craigcurtin:
on my system (i am in Australia) so we use 60hz mains
Australia has 50 Hz mains.
Sorry Nick - brain fade there - thanks for the correction
Craig
craigcurtin:
DC42 - for my education can you explain why we need PWM in this scenario - on my system (i am in Australia) so we use 60hz mains - i just have my SSR turning off and on 10 times per second and let the Zero Crossing nature of the SSR handle the actual drop out from the mains - is there some technical benefit to using PWM (other than maybe freeing up more Arduino Resources ?)
From your description, I believe you are controlling the zero-crossing SSR with a software-generated PWM signal at a fixed frequency of 10Hz. You could generate a 10Hz hardware PWM signal from timer 1 instead, but as it takes very little processing power to generate 10Hz signal in software, there is no particular advantage in doing so.
dc42:
craigcurtin:
DC42 - for my education can you explain why we need PWM in this scenario - on my system (i am in Australia) so we use 60hz mains - i just have my SSR turning off and on 10 times per second and let the Zero Crossing nature of the SSR handle the actual drop out from the mains - is there some technical benefit to using PWM (other than maybe freeing up more Arduino Resources ?)From your description, I believe you are controlling the zero-crossing SSR with a software-generated PWM signal at a fixed frequency of 10Hz. You could generate a 10Hz hardware PWM signal from timer 1 instead, but as it takes very little processing power to generate 10Hz signal in software, there is no particular advantage in doing so.
Aah OK thanks - all i have is a tight loop that runs every 100ms to toggle the SSR on and off which is guess is crude PWM
Craig
I like the idea of burst-fire control. it is what I use in my reflow oven for soldering although I didn't know it was called that. I just think of it as super slow PWM. I use a 1 second period and an SSR that has zero-crossing built in. Seems to work really well. The heating elements have such a slow time response that there is really no need to do this faster.
Regarding the control method. If you desire a controlled temperature increase rate then you could make that your controlled variable instead of setpoint temperature.
I have both types of loops in my controller and I switch between the two. The control loop that handles temperature rise is very stable and this is likely because my desired rate requires pretty much maximum oven power. at first it was a little bit to get my head around it but in the end it seems easy. I only have "PI" terms in the loop that uses a temperature rise setpoint.
For reflow soldering the scheme is:
heat slowly (20% duty) until temperature is 40C
Control slope to 1.5 Degrees C per second until temperature is nearing 140C
Control temperature to 150C for 2 minutes
Control slope to 1.5 Degrees per second rise
Temperature reaches 215C - stop.
so you can see the system changes between the two control loops depending on the goal.
Maybe it does not apply to your application but wanted to mention this just in case.
Hi I agree with Chagrin, keep the temp sensor close to the element as it is critical that the element does not exceed your burn temperature.
You will have to tolerate a long warm up time.
You could increase the heating area by splitting your heating element into two or even four smaller elements, this would mean that you have more heating surface area, hence lower temperature differential between element and oil but same energy transfer.
I would assume if it is a large vat that there is some form of stirring used when the oil becomes liquid.
I have used mk3 method to cook wet brick samples in an electric kiln to 1200DegC , PID will keep the temp constant but you may have to ram-up the setpoint over time to help with start up.
The wet bricks could not be exposed to 1200DegC straight away, they had to be dried by controlled ramp-up in temperature.
I used a MELSEC PLC, only because of industrial requirements re IP rating and industrial hardness.
Tom.
PS These problems are the sort of things that arduino can be very good at.