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PID temperature exceeds limit set

This topic contains 12 replies, has 5 voices, and was last updated by  Stan 1 year ago.

1
Jean Goubert jeanbono

PID temperature exceeds limit set

09/04/2019 at 16:18

Hi there,
I just finished building a compression and an injection machine
For the compression I had no issue with the temperature control. The SSR is turned off when the temperature is reached and then back on again when it falls down a few degrees
For the injection though, the temperature rises really quickly and then occasionally exceeds the set temperature by almost 70C.
I’ve attached a photo
Does anyone have an idea of why that is and how to fix it?
I bought all three PID from the same ebay seller.

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warrior
09/04/2019 at 20:06
1

ssr broken

starter
09/04/2019 at 20:37
1

Hi jerzeek
(Thanks for your great topic The Big Electronic, it’s been really useful)

 

are both SSR broken? they still turn on and off.
I was hoping it was more to do with some settings of the PIDs…

warrior
09/04/2019 at 20:46
1

Hi @jeanbono

Great to hear, then maybe check with the ssr or PID from the compression oven?
You can measure if the ssr is correct with the continuity mode on your multimeter

Greetz, Jerry

warrior
10/04/2019 at 12:43
2

How close is your thermocouple to the band heater? You might have some lag in the conduction of heat and even though the heater turns off, the thermocouple has not yet reached the same temperature. Some overshoot is normal due to tuning of the variables, you can try increasing the value of the ‘D’ term to reduce overshoot but heating may be slower.

starter
10/04/2019 at 12:55
1

Hi andyn, thanks for your reply
The thermocouples are both clamped in the band heaters.
I’m in the workshop tonight so I’ll try to play around with the settings of the PID
Thanks

warrior
10/04/2019 at 19:40
3

I don’t think you want to clamp the thermocouples on or under the band heaters. They should be in a location not too far away but should definitely be measuring the temperature of what you are trying to control without interference from the heater temperature which could be quite different. The PID settings probably can’t cope with the heater response rate.

Some of the cheaper PID controllers (like the units I’m using) don’t allow for tuning the PID parameters. I would try relocating the thermocouples away from the heaters and in good contact with the tube. Just make sure the thermocouple can’t come loose. If it does, your system will grossly overheat. You might want to add a separate thermocouple and display (they are very cheap) to monitor the tube.

A voltmeter test on the SSR control and output connectors should confirm the PID and SSR working

starter
10/04/2019 at 20:06
1

as far i see in the pictures, the tip of the thermocouples has been removed. this is pretty bad, last time i tried this it did nothing but bad; exactly what you’re describing, I stupidly followed the PP video manuals and ended in throwing this 2 sensors into the trash 🙁

I can’t recall german norms about the distances from heat sources to electronics, but to make sure : 30 cm is a good guess though.

starter
10/04/2019 at 23:19
1

thank you guys
I think I’ve sorted it.
I had the sensor stuck on the back of the band heaters where all the electronics are and i moved them to the front of them by the bolts and all of a sudden it was all fine ‘)
Thanks for your comments

warrior
10/04/2019 at 23:38
1

Glad it worked out. I would still consider adding a diagnostic thermocouple away from the bands (maybe near the nozzle) so you can see what the tube temperature is doing. If it is a bead type TC, you can usually attach it with a wrap of aluminum tape and work the tape down around the bead, or you can clamp it.

warrior
11/04/2019 at 12:09
3

With the REX C-X00 type (about the cheapest you can get, in europe anyway) you can adjust the D term from 0-3600. The default value is 60, increasing it should reduce any overshoot, though I’ve never had the need to adjust them, they seem to just work out of the box.

 

Here’s how I mount the heaters and thermocouples, I turn an aluminium sleeve with an ID that matches the barrel, and OD to match the bandheater and a flange on the end I can drill and tap for the thermocouple. This has many advantages apart from just being neater. The sleeve adds thermal mass which smooths out the heating and cooling effects, I can make it a precise fit on the barrel and I can use any diameter bandheater (as long as it’s bigger) so I don’t need to match the tube to the heater and can even swap to a different size barrel without having to replace all the other components.

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starter
11/04/2019 at 14:05
1

Hi Andyn, that’s a very neat solution!
Sadly don’t have the lathe option in my workshop, yet 😉
What is the sleeve between the heatband and the barrel?

warrior
11/04/2019 at 18:20
1

@jeanbono you don’t necessarily need a lathe part. Here is the block I use. It does not matter so much how you attach it to the tube. I screwed it on but you could firmly clamp it. You mount the block first and then use the threaded hole for the thremocouple to compress the tip against the tube wall. I put  a small depression for the tip but you probably don’t need that.

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