Automating Manufacturing Systems - Process Control and ...

plc pid - 25.21
• Block diagrams can be used to describe controlled systems.
• Block diagrams can be converted to equations for analysis.
• Continuous actuator systems can use P, PI, PD, PID controllers.
25.6 PRACTICE PROBLEMS
1. What is the advantage of feedback in a control system?
2. Can PID control solve problems of inaccuracy in a machine?
3. If a control system should respond to long term errors, but not respond to sudden changes, what
type of control equation should be used?
4. Develop a ladder logic program that implements a PID controller using the discrete equation.
5. Why is logical control so popular when continuous control allows more precision?
6. Design the complete ladder logic for a control system that implements the control equation
below for motor speed control. Assume that the motor speed is read from a tachometer, into an
analog input card in rack 0, slot 0, input 1. The tachometer voltage will be between 0 and
8Vdc, for speeds between 0 and 1000rpm. The voltage output to drive the motor controller is
output from an analog output card in rack 0, slot 1, output 1. Assume the desired RPM is stored
in N7:0.
V motor = ( rpm moter – rpm desired )0.02154
where,
V motor
=
rpm moter
rpm desired
The voltage output to the motor
= The RPM of the motor
= The desired RPM of the motor
7. Write a ladder logic control program to keep a water tank at a given height. The control system
will be active after the Start button is pushed, but it can be stopped by a Stop button. The water
height in the tank is measured with an ultrasonic sensor that will output 10V at 1m depth, and
1V at 10cm depth. A solenoid controlled valve will open and close to allow water to enter. The
water height setpoint is put in N7:0, in centimeters, and the actual height should be +/-5cm.
8. Implement a program that will input (from I:000) an analog voltage Vi and output (to O:001)
half that voltage, Vi/2. If the input voltage is between 3V and 5V the output O:002/0 will be
turned on. Include start and stop buttons that will force the output voltage to zero when not
running. Do not show the bits that would be set in memory, but list the settings that should be
made for the cards (e.g. voltage range).
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