A customer is getting axes stuck in the PLC Open state STANDSTILL_PENDING during machine commissioning. How can this happen and how can it be avoided?
Motion/PLC 2.6: How to avoid getting axis stuck in STANDSTILL PENDING
Best reply by bschmidt
This state occurs when ctrlX Motion has commanded the drive to AH/AF (S-0-0134 bits 15-14 = 11) and is waiting for acknowledgement from the drive (S-0-0135 bits 15-14 = 11). This is normally a brief transitional state, but in rare cases the axis can get stuck in this state.
One case is if the drive is in Easy Startup Mode. Drive shows Ab. MC_Power is executed (S-0-0134 bits 15-14 = 11) but S-0-0135 bits 15-14 stays 10. This is because the enabling is overwritten internally by the drive. There is no error but the axis is stuck in STANDSTILL PENDING.
A more realistic case (and the actual situation of my customer) is a timing issue coming out of STO condition. The drive status word indicates ‘Ab’ if drive is in STO, no different than if the drive is in Ab and STO is not active. If I turn on an Axis Interface mode bit (for example MODE_AXS_VELOCITY) while the drive is in STO, then the axis gets stuck in STANDSTILL PENDING. (same result if you execute MC_Power)
Here the drive shows E8027, but as soon the the STO contacts are closed the drive is back to "A0012 Control and power sections ready for operation" and the axis remains stuck in STANDSTILL PENDING with no error. At this point you can toggle the Axis Interface mode bits (or MC_Power) many times and it remains stuck. But the situation that caused this could be very brief and you don't know why it's stuck. You end up rebooting the ctrlX CORE to recover.
To avoid this possibility you can add the actual STO status P-0-0106 to the AT and subsequently DataLayer Realtime of the PLC and check bits 2/1 = 00 as a necessary condition for enabling the axis.
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