25.7 ASSIGNMENT PROBLEMS

1. Design a basic feedback control system for temperature control of an oven. Indicate major components, and where they are used.

2. Develop ladder logic for a system that adjusts the height of a box of plastic pellets. An ultrasonic sensor detects the top surface of the plastic pellets. The ultrasonic sensor has been calibrated so that when the output is above 5V the box is in the right height range. When it is less than 5V, a motor should be turned on until the box height results in an input of 6V.

3. Write a program that implements a simple proportional controller. The analog input card is in slot 0 of the PLC rack, and the analog output card is in slot 1. The setpoint for the controller is stored in 'Setpoint'. The gain constant is stored in 'Kgain'.

4. A conveyor line is to be controlled with either a variable frequency drive, or a brushless servo motor. Workers will place boxes on the inlet side of the conveyor, these will be detected with a `box present' sensor. The box position is also detected with an ultrasonic sensor with a range from 10cm to 1m . When present, boxes on the conveyor will be moved until they are 55cm from the sensor. Once in place, the system will stop until the box is removed. After this, the process can begin again when a new box is detected. Design all of the required ladder logic for the process.

5. A temperature control system is being developed to control the water flow rate for cooling a mold set. Unfortunately the sensor in the dies doesn't allow us to measure the temperature. But it does provide a set of bimetallic contacts that close when the die is above 110C. Luckily a Variable Frequency Drive (VFD) is available for controlling the flow rate of the water. The control scheme will increase the water flow rate when the die temperature input, HOT, is active. When the HOT input if off the flow rate will be decreased, until the flow rate is zero. In other words, when the HOT input is on, a timer will start. The time accumulated, DELAY, will be proportional to a voltage output to control the VFD. If the HOT sensors turns off the DELAY value will be decreased until it has a value of zero. Write the ladder logic for this controller.