Induction Furnace Crucible Break Detection Design Log 2


The crucible break detection (CBD) detects if the crucible has broken and shuts off the induction furnace. It accomplishes it by having two parallel metal plates underneath the crucible. If the crucible were to break. the metal would pour out and short out the coil. An electrical circuit will then shut off the relay that provides power to energize the 3-phase contractor and thus the entire work coil driving circuit.

First Circuit

The first stage consists of a comparator circuit which compares a voltage set by a potentiometer to the voltage on one side of the plate. The other plate is charged to 5V. When the plates are shorted, the voltage rises on the bottom plate. When the voltage rises above the threshold set by the comparator, the comparator outputs a one. This is then output to the set pin of the SR latch circuit.

The latch circuit takes a pulse from the comparator and sets the output to always be high until the circuit is reset. The SR latch circuit is made with two NOR gates. For more information about the latch visit Wikipedia. The reset pin is tied low with a pull-down resistor and a switch on the control panel resets the output. Led indicators on the control panel indicate the status of the break detection. The green led will light up when Q is low and the red led will light up when Q is high. The Qnot output goes to a single transistor level shifter. I later found out that most pins on the Tiva microcontroller are 5V tolerant so this circuit is not needed. There is also a simple zener diode power supply to provide the 3.3V logic for the microcontroller.

This circuit was very sensitive to stray fields from the induction coil. When I was doing the 3-phase power tests on the circuit, the detection circuit would always trip on startup. This would immediately shut off coil driving circuit. That was not good because I would have to reset the detection circuit after turning on the furnace circuit breaker. Even then it would shut off the furnace randomly. I could tell because I programmed an RGB led to turn purple when the CBD was tripped. I then made a second revision to the circuit to address these issues.


 Second Revision

This circuit was mostly successful in noise immunity. The top plate is charged to 12V provided by a separate supply. The bottom plate is connected to the relay coil. When the coils are shorted, the relay is energized. This makes the output go from high to low. The microcontroller is interrupted at the transition and shuts off the furnace. There is a low pass filter on the output to prevent stray pulses from creating false triggers.

I had another circuit before this one that used an optocoupler instead of a relay. This resulted in the CBD to always trip when the induction coil was energized. The current required to turn on the led in the optocoupler is in the 10s on mA while the current to energize the relay was in the 100s of mA.

This proved to be more reliable than the first circuit but still suffered from intermittent turn offs of the coil driving circuit. I suspected that the microcontroller was at fault. I thought that the high impedance of the GPIO input created a path for stray EMI emissions to enter the circuit. This would cause the microcontroller to shut off the circuit randomly.

Third Revision

In this revision, I get rid of the microcontroller and design a hardware only route. It currently is working great, although I have not tested it with 3-phase yet.

The main element of the circuit consists of a latching relay (part#:G5RL-K1A-E-DC12). This relay is special because it has two coils; one for setting the relay, and one for resetting. The set and reset coils are connected together at pin 9, this is connected to 12V. The other side of the set coil is connected to the top of the CBD, and the bottom plate is connected to ground. When the plates are shorted, the set coil is energized. The test button on the control panel also energizes the set coil. The output switch is closed when the set coil is energized. The only way to reset the circuit is to energize the reset coil with a switch on the control panel.

The output from the relay goes into an OR gate which allows either the microcontroller or the CBD to turn off the furnace. The microcontroller output goes into a single transistor 3.3V to 5V level shifter. The leds on the output indicate the status of the circuit on the control panel. A capacitor on the output of the relay reduces noise from the mechanical switch on the node. The pull-down resistor ties the voltage to ground when the relay is open.