Sensorless homing stepper without an encoder

Questions
1

I have a question regarding the IONICube (4 axis) motor controller regarding sensorless (hardstop) homing.

Is sensorless (hard-stop) homing possible with a stepper motor without an encoder installed for that axis?

My hardware configuration:
Drive Type: IONI Drive (controlled by IONICube Servo Driver)
Motor type/Model:
Z-AXIS: PN: MOT-AN-S-060-005-042-L-A-AAAA
Encoder: NO
Software: Granity
Firmware: 10720
Other details:

  • Homing X & Y using external homing sensor works
  • Sensorless homing Axis A working
    Axis A motor: PN: LEPY10K-25 (Encoder: Yes)

Problems arise when:
Configuring for sensorless homing, I can not seem to get hard-stop (sensorless) homing working with the Z-Axis. This axis does not have an encoder. As there is no position feedback (encoder), rather than relying on a tracking error (FPT) as the hard-stop homing mechanism, I was hoping setting a homing current limit (HMT) would work. Testing with a current limit of 0.25A does not work, the motor just continues to drive the actuator into the hard stop.

How it behaves:
The motor just continues to drive the actuator into its hard stop.
Need to hit e-stop to prevent damage.
Drive movements are smooth and no oscillation is seen in data capture.

How I would expect it to behave:

  • With a low homing current limit (HMT), the controller would detect hard-stop homing as a result of increased current resulting from the resistance of the mechanical stop being reached.

I have set the following configuration:
HMT=0.25A
HMV=5
HMA=1
FPT=400
HMH=150 [Edit: corrected value]

Thoughts?

2

Hard stop homing does not work without an encoder.

Why the drive would increase the current? I mean to ask that how the servo drive would know that there is a hard stop if there is no feedback from the shaft movement to the servo drive?

3

Hi Tommi,

Thanks for the clear answer to my question, that’s really all I need answered.

The source of my confusion is that you website refers to mechanical homing as sensorless. There is no mention that an encoder is required. An encoder is a motion sensor, so a minor update to the website to clarify this dependency for all might be helpful .

As for my wondering if there was a way for a motor overcurrent event to trigger a hard stop, it is not really relevant. As you say, your controller must have some sensor present when a stepper is used. We will either add a homing sensor or an encoder.

Thank you - John