a device that converts a physical stimulus or variable of interest (such as temperature, force, pressure, or displacement) into a more convenient form (usually an electrical quantity such as voltage) for the purpose of measuring the stimulus.
3 What is the difference between an active sensor and a passive sensor?
Answer: An active sensor is one that responds to a stimulus without the need for any external power. A passive sensor is one that requires an external source of power in order to operate.
5 What is an actuator?
Answer: An actuator is a hardware device that converts a controller command signal into a change in a physical parameter. An actuator is a transducer, because it changes one type of physical quantity, such as electric current, into another type of physical quantity, such as rotational speed of an electric motor.
8 In a DC motor, what is a commutator?
Answer: called A commutator is a rotary switching device that rotates with the rotor and picks up current from a set of carbon brushes that are components of the stator assembly. Its function is to continually change the relative polarity between the rotor and the stator, so that the magnetic field produces a torque to continuously turn