Controlling electric power steering using an Arduino to emulate torque sensor

Question

I am trying to control an electric power steering rack with an Arduino. To do this I need to "trick" the EPS module that there is torque being applied when there really isn't.

On the rack there are 4 wires coming from the torque sensors where the torsion bar is which go into the EPS module. I cut these wires so I could splice in and play around.

I determined each wire is 5+V, T1, T2, and GND. When I connect my own 5V and GND to the torque sensor, I am able to read T1 and T2 via the two analog-in pins on my Arduino. When no force is on the wheel, T1 reads about 2.3V and T2 reads about 2.6V. When trying to steer left, T1 drops while T2 rises and vice-versa.

I have a shield with 4 (100k) digital potentiometers. I figured it would be easy enough to have the Arduino "emulate" the torque sensor, so I connected the EPS module 5+V to POT1A, GND to POT1B, and T1 to POT1Wiper. I did the same for T2 (5+V to POT2A, GND to POT2B, and T2 To POT2Wiper).

My Arduino sketch simply reads the analog pins for T1 and T2 connected to the torque sensor, scales them into a float ranging from 0.0-1.0, and then I tell each digit-pot to adjust the wiper accordingly. What should result is the steering be close to the same as if it were directly connected to the torque sensor.

When I try applying my own 5V source to the POTs, the wiper voltage reading is correct (i.e. I tell the digi-pot to set wiper position to 128, meter reads 2.5V). But, when connected to the EPS module, the reading is off (at 128 it reads around 4V and only changes slightly as from 0 to 256).

I then tested the EPS module wires. Connecting my multimeter to 5+V and GND reads 5V (expected), but also reads 5+V on T1 or T2. T1 and T2 show no voltage.

Also to note when testing voltages on T1 and T2 while the torque sensor is connected to the module like normal, the voltage readings are what I get when I power and read the torque sensor directly.

I have confirmed that the signal from the torque sensor is pure analog, no PWM or digital signals at all.

Any ideas on what the torque sensor is doing to drive the ECU properly to get this working would be much appreciated.

Thanks, Andrew

Answer 1

Looks like you are not driving the output drivers are not strong enough. Many of the sensors have low output impedance (about a kiloohm or less) and thus can provide tens of mA's of current. Your 100K digipot can provide much less than that.

Try adding op-amps to each output in a simple buffer connection. Make sure the op-amp can have the desired input and output voltage (you likely want 'rail-to-rail' opamp) -- for example LM324 will only accept 0V to 3.5V input when powered from +5V.

Answer 2

To me it seems that is is driving a differential signal. You could try using a Single-Ended to Differential Converter to try to emulate the signal that you are seeing with a low output impedance that will not get any loading effects from the ECU as you seem to be experiencing with the digital POTs

Answer 3

It is my understanding of most EPS sensors that the difference in voltage from T1 and T2 tells the controller two things. Which direction for the motor to turn and with how much force, current. The controller needs both, rather the difference in the two.

Answer 4

You are half way there. The torque sensor is activated by the EPS module, but first you have to activate the EPS module by feeding it data as ignition switch, RPM data and the constant 12 volts for the DC motor.

Answer 5

Use a Ratiometric DAC with 12 bit resolution and bandwidth 3.2kHz with a digital low pass filter to generate the emulated sensor output. Make sure your circuit doesn't pull more than 30mA.