Controllers & Positioners

Question 1

List the five elements of a control system.

Answer 1

1. Detector: senses the change in value of the parameter being measured (level, pressure,
2. Transducer: converts the output of the detector to a usable electronic or pneumatic signal.
3. Transmitter: sends the signal to the controller.
4. Controller compares the value of the parameter from the transmitter with the desired value (setpoint). The error signal is the difference between these two signals and is used to regulate the control signal to the final control element.
5. Final control element: varies the process so that the value of the parameter being measured may be kept within the desired band.

Question 2

What is dead time?

Answer 2

The time delay between the change in a process parameter and the process controller sensing of that change

Question 3

As it applies to control systems, explain the term “feedback”.

Answer 3

Information on the controlled variable that is sent, or fed,

back to the controller via the measuring element

Question 4

Describe the operation of a closed-loop system.

Answer 4

The controlled variable change is sensed by the measuring element. The controller compares the measured variable with the setpoint and generates an
error signal. The final control element regulates the
manipulated variable. The change is detected by the measuring element.

Question 5

Describe the operation of an open-loop system.

Answer 5

Another method of control is to regulate all the inputs to the process by the use of separate controllers and separate final control elements. If the controlled variable is not used to adjust any of the inputs to the process, the system is considered to be an open-loop control system.

Question 6

State the purpose of a controller.

Answer 6

A controller is a device that compares an input signal with a setpoint and generates an output based on the difference (error signal).

Question 7

What is a direct acting controller?

Answer 7

Controller output increases as the measured variable increases

Question 8

What is a reverse acting controller?

Answer 8

The controller output decreases as the measured variable increases,

Question 9

Describe the theory of operation of the two-position controller.

Answer 9

The device has two positions: on or off.

Question 10

What is dead band?

Answer 10

The deadband (or neutral zone) represents the region of measured variable overlap within which the controller output can be either on or off, depending on the state of controller output prior to the measured variable entering the deadband.

Question 11

Describe the theory of operation of the proportional controller.

Answer 11

It provides a stepless output that can position a control valve at intermediate positions as well as fully open and fully shut. With proportional control, the final control
element has a definite position for each value of the measured variable.

Question 12

What is gain?

Answer 12

It is the ratio of output change to input change for a proportional controller.

Question 13

What is proportional band?

Answer 13

The change in the input required to produce a full-range change in output.

Question 14

How are gain and proportional band related?

Answer 14

Gain and proportional band are inversely related. Gain = 100%/PB

Question 15

What is offset error?

Answer 15

The difference between the setpoint and the measured
variable is called “error”. This error signal is used to maintain the control valve at a specific position in response to a change in demand of the system. This inherent error means that the measured variable will equal the setpoint at one condition only. At all other conditions an error, called the “offset error”, will exist between the setpoint and the measured variable.

Question 16

How are gain and offset error related?

Answer 16

The larger the gain, the smaller the offset error. The smaller the gain, the larger the offset error.

Question 17

Describe the theory of operation of the proportional-plus-reset (PI) controller.

Answer 17

It automatically resets the measured variable to the setpoint. Thus, offset error is eliminated. The controller
accomplishes this by continuing to change the output after the proportional action has been made. The reset portion of the controller performs the mathematical operation of integration. The integral portion is defined in
“repeats per minute”. This is in reference to how often the integral gain term is calculated. Essentially, the integral gain term is added to the proportional gain term.

Question 18

Describe the theory of operation of the proportional-plus-reset-plus-rate (PID) controller.

Answer 18

To overcome the disadvantages of a PI controller, a “rate section” may be added. The rate section responds to the rate of change of the error signal. It accomplishes this by increasing the controller output as the error signal rate of
change increases. Thus, the controller anticipates large error signals. The rate portion of the controller actually performs the mathematical operation of differentiation. A
gain term is calculated based on the rate of change of the error signal. A larger rate of change results in a larger derivative gain term. This derivative gain term is added to the proportional and integral gain terms.

Question 19

What is the flow characteristic of a control valve

Answer 19

It is the relationship between the flow rate through the valve and the percentage of valve travel.

Question 20

What are the characteristics of a flow control valve that is described as linear?

Answer 20

The flow rate is directly proportional to the valve position.
With these types of flow characteristics, an incremental change in valve position will cause a constant incremental change in flow rate at any given point in the valve travel.

Question 21

What are the characteristics of a flow control valve that is described as linear?

Answer 21

The flow rate is directly proportional to the valve position.
With these types of flow characteristics, an incremental change in valve position will cause a constant incremental change in flow rate at any given point in the valve travel.

Question 22

What are the characteristics of a flow control valve that is described as quick opening?

Answer 22

It produces a large linear change in flow rate at low valve

travels to obtain about 90% of the total flow at only about 60% of total valve travel.

Question 23

What are the characteristics of a flow control valve that is described as equal percentage?

Answer 23

It exhibits flow rates that increase exponentially with valve travel. The name derives from the fact that for a given percentage increase in valve travel; the valve produces an equal percentage increase in flow rate. In other words, a change in flow rate is always proportional to the flow rate just before the change in valve disk position is made. For example, when the valve is slightly
open and flow through the valve is small, the change in flow rate obtained by opening the valve a given amount is small. On the other hand, when the valve allows a large flow, the change in flow rate obtained by opening the valve the same given amount is large.

Question 24

Describe the operation of controllers.

Answer 24

The measured variable and the setpoint are “summed” at the input to the controller. This block generates the error signal that the controller will act upon. The setpoint is set on the front of the controller using a thumbwheel. The resultant error signal is displayed on an indicator on the front of the controller and applied to the automatic control network. When the controller is in automatic,
the network conditions the input as determined by the characteristics selected for control of the particular process (proportional, integral, and/or derivative). The transfer switch permits the operator to use the automatic signal or a manual signal to control the operation of the final element. The auto/manual transfer switch and the manual control pushbuttons are on the front of the controller. The output signal is amplified and sent to operate the final control element, and is also displayed
on a meter on the controller front.

Question 25

State the function and describe the characteristics of valve positioners.

Answer 25

To maintain the control valve disk at a position that is directly proportional to its controller output pressure. Additionally, the valve positioner may function to supply a proportionally larger pneumatic signal in order to move the valve.