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Hank needs a simple mechanism to manage the heating system in a home.
Mr. Jones gives Hank a call because his furnace isn’t provding the heat he and his family needs for the evening. He has set the temperature to 70 degrees but the rooms are not warming.
In this instance, course control with an acceptable tolerance is more than adequate. This is where on-off control comes into the mix.
On/off control is the most basic and inexpensive method to control temperature. It operates on the principle of supplying full power to a heater when the temperature is below the desired setpoint.
Once the temperature rises and surpasses the setpoint, power is then turned off until it drops below the hysteresis setpoint. Power is then turn on to restore it to the desired setpoint.
If hysteresis is not implemented the system can, and most likely will chatter. Chattering occurs when the setpoint and feedback are very close to one another and causes relays to turn on and off very quickly which is hard on the components.
The easyE4 makes it very simple to implement this technique.
The software used to program the device contains a function block called “Three Step Controller” that can be used for on-off control. Also, the function block contains all the important parameters such as setpoint, feedback, and hysteresis values making the programming take just minutes.
Likewise, the PLC has built in relay outputs that can handle up to 8 amps, meaning the device can either directly power a heater, or it can pilot contactors for high power heaters.
XH/-XH: Distance X from switching point
H1: Hysteresis 1 for XH
H2: Hysteresis 2 for -XH
Y(t): Operating points for Q1/Q2
Q1: Switch output X = positive
Q2: Switch output X = negative
Let’s take a closer look at how to configure the parameters on the faceplate of the function block.
In the example below we chose a setpoint of 1200 and a hysteresis of 50. Either the positive x, or negative x direction can be used to turn the output on and off. We chose to utilize the positive x direction.
Notice however, that the output is off until the error is greater than XH and has a hysteresis of H1.
Therefore, we inverted the output to function in the opposite way so that Q1 is on until the feedback reaches XH, and then stays off until the feedback decreases by H1 amount.
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