Temperature control; motorised gas valve

[ParaffinPower]

Hello

I have a motorised gas valve. There is no feedback.

I'd like to control a temperature with it. Is there a preferred methodology?  I'm still at the 'open it, wait, gauge the reaction'. Is this the only option open to me?

Thanks 

[Jay Anthony]

Without feedback, the normal method is to time the valve from full open to full close and use the time as your manipulated variable.

[DanW]

Quote

 

Presumably it is not analog input (0-10V or 4-20mA) from your statement.  Such an actuator typically has no feedback because internal feedback is used for positioning.   A feedback analog signal is sometimes an option.

Sometimes actuators can be converted from one control signal to another.

What control signal does the actuator respond to ?

Is voltage applied for valve opening, it springs closed when voltage is released?

Is a voltage applied to one terminal which drives the actuator one direction; voltage applied to the other terminal drives the actuator in the other direction?  Actuator holds position when no voltage is applied?

Edited June 1, 2016 by DanW

[ParaffinPower]

Thanks for the replies. 

Your last guess is right. It stays where I leave it.

I can move the actuator but then do I have to wait to monitor the temperature response? 

[kaiser_will]

You have the building blocks for a PID loop, with temperature as your feedback.  Does your controller support PID control?

[DanW]

>'open it, wait, gauge the reaction'

That's essentially what proportional control P, PI or PID, does.

The task for PID control is to determine whether a change in output (the position of the valve) is needed and how much change.

It does that by comparing the process variable (PV), the measured temperature at the process, to the setpoint (SP), the value the operator wants the temperature to be.  If the PV does not equal the SP, the difference is considered to be the "error".   A positive error drives the output in on direction (say, increases the output), a negative error drives the output in the opposite direction.

How big an output change is made is determined by the tuning constants, the values used for Proportional band/gain, Integral/reset and Derivative/rate; which is the P and the I and the D in PID.

"open it" (or close) is the change in output, which covers the range of 0-100% valve travel (closed to full open)

A proportional controller doesn't 'wait' but continues to calculate based on the error.  Zero error is when the feedback temperature is the same as the setpoint temperature.

When the feedback temperature is not the same as the setpoint temperature, then the PID alogrithm 'gauges the reaction' and calculates how much change is needed.

To do PID, you need PID functionality in the PLC, a temperature sensor (thermocouple or RTD) in the process, an analog input for the temperature sensor, and the logic to control that actuator based on the PID's 0-100% output value.  

Developing a logic algorithm to drive that type of actuator can be a project in itself (there's probably a thread on this forum discussing it).  If that actuator has an internal 'slidewire' (check and decode the model number) there are commercial converter modules that convert a conventional 4-20mA (analog output) proportional signal to the on-off logic needed to drive the motor based on the motor's slidewire feedback.