How is the output of PID controller converted into Relay?


Thread Starter


I am a college student. My project is designing a temperature controller and i want to use PID algorithm. I have used a Omron temperature
controller. I find its output is Relay, but I know the PID's output is continuous. How is it converted into Relay. I do not understand it.
Thank you for your help.
Let me start by saying that in the 20+ years that I have been involved it process control, the normal output of a PID controller has almost always been an analog signal, (such as 4-20ma or 3-15 psi.) In temperature loops the output usually drives a steam, fuel or cooling water control valve.

A year or so ago I ran across a controller used on a small research project oven which used a relay output for PID control. I don't mean a on/off control scheme like a thermostat in your house where the heat is on steady at 100% until you reach setpoint and then shuts off to 0% until the temperature falls below the deadband and then cycles on again.

If this PID controller was calling for 40% of the heat element's capacity in order hold setpoint, an on/off duty cycle was continually repeated even when steady at setpoint. In a 10 sec cycle 4 seconds on and 6 seconds off would provide 40% of heat output. If the temperature began to drop, the output could increase to 5 seconds on and 5 seconds off which increased the output to 50%.
This allowed for PID control without a current controller for the heat element.

I hope this helps.

Mike Copelin

One method would be to convert the PID output into a duty cycle. In other words, convert the magnitude of the PID output into a proportional on and off time for the relay control.

Zan Von Flue

Pulse width control.
Basically 2 factor's are needed, Basic frequence and output(%).
The basic frequence is the 'reset time'. For example if your basic frequence is 1 sec, and the controller has a 50% output. 0.5 sec on, 0.5 sec out, then start again. Of course 100% is then always on (no out time). There are however also other times for example min. time.- i.e. the relay can't responed to a 10ms signal. And any other alarm times you can think of.
The shorter the basic time the fast you will cycle the output. A too long of a basic time, regulation will be hard.
In most PLC's there should be either a Pulse width PID controller or at least a built in convertor.


Steve Snodgrass

Some Temp controllers use a relay or digital output instead of an analog. The idea is that
somewhere in the set up of the PID, there is a time proportional output. When the loop calls for an increase in PV, it pulses the output to energize the heaters and depending on the duty cycle, how fast the loop reacts. This is not an absolute answer but should give you the general idea.

you must examine the siemens s7-200 PID wizard.
it can be either analog output or digital output.

software : siemens microwin
What no one has told you, is that if you use a relay that switches any large resistive or inductive load then unfortunately the output relay will die well before its life of approx 2-3 k mechanical operations.
If you plan to use this then the relay will perhaps only last for up to 3 monthhs before the contacts are damaged.
Better to connect up an ssr (solid state relay)
A really high quality programmable temp controller has all parameters available to limit time on\off and cycle rate, but it will probably already have a high speed analogue output to control the heat/cooling function.


Bob Gillooly

The PID of any controller is an internal process that controls and output. The output can be an NPN, PNP, TRIAC, or RELAY type etc. In most Omrons the process type is selectable, and the output module can be purchased and installed to suit your needs. If you run the auto tune feature (if your model support this) it will adjust according to the cycle time value you selected. I have a relay out Omron E5EK-DRT I selected a 20 second duty cycle so the relay will not get hammered on and off every one or two seconds. (this will extend the life of the relay) this relay replaced a failed PNP output (which will fail closed) I am currently waiting to install an NPN output (which would fail open) but I do not anticipate changing my PID. the description "Rx2tee" has given is right on the mark for relay outputs on/off time percentage in a window of time. Other output modules will give an actual percentage of continuos output current or voltage.
The PID temperature controller has a "cycle time" adjustment. Example: set it to one second. Then set the PID settings (or "auto tune" these if you want). As you approach your setpoint, you will get a time proportioning output. Example: 90% output = .9 seconds "on" and .1 "off". Your process will stabilize at some level such as 40% (.4 on and .6 off). The PID numbers will determine how quickly that
percentage gets changed when a disturbance occurs.

Caution. A one second cycle time is used with solid state relays (SSRs). For slower devices use: 10-20 seconds for mercury contactors or
60 seconds for mechanical contactors. Fast cycle times offer the best control and minimize thermal stresses in the heaters.