25.6 PRACTICE PROBLEMS
(Note: Problem solutions are available at http://sites.google.com/site/automatedmanufacturingsystems/)
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 'rpm'.
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
height, in centimeters, and the actual height should be +/-5cm.
8. Implement a program that will input an analog voltage Vi and output half that voltage, Vi/2. If the input voltage is between 3V and 5V the output 'warning' 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).
9. List and describe the most important control memory parameters required to enable a PID function.
10. Implement the system in the block diagram below. Indicate all of the settings required for the analog IO cards. The calculations are to be done with voltage values, therefore input values must be converted from their integer values.
