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PLC control programming for two heaters, TCK, s/s relays

PLC control programming for two heaters, TCK, s/s relays

I would like to provide the PLC control for a scenario where the temperature in a machine, heated by the two electric heaters, is controlled with use of 1 TCK and 2 s/s relays. Please see draft “PLC4” sent attach. The TCK is fixed to the machine and is not in a physical contact with the heaters. Nominal temperature of machine is 200degC. The machine temperature tolerance is +/-2degC. This means that the heaters should be turned on when machine T drops to 198degC and turned off when T reaches 202degC.
For this moment I have created the start PLC diagram outlined on draft “PLC4”.
I am still learning PLC. Therefore, I would be very grateful for your opinion whether the PLC diagram shown on picture “PLC5”, in terms of the tools used (direct contact, direct coil, etc.) and their layout, is of proper design.
I know that there is much more work needed to complete this task (e.g. set up of the variables, etc.).

Please help.
Thank you.

RE: PLC control programming for two heaters, TCK, s/s relays

Good Practice says that you need two control loops - a temperature control loop and a high limit shutdown control loop.

Temperature control
The temperature control loop needs three elements
1. an input - the thermocouple (TCK)
2. a logic solver, like a comparator or an on-off control function block, which has a setpoint (SP). In reverse action, (PV below SP) the output is ON when the PV (Temp) is below SP, OFF when PV is above SP.
3. an output element - a digital output that drives the SSR

In your logic diagram, the thermocouple should be on a separate rung of logic as an input to the logic solver block/function. The output of that logic solver function is what you call the switch, a coil that drives the SSRs.

High limit
The failure mode of an SSR is the ON state - the OFF state would be "failsafe" - if the SSR fails, it would not conduct current to the heaters. But since SSR's fail in the ON state, a second loop is required to sense the temperature and turn off an electromechanical switch (called a Safety Contactor in the graphic below), a relay, a contactor or a shunt-trip contactor which is 'upstream' of the SSR (Thyristor in the graphic below) in order to prevent electrical power from reaching the heaters, since the SSR can no longer 'open' in its failed closed position.

High limit control loops are independent control loops and use a temperature sensor (thermocouple) of their own and an independent control loop so that the ability to shut down power to the heaters is not contingent on a single point of failure.

Hence, the high limit function is not shown in the PLC logic because it is done externally as a separate control loop.

RE: PLC control programming for two heaters, TCK, s/s relays

If you have two heaters and control elements you can provide "two stage control".

If the system is 2 degrees below setpoint both heaters are used.
If the system is 1 degree below setpoint one heater is used.
Or some best pattern like this.

Alternatively, write a simple PID controller into the PLC which is fairly unfettered since the control elements are solid state and can be cycled with impunity.

Do note that some systems cannot stand full heater power continuously during startup as it can slag the heating elements. In those cases it requires some amount of PWM until the system is up to some minimum temp.

Keith Cress
kcress -

RE: PLC control programming for two heaters, TCK, s/s relays

I'd definitely do some form of PID controller which results in continuously varying the average voltage applied to the heaters, via PWM or cycle by cycle switching.

RE: PLC control programming for two heaters, TCK, s/s relays

Thank you for your detailed info on the PLC layout for the scenario. I am grateful, it helped me a lot.
I have amended the PLC logic. I send it attached.
I will think about the high limit control loops for the system, thank you for your advice.

Indeed, the two stage control is good option. I think I am able to use PID controller within PLC logic. I send the logic attached.

RE: PLC control programming for two heaters, TCK, s/s relays

Thank you for the reply.
I have added the PID controller into my logic diagram. I send it attached.

RE: PLC control programming for two heaters, TCK, s/s relays

I haven't a clue what the notations like EN, ENO or A mean. I have no idea what T Control H1Z1 is or does. Cryptic notations do not convey meaningful information.

For a PID block, I would guess that A is an analog input to a function block and EN is a logic enable, but who knows?

PLCs require a specific thermocouple input card that applies cold junction compensation and linearizes the thermocouple's mV signal.

I doubt that all that signal conditioning is represented with a binary N.O. contact symbol and label, but again, who knows?

If ENO is a binary output, then yes, it would drive a solid state relay which can handle the heater current.

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