Homing problem

From the original Synthetos forum (2011–2025)

Started by Bill on Nov 23, 2015 in TinyG Support. 1 reply, last activity Nov 23, 2015.

Bill Nov 23, 2015 #

I found the following two issues when I repeated doing homing operation:

1. The homing operation could sometimes fail to complete correctly. A typical outcome when that happens is that, the motor would not reach the specified Zero Back-off position. For example, instead of drive a further Zero Back-off of 14 mm after the switch was released at the coordinate about -4mm, it only moved about 4 mm when the coordinate went to 0. In this situation, the TinyG seems to know the table coordinates correctly but for some reason it stops early before the specified Zero Back-off was reached. For a reason unknown to me, if I reduce the current limit to that axis to a lower value, the homing operation can then be completed correctly. I don't understand that why the higher current would make it incorrect, but the lower current could make it work? Also, the current limit that made the homing work last time may fail to work in a few days like what happend to me today, and I needed to lower the limit again to make it to work. Would you be able to advise why the homing operation would perform like this, and what should I do to ensure a reliable homing operation?

2. The current limit for each axis seems not independent to each other. When I adjusted the current limit of axis X via its potentiometer, it also affected the actual current in axis Y, is it what you would expect? Or how to ensure an independent adjustment of current limit for each axis?

cmcgrath5035Staff Nov 23, 2015 #

2. Can you describe why you believe X current setting affects Y? I.E., how are you measuring current?
Here is a typical schematic page showing two of the 4 drivers.

https://github.com/synthetos/TinyG/blob/master/hardware/v8schematics/v8g/tinyGv8g%20-%20schematic%20page3.pdf

The wiper of each pot connects to one, and only one, motor driver and driver internals control the actual current limiting for that device(only)
The only things common among the current pot circuits is 3.3V and ground.
Have you tried monitoring 3.3V? Based on your description, I might suspect that your Vmot Power supply is going into current limiting when motors turn on, affecting 3.3v which is derived from Vmot input. Perhaps look at 3.3V with an Oscilloscope when doing a move or running a simple Gcode?
Current to the motors (and draw on the Power supply) is narrow pulses - detecting with just a VOM could be difficult.

1. I suggest chasing #2 first. Base on your description, 2 & 1 could be explained by a dynamicaly current limiting Vmot power supply.
Very difficult to predict how significant variation in 3.3V supply might affect normal tinyGoperation.
Folks over time have reported Vmot supplies that looked OK supplying rated current to a static (constant, e.g. a resistor) load but the Supply current limiter was failing under a heavily pulsed load.

This topic is from the original forum (November 2015). Your reply starts a new discussion linked to it.

Different problem? Ask a new question

Back to TinyG Support or latest activity.