Making a control Loop for an induction heater - PLCTalk.net

The Power supply is controlled linearly with 0V meaning 0 kW and 10V meaning 25 kW.

To add some context, the induction heater is heating up a stabilizer bar for a car to bond a bushing. The internal temperature of the bushing needs to be 170 degrees C + for a good bond. The temperature does rise slightly after the induction unit is turned off, but it's not an issue.

Currently, we manually set up the system by creating a relationship between the internal and external temperatures of the bar using a pyrometer. For example, we adjust the power settings until we achieve a desirable cycle time, and then measure the external temperature when the internal temperature reaches 170 degrees C. This external temperature then becomes the setpoint for the PLC. The PLC will monitor the external temperature to reach the setpoint and then turn off the induction unit.

My goal is to provide the PLC with the internal temperature, external temperature, and a set amount of time. The PLC will then command the heaters to heat the bar by adjusting the power supplies' kW settings until the internal temperature is reached. The PLC will essentially record the cycle it just performed in an array. When switched to auto mode, the PLC will replay the calibrated array, and the power supplies will run the same power settings as during calibration.

Hopefully, this gives more insight into the setup I'm working with.

Currently, I have created a hysteresis control program that turns the heater on to a set kW rating if the internal temperature is below the desired point at a given time, and turns it off if it's above. I haven't tested it yet, but I think it might work. Probably not the most ideal method, but I'm not too familiar with PID control, which I believe would be the proper way to do things.

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Induction Heating is a process where a metal object, magnetic or non-magnetic, is placed in a varying magnetic field, technically known as Eddy Currents. Imagine you have a round copper coil and then you pass an alternating current through this coil. That would generate a magnetic field within the coil.

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Then imagine you interrupt this field by inserting a piece of metal inside this coil. The magnetic field would begin to wrap itself around the workpiece at a very high rate. The rate depends on the frequency of the applied RF field. For instance, if the RF frequency is 50kHz, then the field will wrap around the workpiece 50 times per second.

As the field continues to “spin” around the metal, it begins to generate heat within the workpiece. How? Because every metal has a certain amount of resistance within it, and this resistance causes the applied field to create friction within the metal. This, in turn, generates heat on the surface of the metal, commonly known as a Skin Effect. Depending on how long you leave the field, the heat will begin to travel through the metal by conduction. The cool part is that the Coil is water-cooled while the metal piece gets hot!

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