PID musings

[Mike W1]

Noticed a couple comments lately and can't find them to reply......so

It appears to me that mine use what I'd call ON/OFF control as you can visibly see when the power is going to the controlled appliance. It figures out a cycle that will maintain that temperature pretty closely.

Another soul mentioned it took longer to bring the pot up to temperature using his new PID. I guess it would because it's going to shoot out power in that ON/OFF cycle as above.

I hadn't paid any particular attention to it on the pots as I'm usually getting things collected to use and set in place. But yesterday while pouring some mold laps it was readily apparent that the hot plate was going through just that sequence and took quite a bit to get where I wanted it at.

So today I wired in a SPDT switch. Now I can still see the temperature but with the switch towards the outlet it just keeps climbing constantly. When you get close to temperature flipping the switch allows the PID to take control. Best of both world's for the $5 toggle switch. BTW it keeps that hot plate between 379°-381° F when set at 380°to get that particular mould where I want it. Fiddling with the nuts for the lap and getting ready for the next pour usually dropped mould temperature about 25°, and easily remedied with a short perch on the hot plate. Getting to like that plate more and more as I use it.

[el34]

That on-off is called PWM for Pulse Width Modulation and is the common method for controlling motor speed and heaters and other things with a digital 'total on or total off' method as opposed to putting out half the voltage or whatever fraction would otherwise be appropriate. The huge advantage has to do with power dissipation/heat generation of whatever is doing the controlling, and it works great. Even light dimmers from decades ago use that technique. The amount of effective power going to the thing is the ratio of on time to off time. Or on time to total cycle time, whichever.

If the controller has been 'trained' it should keep power on continuously until it gets close to the set temp, maybe 20 deg shy. It starts to cycle on/off so that it won't overshoot but that's a big part of the P, I, and D values and their interaction. Even when the power is off the residual heat in the heating element will continue to heat up the pot, that's what the PID is counting on. The P, I, and D values are 'discovered' by the autotune and tells the controller the heat movement characteristics between element and TC such as how much heat is still in the element and how much will it continue to heat up the lead and how long will the heat transfer take.

[RP]

Interesting I just got my PIDs going for the most part and I guess its leaning as the temp swings are not as much as they were when I first turned it on. I did add a LED light for power on yes I know the pid has one but I wanted a quick glance to tell. I also added a volt/amp meter so I know that my unit is working not just having power sent to it.
My ? is did this start as soon as you got your pid in line or happened later on. Also my thoughts for my hot plate to warm molds was some plate steel drilled and tapped for my TC sitting on the hotplate but I prefer Alum. for the weight. Which would be the better if I got the choice?

[edctexas]

On my hotplate, there was a stud with a nut in the center bottom of the hot plate. The heating coil goes around the hot plate's bottom about 3" from the center. I mounted a TC that has a terminal lug on to the center stud and brought the TC's wires out thru an existing hole. I plug the TC into my PID. This works better fro me than attaching it to an aluminum plate. The temp is not even across the hot plate surface. The AL plate had more even temp across the area but seemed to struggle to get up to 380-390F. I think the plate lost more to the air. I tried a shield around the hotplate, but that shield got in the way. So I am still using the TC bolted to the bottom center of the plate. The temp right above the heater element is much hotter than the center. To heat an alloy ingot, I set it on a slab of AL. Setting the ingot above the heater element sometines causes melting. So I use the AL pad which seems to prevent that.

[el34]

Interesting I just got my PIDs going for the most part and I guess its leaning as the temp swings are not as much as they were when I first turned it on. I did add a LED light for power on yes I know the pid has one but I wanted a quick glance to tell. I also added a volt/amp meter so I know that my unit is working not just having power sent to it.
My ? is did this start as soon as you got your pid in line or happened later on. Also my thoughts for my hot plate to warm molds was some plate steel drilled and tapped for my TC sitting on the hotplate but I prefer Alum. for the weight. Which would be the better if I got the choice?

The first thing I did was run the Autotune thing. And for no intelligent reason I did it several times since but can't say they made much difference other than the initial one.

I read somewhere to start the Autotune when it's 30 or so degrees below the setpoint so it can measure what it measures when it's kinda close. But there should be no reason to run Autotune every time you set a different temp.

Steel vs aluminum- heat flow will happen much faster in aluminum than steel, meaning whatever is supposed to be getting hotter will take less time. But steel will hold its temp longer and be more temperature stable.

[georgerkahn]

Referencing http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html, they define thermal conductivity as," the quantity of heat transmitted through a unit thickness of a material - in a direction normal to a surface of unit area - due to a unit temperature gradient under steady state conditions"
The thermal conductivity for ALUMINUM is 205; for carbon STEEL is 43.
I have an 8" diameter by 3" thick aluminum disc atop my General Electric el cheapo, and PID governed hot plate. Set most of the time for 400*F, it seems to range from 393* to 410*. I have a hole I drilled longitudinally midway -- 1 1/2" from both top and bottom, in which I have my type K T/C inserted. I squeshed in some thermal conductivity compound first, and I believe the over-temp is due to the mass of my block. But... I'm ecstatic re it's operation. ('Specially using moulds with pins for HP or HB!) If I had to do it again, again I'd use aluminum.
georgerkahn

[Mike W1]

http://i.imgur.com/jT04Daq.jpg
I'd still choose aluminum, thicker if I had it, but 1/4" was what was on hand. Drilled and tapped that thicker piece on for the TC. Next time out I'll take some surface readings around the plate as that had never occurred to me before. As I mentioned above I set my PID at 380° and that keeps my mould in the 310° range.

I record my readings for particular chores so they're inserted before I even fire things up. The other day while raising the set temperatures by 20° the unit still stayed in that ± 1° range. But before installing the SPDT switch and waiting to get up to set temp there was a considerable amount of time no power was being applied out. Have always watched that somewhat anyway as I have a neon indicator that glows when power is going out.

And I'm real sure once the PID has AT'd the learning is OVER. Usually over/undershoots for a couple cycles after it's finished and then what you have is what you get.

[el34]

And I'm real sure once the PID has AT'd the learning is OVER. Usually over/undershoots for a couple cycles after it's finished and then what you have is what you get.

I'm pretty sure that's right. I've made 3 PID controllers, the Lee pot and two more for glasswork kilns. One of the kilns tuned well but like yours it took a while to heat up the last 50 deg or so, on its way to 1050, lots of on-off-on-off. My pot controller jumps right on up there with only a few deg overshoot.