Load resistance change detection

currentresistancevoltage

I am trying to detect load resistance changes.

schematic

simulate this circuit – Schematic created using CircuitLab

I need to monitor R load resistance (glow plug).

PWM duty cycle (max) varies from 100% when V1 is 1.2V to 3.7% (8V)

I need to keep the temperature of the glow plug constant. The actual resistance of the glow plug is unknown (0.2 to 0.3R when cold). On the start-up I measure the voltage of the power supply – and power the glow accordingly (glow plus are 1.2V – for higher voltages the PWM duty cycle is k = (V1)^2/(1.2)^2) ). When the engine is running I need to keep the glow temperature constant. So I need to detect changes in glow plug resistance – if it goes down (glow is getting colder) I need to increase the k, when it goes up (glow is getting warmer) I need to reduce the k.

Users can use any battery packs up to 8V.

The problem is that those changes are extremely low. For the particular V1 it quite easy to create the circuit which amplifies them (I was experimenting with the low pass filter and differential amplifier).

schematic

simulate this circuit

But I can't find any way to make it universal for the different V1.

I don't need to measure the actual resistance – I just need to detect the change of the resistance (if is going down I need to increase the duty cycle and vice versa)

Best Answer

Here is a sketch of what the solution might look like. Using some clever software, you can use this to stabilize the duty cycle on some particular value of resistance (current flow) no matter what the battery voltage is. You can sample the current during the ON state to make sure you're still close, then increase and decrease the duty cycle from there.

I just winged this circuit together from memory, so please read up on 'constant current circuits' to see the specifics of how to design the op-amp circuit and how to get the gain you need and how to select a correctly sized sense resistor.

Hope this helps!

schematic

simulate this circuit – Schematic created using CircuitLab