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    Setting up the SET-620 temperature controller for a heat-treating oven.

    The SET-620 is a low-cost PID controller available on Ebay. Because of its low cost and ready availability, it seems to be a popular choice for home-built heat-treat ovens.

    The same controller also appears to be available badged as an N2006SP, and probably has several other numbers as well; I can’t be sure as I’ve only played with a SET-620.

    It seems that one of the reasons for its low cost is that there has not been a great deal spent on writing easy-to-follow manuals.

    The manual can be found at http://www.tridonic.ic.cz/aukro/temp/set620.htm among other places.

    This manual is adequate for anyone who is reasonably conversant with PID controllers in general, but is probably not so easy to understand for someone new to the subject. To be fair, the “Device Application Example” in section 7 of the manual is close to being “our” process and setting it as in the example will probably work OK.

    If you’re interested, you can google PID control and read up on the subject, but the autotune on the SET-620 means you don’t need to understand it in order to use it.

    If you are using it for a heat-treat oven, you will need the SET-620 controller, an SSR (Solid-State Relay) rated for at least the maximum current drawn by your element(s), and a suitable thermocouple. The SSR I used was a “Fotek” 40Amp unit, supplied with the controller.

    The thermocouple was sourced elsewhere. You will need to look for a Type “K”, Mineral-Insulated thermocouple, suitable for use up to at least 1100 degreesC, and it will need to have suitable cable to reach the controller.

    The controller can be programmed with the SSR output side disconnected, but must be autotuned with the process running.

    The way I set it up was as follows:

    Connect the controller mains supply, the low-voltage output to the SSR, and the thermocouple. Do not connect the Output side of the SSR.

    Power up the controller. Let the display initialize.

    Press “Set” (left hand button) to enter the setting mode.

    Four Zeros will appear with the left-hand one (the thousands digit) flashing.

    The configuration password is 0089.

    Use the second (>) button twice to move the flashing highlight to the tens digit, then press the third (down arrow) button twice to set it to 8. Press the > button to move the flashing highlight to the units digit and the down arrow once to get it to 9.

    Press the “Set” button and you are into the parameter setting menu.

    With “INTY” on the top line, press “Set”. The default is “P10.0”. Use the up or down arrow keys to cycle through the input types until you get to “K”, then press “Set” to store it.

    Move “OUTY” to the top line and press “Set”. Default is “2”, which is what you want; The PID output is to the SSR, with the relays used for alarms, which you probably don’t need.

    If it doesn’t show “OUTY” as 2, use the up or down keys to make it so, then press “Set” to store.

    Move “ATDU” to the top line and press “Set”. I used a setting of 20 and it worked fine, so I’d suggest you do the same. I’ll try to explain what this parameter does later.

    Keep going, setting “PSB” to Zero, “RD” to Zero and “CORF” to your preferred units. I used a setting of Zero, for degreesC. For degreesF, you’d use a setting of 1.

    To leave the Parameter setting menu, get “END” to the top line and press “Set”.

    This will return you to the normal operating screen. The top line will show the measured temperature. The bottom line is the setpoint. The setpoint is adjustable with the up and down arrow keys. Set it to Zero.

    By not having had power to the output side of the SSR, you will have made sure the oven couldn’t start heating up while you were faffing about setting up the controller. Making sure it is set to zero at this stage will mean that it won’t start heating up as soon as the output side of the SSR is connected and powered up.

    Power down and connect the SSR output. Double check everything. Power up.

    The controller should initialize, then show the oven temperature (around ambient temperature) on the top line and the setpoint of zero on the bottom line.

    You now need to autotune. As far as I can tell, during autotune, the controller applies full (i.e. continuous) power until it reaches a temperature of (setpoint-ATDU), then it applies zero power until the temperature falls below (setpoint-ATDU). It repeats this, calculates the recommended settings from the amount of overshoot and the time taken, and stores the calculated values. It then controls to the setpoint using those values.

    I autotuned at a setpoint of 220 degreesC.

    My main reasoning was that I would want to temper in the oven as well as Austenize. Overshooting the Austenizing temperature by a few degrees would not be a problem, but overshooting the tempering temperature might lose a Rockwell point.

    To Autotune, raise the temperature setpoint to a suitable value (I used 220 degrees: the temperature was reading about 20 degreesC when I powered up. I added 200 and set it to that. If I’d been using degreesF, I might have added 300) and immediately press the second (>) button, holding it in until the Autotune indicator flashes. Release it and wait. Autotune will take a few minutes to complete. Once complete, the Autotune indicator will stop flashing and the oven should hold temperature at setpoint.

    To see what settings the Autotune has come up with, press “set”, enter the PID menu password “0036” and work through the parameters as you did for the configuration parameters. Don’t change anything, just note them down.

    The values I got were:

    P = 4.4
    I = 0063
    D = 015
    SouF = 0.2
    Ot = 002
    Filt = 0

    Once you get to “End”, exit the PID menu and get back to the normal operating screen.

    If you are anything like me, you’ll want to play at this point. It’s probably worth trying to get a feel for what happens at different temperatures. You will almost certainly find that the rate at which the temperature rises drops off dramatically at higher temperatures. You will probably also find that overshoot also drops off at higher temperatures, to the point where it is negligible at Austenizing temperatures.

    If the overshoot seems to be an issue at tempering temperatures, there may be some scope for tweaking “SouF” and the P,I and D values to reduce it. Personally, I wouldn’t even bother trying. I’d bury anything I wanted to temper in a tray of dry sand and put the whole lot in overnight. The sand will provide a large heat sink to soak up the initial temperature peak, and a long temper with very good temperature control is much more predictable than a quick temper with poorer temperature control.

 

 

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