I have a roboclaw 60amp motor controller connected closed loop to an Electric Power Steer motor/ gearbox by using potentiometers [ one on the control input and another on the steer motor gearbox output shaft] Everything works fine.
I now want to use a force feedback motor attached to the control that will provide a counter torque to the drivers input. One way [ I have read] is to put a current sensor on the steering motor to sense current flow [ bi directional] , the current sensor putting out a analog voltage signal. If I can get that to happen I think I can control the force feedback motor to apply a resistance torque against any applied driver torque at the control using a second controller like a Simple motor controller 18v which I can then program to vary the feedback force.motor
will this : ACS711LC Current Sensor Carrier -12.5A to +12.5A or something similar do the job?.
I assume that if the current sensor is placed in one of the wires of the steering motor it then will sense current flow direction and amplitude… and then output a ratiometric voltage signal?
Problem with the above item is that’s its rated at 12.5 amps whereas I am using max 20 amps [ programmed in the controller ] but I think I can get larger amperage current sensor units.
Does this plan sound right? cheers ad thanks Phillip
If I’ve read this correctly your using a second motor to limit torque? If that’s correct that’s not the way to do it. Forgive me if I read it wrong.
What you should be doing is using the current feedback to limit the input to the main motor. The Roboclaw controller has a built in current sensor and I believe the current limiting is software controllable. Limiting the current also limits the torque and thus force of the gripper.
If you still want to go down the external sensor path there’s a sensor for the ±20Amp range. You can use the ACS711EX wich is a ±31AMP sensor. You’ll need to feed this back into your controller to reduce the output of the Roboclaw controller.
Hi Shaun No, I am using the second motor to give the human
operator a sense of [artificial] feedback when he moves his control to
control the main steer motor to actuate steerable wheels. Say for
example the wheels are off the ground… this will mean that the main
steer motor will use low amperage to steer them and so the low amperage
will give a low voltage signal change[ from the current sensor on the
steer motor wires] and this voltage signal will then create light
feedback resistance in the control feedback motor. On the other hand if
the wheels are taking the weight of the vehicle, more amperage will be
required to steer them… more voltage signal from the current sensor
and so, more resistance torque form the feedback motor attached to the
control. Then, say the wheels get to full lock stops… the amperage
will peak if the driver keeps the wheels against the stops… the
current sensor signal voltage will peak… and the feedback motor will
apply maximum restoring torque on the control so the human knows he has
reached the limits of actuation. " the stops"
The feedback motor is driven by its own controller which is connected
to the voltage signal from the current sensor on the main steer motor
wires and the feedback motor will operate in a stalled condition to
apply its torque to the humans control. [ No torque when the wheels are
straight ahead]. When the wheels are steered the trail of the contact
patch always tends to restore the wheels to the straight ahead [ like on
a conventional car] and this is a way to provide artificial feedback
to the driver although there is no mechanical connection between the
driver and the wheels.
My main enquiry is now about
This can sense bi directional current flow up to 30 amps and produces
a voltage signal which [ I assume] I can then drive a controller
[say a polulo simple motor controller 18/7] to power the feedback
motor. I am " fairly confident" this will work… just not sure.
Cheers Phillip
You could drive it directly with a voltage divider across the sensor output. The current sensor outputs a signal from 0v (-30A) to 5v (+30A) with 2.5v center (0A). The control signal for the motor is 0v to 3.3v with a configurable range and centre.
Make sure your resistor divider combined resistance is above the minimum sensor output resistance of 4.7k.
Thanks Shaun , I feel more confident now. One more question : I
have found the same thing with a higher rating [ 75 amps]
My concern is : what happens if these devices fail? It is well
established that Roboclaw will likely burn if one of its motor leads
becomes disconnected while being powered. So, if the current sensor
fails does this failure result in an open condition?.
I have seen other types of current sensors where the current carrying
lead … simply passes through a hole in the sensor so that the power
cant be lost. I have limited the output of roboclaw to 30 amps and
will" try something"… but any further comments on these variations?
The controller burnout on any inductive load is from a connection connecting and disconnecting repeatedly causing surge currents and voltage spikes at the controller. Many controllers are protected against this but not all of them aren’t. The fault is usually going to be the connector to the sensor which you should check periodically.
You can get AC/DC open loop current sensors where they have a hall sensor in a toroid core but they’re usually a lot more expensive. I haven’t seen any hobby sensors of this type.
Silicon Chip magazine had a DIY unit like this many years ago. I think you can still get kits for it.
