Test whether one value is less than a second value.
Let say we have a HPU system that we want to monitor the temperature of the oil. When we start the HPU system, we want to check if the temperature of the oil is below 70C(Celsius). If it is then we should turn the heater on. We should also disallow the HPU to run till minimum temperature is reached.
Ladder Logic Solution
- Temperature element I:1.0
- Heater O:2.0/0
- HPU No permission B3:0/0
- Push button I3.0/0
Once the HPU start button is pushed then we verify if temperature < 70. It this is not true then we energize the heater and HPU no permission becomes true. Hence we are not allowing the HPU to run.
Once the temperature is greater or equal to 70 then HPU no permission will be false hence the HPU will start.
- For simplicity reasons, we are assuming that the push buttons are maintained signals.
- Notice that the temperature value is passed via an analog input (16 bit word).