This depends on time constant of heating process and allowable short time deviations in temperature gradient. Generally, you didn't tell about accuracy requirements and if utilizing a feedback controller or simple feedforward power control. If process timeconstant are shorter than a few seconds, likely with air/gas heating or small heated zones, you would see at least part of the power switching in temperature graph. I guess, although not mentioned explicitely, you are using full-wave packet switching with mains voltage. To achieve shorter switching periods, you could use a sigma-delta like switching of single full-periods instead.Will it have an effect to the heating process if we control the heater by PWM?
Sinisa said:First of all I would think that you don't have enough resolution from your temperature reading. With 20mV you are reading temperature in 2º increments and you could expect that your system will not be abe to stabilize and at very minimum would oscilate between two values 2º appart.
Second, if you are using opto isolator with triac output you should make sure that is not zero crossing kind. This kind of opto isolator will cripple your time resolution to 100 steps in 50Hz system or 120 steps in 60Hz system.
Third, you should define if thermal inertia of your system (quantity and/or type of material heated) is changing and how much. Depending on this you can choose type of control.
4. You should have sufficient heater power, but not too much. Maximum ramp rate and thermal inertia of the system will determine that. Add some power to spare.[/quote ]
There are three nichrome wires as load connected in parallel so we only have one control with the output. Every load has 1500 Watts and the three heating elements will have 4500 Watts, is this sufficient? When we tried to control the temperature, in every on and off of the circuit, there is a great fluctuation of current that even the monitor of the computer flickers out.
6. Choosing control method:
- For system that can have a bit of temperature variation, ON/OFF system is simplest. Hysteresis is introduced to reduce switching frequency to desired level. Since hystersis means turning on heat on lower level than turning off, temperature will vary between two points. Higher hysteresis, lower switching rate and higher temperature variation.
Yes we are using the conventional ON/OFF temperature control system but we only incorporated the artificial PWM from the program. The algorithm of the program determines how to control the temperature. In one second, there are only two periods - the ON period and the OFF period. Suppose you say that there is a 10% duty cycle to control the heater at first five minutes, the only ON period will just be at the 100 milliseconds and 900milliseconds OFF of the 1 cycle which is one second. If 60% duty cycle, 600 milliseconds ON and 400 milliseconds OFF. So I don't see no problem with the frequency of the signal and if that would synchronize with the frequency of AC which is 60 Hertz.
Thanks for the help.
Yes there is a 2°C step on the ADC 0808 part but we used op-amp to double the output voltage from 10mv to 20mv to make it 1°C per step.
We used the optoisolator MOC3041, is this a non-zero crossing kind?
What is a thermal inertia? We did not consider this parameter when we were designing the system.
At one stable temperature measure On/Off ratio and multiply by heater power. That is power (Pmin) needed for maintaining that temperature. Note that this might be different at other temperatures.There are three nichrome wires as load connected in parallel so we only have one control with the output. Every load has 1500 Watts and the three heating elements will have 4500 Watts, is this sufficient? When we tried to control the temperature, in every on and off of the circuit, there is a great fluctuation of current that even the monitor of the computer flickers out.
Yes we are using the conventional ON/OFF temperature control system but we only incorporated the artificial PWM from the program. The algorithm of the program determines how to control the temperature. In one second, there are only two periods - the ON period and the OFF period. Suppose you say that there is a 10% duty cycle to control the heater at first five minutes, the only ON period will just be at the 100 milliseconds and 900milliseconds OFF of the 1 cycle which is one second. If 60% duty cycle, 600 milliseconds ON and 400 milliseconds OFF. So I don't see no problem with the frequency of the signal and if that would synchronize with the frequency of AC which is 60 Hertz.
Thanks for the help.
Apart from several meaningful things that have been mentioned, e. g . about sufficient resolution in temperature measurement, the discussion to my opinion is somewhat up in the air, cause not a serious specification has yet been given: intended temperature rate, required accuracy in reproduction of setpoint ramp and heated medium (air?) are completely unknown.
Bigger problem you will have if you have large delay between change in output and reading that change on the sensor. As heat propagates through body from heater to sensor, when sensor gives information that output should be decreased, too much energy might already be given by heater to body. That is where overshoot will occur when ramping. PID algoritm can be implemented to compensate for that delay. If body mass changes and this delay chages, you are in trouble if you require tight control of temperature.
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