WorksheetsSensors and Control Systems Worksheet
Total questions: 50
Worksheet time: 50mins
A sensor that generates its own electrical signal without requiring an external power source is known as a(n):
Passive sensor
Active sensor
Digital sensor
Contact sensor
Which of the following is a drawback of using an RTD (Resistance Temperature Detector) compared to a thermocouple?
Wider temperature measurement range
Higher sensitivity to temperature changes
Slower response time
Lower accuracy
What is the main principle of operation for a capacitive proximity sensor?
Detecting changes in a magnetic field
Measuring the time it takes for a sound wave to return
Sensing changes in the electrical capacitance
Detecting light reflected from an object
A linear variable differential transformer (LVDT) measures:
Angular velocity
Linear displacement
Force
Light intensity
What property makes an inductive proximity sensor suitable for detecting metal objects only?
Its reliance on a light beam
Its generation of a magnetic field
Its ability to detect changes in pressure
Its use of a sonic transmitter
The smallest change in a measured variable that a sensor can reliably detect is its:
Accuracy
Hysteresis
Range
Resolution
A strain gauge works by:
Measuring temperature and converting it to voltage
Generating voltage through a piezoelectric effect
Changing its resistance in proportion to applied strain
Emitting light and measuring the reflection
Why is a 4–20 mA signal loop commonly used for process control transmitters?
It is insensitive to line resistance, especially over long distances
It allows for higher data transfer speeds
It is a purely digital signal, eliminating conversion errors
It is the only standard that PLCs can accept
Which of these sensors is best suited for a dirty industrial environment where dust or grime might interfere with light-based sensors?
Photoelectric sensor
Ultrasonic sensor
Optical encoder
Laser sensor
What is the phenomenon where a sensor's output value for a given input differs depending on whether the input value is increasing or decreasing?
Repeatability
Drift
Linearity
Hysteresis
A control system that measures the output and feeds it back to the input for comparison is known as a:
Feedforward system
Cascade system
Open-loop system
Closed-loop system
The "I" in a PID controller stands for Integral and is responsible for:
Dampening oscillations
Eliminating steady-state error
Increasing the speed of response
Responding to the current error
What is the primary characteristic of an open-loop control system?
It is more accurate and complex
Its control action is independent of the output
It is immune to external disturbances
It has a fast response time without any offset
Increasing the proportional gain (Kp) in a PID controller will generally:
Increase steady-state error
Decrease the speed of response
Decrease the stability and increase overshoot
Eliminate oscillations
What is the effect of increasing the derivative gain (Kd) in a PID controller?
Eliminates steady-state error
Increases oscillations
Provides a dampening effect to reduce overshoot
Slows down the system's response
What is the main purpose of signal conditioning for a sensor signal?
To increase the signal's speed
To convert the signal from analog to digital
To filter noise and amplify the signal to a usable level
To change the signal's frequency
In a cascade control system, the output of the master controller is used to:
Directly control the final control element.
Act as the setpoint for the slave controller.
Provide feedback to the master controller itself.
Perform arithmetic operations.
What type of controller would you use for a process that only needs to be turned completely ON or completely OFF?
Proportional controller
PID controller
On/Off (Bang-Bang) controller
Integral controller
A system where the controlled variable is driven towards the setpoint with a continuous, proportional output is a proportional (P) control system. What is a common limitation of a proportional-only controller?
It can eliminate steady-state error.
It can never reach the setpoint.
It often results in an offset (steady-state error).
It is always unstable.
A feedforward control system is used to:
Correct for errors after they have occurred.
Anticipate and correct for the effects of disturbances before they impact the process.
Provide a more accurate final output.
Measure the output and compare it to the input.
Which programming language for PLCs most closely resembles an electrical relay ladder diagram?
Structured Text (ST)
Function Block Diagram (FBD)
Ladder Logic (LD)
Sequential Function Chart (SFC)
What is the function of the Central Processing Unit (CPU) in a PLC?
To convert analog signals to digital signals.
To provide power to the PLC modules.
To execute the user-programmed control logic.
To connect the PLC to external field devices.
In a PLC program, a Timer On-Delay (TON) instruction is used to:
Immediately turn on an output and then turn it off after a delay.
Turn on an output after its input has been true for a set time.
Count up to a preset value.
Retain its accumulated time even if its input goes false.
A digital input to a PLC is typically used to monitor:
The temperature of a process fluid.
The speed of a motor.
A switch or button that is either ON or OFF.
The pressure in a tank.
What is the purpose of the "scan cycle" in a PLC's operation?
To perform a self-diagnostic check once per day.
To continuously read inputs, execute logic, and update outputs.
To download a new program from a computer.
To display the program on an HMI.
A retentive timer (RTO) in a PLC is different from a regular timer because it:
Has a longer time delay.
Does not reset its accumulated time when its input goes false.
Operates only when the input is pulsed.
Can only be used for delays of less than one second.
If an output device needs to stay active after the initial trigger signal is removed, which PLC programming technique is most commonly used?
Latching (seal-in circuit)
A simple timer
A counter instruction
Direct output addressing
The main benefit of using a modular PLC over a fixed PLC is:
Lower cost for small applications.
Ease of programming with Ladder Logic.
High scalability and easier maintenance.
Smaller physical footprint.
An analog output module on a PLC is used to control a:
Limit switch.
Motor that needs to be either ON or OFF.
Variable speed drive (VFD).
Push button.
In PLC addressing, what does the term "image table" refer to?
A table of historical data.
A graphical representation of the process.
A memory area holding the status of inputs and outputs during a scan.
A table listing all connected field devices.
A technician notices that a temperature control system is constantly oscillating around the setpoint. What is the most likely cause?
The proportional gain (Kp) is too low.
The integral action is too slow.
The derivative action is too high.
The proportional gain (Kp) is too high.
When installing a photoelectric sensor with a reflector, the most critical aspect for reliable operation is:
The color of the object.
The weight of the object.
The alignment of the sensor and the reflector.
The temperature of the object.
A 4–20 mA transmitter's signal suddenly drops to 0 mA. What does this most likely indicate?
The process variable is at its minimum value.
The process variable is at its maximum value.
There is an open circuit (e.g., a broken wire) in the loop.
The transmitter is uncalibrated.
A motor is controlled by a PLC via a standard motor starter. If the motor fails to start but the PLC output signal is ON, what is the most logical next step for troubleshooting?
Check the PLC program for a fault.
Replace the sensor connected to the PLC.
Check the wiring and power supply to the motor starter.
Increase the PLC's scan time.
A technician needs to implement a safety feature that requires an operator to press two buttons simultaneously to activate a machine. Which type of PLC logic is needed?
OR logic
AND logic
NOT logic
XOR logic
An ultrasonic sensor is used to measure the level of a liquid in a tank. If the sensor starts giving erratic readings, what should a technician investigate?
The liquid's color.
The liquid's specific gravity.
The presence of foam or condensation on the sensor.
The ambient temperature around the tank.
A pneumatic actuator connected to a PLC is not extending when commanded. The first step in troubleshooting should be to check:
The pressure of the compressed air supply.
The PLC's power supply.
The wiring to the analog output module.
The PLC's network connection.
A limit switch is used to detect the position of a moving part. The switch is wired as a normally-closed (NC) input to the PLC. What is the expected input status when the moving part is pressing the switch?
The input is ON (logic 1).
The input is OFF (logic 0).
The input state is irrelevant.
The PLC will enter a fault state.
A Variable Frequency Drive (VFD) is an example of an actuator that is often controlled by a PLC using an:
Analog output.
Digital input.
Discrete output.
Timer function.
When commissioning a new system, why is it important to perform an I/O check before writing the control program?
To test the speed of the PLC.
To verify that all sensors and actuators are correctly wired and functioning.
To measure the system's power consumption.
To simulate the process behavior.
A transducer is a device that:
Provides a constant output signal.
Converts one form of energy into another.
Measures only electrical quantities.
Is only used in open-loop systems.
What is the primary difference between a fixed PLC and a modular PLC?
A fixed PLC is more expensive.
A modular PLC can be expanded with additional I/O and communication modules.
A fixed PLC has a faster scan time.
A modular PLC is only used for very small applications.
In a process control system, the "setpoint" is the:
Current value of the process variable.
Output signal from the controller.
Desired or target value for the process variable.
Error between the process variable and the desired value.
Which programming language is best for implementing complex mathematical calculations within a PLC?
Ladder Logic (LD)
Function Block Diagram (FBD)
Structured Text (ST)
Sequential Function Chart (SFC)
The term "gain" in control systems refers to:
The speed of the controller's response.
The ratio of the change in output to the change in input.
The overall energy consumed by the system.
The delay between the input and output.
What is the main function of an encoder in a motor control application?
To provide feedback on the motor's speed and position.
To turn the motor ON and OFF.
To amplify the power signal to the motor.
To measure the motor's temperature.
What is the main advantage of using a solid-state relay (SSR) over a mechanical relay?
Lower cost.
Longer lifespan due to no moving parts.
Higher current capacity.
Simpler wiring.
A SCADA (Supervisory Control and Data Acquisition) system's primary role is to:
Directly control all field devices.
Provide a high-level, centralized monitoring and control interface for a plant or process.
Write and debug PLC programs.
Perform high-speed logic operations.
What is the most common cause of integral wind-up in a PID controller?
The derivative term is set too high.
A sudden change in the setpoint.
The controller output is saturated (at its limit) while the error is still present.
The proportional gain is too low.
The purpose of a Human-Machine Interface (HMI) in an automation system is to:
Control the motion of a robot.
Execute the core control logic.
Provide a graphical interface for operators to monitor and interact with the process.
Act as a safety interlock.
