Heated Bed Tuning

[perlguy 0]

I got me temperature working for my heated bed. It was a bad thermistor or connection to the thermistor. I replaced it and I now have the temperature.

I am trying to now tune the temperature for the bed, and I am not sure if I am doing it properly - so I am hoping someone can help out!

I used a spare thermistor that I had for our Solidoodle printer, I checked the Configuration.h settings on it and it was set as a #6 thermistor.

I have the power connected via a relay so that I can power a QU-BD silicone heat pad using an old computer power supply.

It all seems to be "working", however when I try to do the PID tune using this command:

M303 E-1 C8 S60

It runs through about 4 of the cycles seemingly just fine. The next 4 cycles is repeatedly turns my relay on-and-off rapidly.

Also, the results seem to be pretty different between PID tuning sessions.

Here are the 3 PID tuning session results that I have so far:

(Session 1 & 3 seem pretty close. Session 2 seems to be way off).

bias: 148 d: 107 min: 59.48 max: 60.34

Ku: 319.14 Tu: 26.61

Clasic PID

Kp: 191.48

Ki: 14.39

Kd: 636.87

PID Autotune finished ! Place the Kp, Ki and Kd constants in the configuration.h

ok B:60.00 @:64

bias: 169 d: 86 min: 59.97 max: 60.17

Ku: 1077.39 Tu: 21.23

Clasic PID

Kp: 646.44

Ki: 60.89

Kd: 1715.80

PID Autotune finished ! Place the Kp, Ki and Kd constants in the configuration.h

### ENABLED BED_LIMIT_SWITCHING

bias: 174 d: 81 min: 59.66 max: 60.26

Ku: 343.07 Tu: 27.13

Clasic PID

Kp: 205.84

Ki: 15.17

Kd: 698.11

PID Autotune finished ! Place the Kp, Ki and Kd constants in the configuration.h

Am I even doing this properly?

Also, when I am tuning at 60C - the actual temperature is about 75C (using a touchless thermometer).

Thanks!

 

[joergen 2]

Also, when I am tuning at 60C - the actual temperature is about 75C (using a touchless thermometer).

 

Since your thermistor is not where it actually needs to be, you see a much different temperature (I taped my TC cabe for exact that reason on top of my glass). you need to put a piece of black tape on top of your printing surface, and use that spot with your IR thermometer. using a IR thermometer on shiny surfaces doesn't work. Then dial in the temp you need (PLA needs about 70C)

 

[perlguy 0]

Awesome, thanks!

What about the relay switching on-and-off rapidly? How can I stop that from happening?

It happens only when the target temperature has been reached. Then, it seems to do the rapid switching to try and "maintain" the temperature.

 

[perlguy 0]

Anybody else have any ideas on how to fix the rapid-relaying-relay problem?

Personally, I find the comments about the "bed temperature control" a bit confusing.

Should I be using PID or Bang-bang?

 

[owen 19]

I think you are using PWM when you shouldn't be.

 

[mrjohnk 0]

To build upon what Owen said, you shoud not use PID/PWM controls on a mechanical relay. If you have a solid state relay or MOSFET switched power, PID/PWM can be used as those can take the rapid switching. The mechanical relay cannot. The Marlin firmware can control this via the setting documented below from the Marlin configuration file:

// Bed Temperature Control

// Select PID or bang-bang with PIDTEMPBED. If bang-bang, BED_LIMIT_SWITCHING will enable hysteresis

//

// uncomment this to enable PID on the bed. It uses the same ferquency PWM as the extruder.

// If your PID_dT above is the default, and correct for your hardware/configuration, that means 7.689Hz,

// which is fine for driving a square wave into a resistive load and does not significantly impact you FET heating.

// This also works fine on a Fotek SSR-10DA Solid State Relay into a 250W heater.

// If your configuration is significantly different than this and you don't understand the issues involved, you proabaly

// shouldn't use bed PID until someone else verifies your hardware works.

// If this is enabled, find your own PID constants below.

#define PIDTEMPBED

 

[gr5 2,172]

In configuration.h in the marlin build you want the following commented or not commented as shown:

//#define PIDTEMPBED

//#define BED_LIMIT_SWITCHING

#define MAX_BED_POWER 255 // limits duty cycle to bed; 255=full current

If you haven't built your own Marlin yet, well, I recommend it. It's not too hard. You have to install the arduine IDE and you have to download the latest Marlin (which I'm pretty sure has some key bugs fixed that aren't in cura yet). There are instructions if you google about how to build marlin. Very detailed instructions. It ended up not being so hard.

To get help on what to comment in and out in configuration.h I read the comments in there and I also used daid's web page and looked at the configuration.h that came out of that.

 

[perlguy 0]

That worked, thanks!

And idea how to better control the temperature? It currently seems to be running about 10-15 degrees higher than the thermistor is reporting.

Should I try changing the thermistor settings until I find one that is the closest?

I really do appreciate all of the help being offered!

Brent

 

[owen 19]

Should I try changing the thermistor settings until I find one that is the closest?

Brent

 

That's what I did. Mine is still 15 degrees out but it's consistent and working OK so I'm sticking with it. You may find a closer match with yours. You can put an entry in the Thermistor tables of the firmware too I think but I've never seen instructions or bothered going down that path. Another thing you can do is adjust your 4.7k resistor.

 

[perlguy 0]

Awesome, thank you Owen!

 

[gr5 2,172]

Don't just trial and error your thermister. Presumably you bought it from somewhere and there are specs for it? These thermisters come with a formula. You can use the formula to generate your own table if there isn't one in the firmware already. There's a bunch in thermistertables.h.

Or if you have an accurate thermometer you could calibrate the thermister yourself at 10C intervals

 

[smoothol 0]

Hey guys,

I need some help and am wondering if you guys have the knowledge!

basically i've got a 25amp DC-AC SSR connected to 1000w heatbed and mains power.. its all connected properly - the rating is 250V on the ac terminals of the relay and there is hardly any V on the dc terminals, the multi-meter says a pesky 18 when its on the 2000m DCV setting.

so I think it is connected properly, however im unsure of what firmware setting i would have to change to get it switching?

i've uncommented

#define PIDTEMPBED

#define BED_LIMIT_SWITCHING

however when turned on it still doesn't allow enough V to the mks base.

any ideas how i can get it up and running??

**also i have connected the + - DC to the 12V of my MKS base, is this correct or should it be 5V?? (would i have to change pins in firmware?)

Edited August 20, 2016 by Guest