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Open Load status not resetting in all situations

Russell Gunn 5 months ago in Master modules / MC4x updated by Gustav Widén (System support) 5 months ago 1

I am having a problem with the open load detection on a COUT on am MC43FS

  1. I am running IQANdesign 5.03.14.5040
  2. The manual indicates that the output detects open load two different ways which may be true, but the controller responds in three different ways.
    1. When the output is off, if the load is disconnected, the controller will set the open load status to TRUE, when the load is reconnected, the controller will set the open load status to FALSE and allow normal operation.
    2. When the output is on and passing some normal current, if the load is disconnected the controller will set the open load status to TRUE and turn off the output, if the load is reconnected, the open load status remains FALSE, and the output remains off until the command is set to zero. Setting the command to zero causes the controller to reset the open load status to FALSE, after which the output will respond to a normal current command.
    3. When the output is on, AND some fault causes the resistance of the load to increase such that less than 50% of the commanded current can be pushed through the load, the controller will, set the open load status to TRUE and turn off the output. Unfortunately commanding the output to zero does not cause the controller to reset the open load status to FALSE. This acts like the open load status is latched to TRUE if it was tripped by “extreme saturation”
  3. It is my belief the third response is a bug in MC43FS because, a MC2 running IQANdesign 4.07.7.4605 works as expected I.E. commanding the current to zero would reset the open load status to FALSE Such that the controller could lower the current and try again.

I believe this question was already answered in a support case to one of our RAEs, but for anyone else reading the post, here is a brief explanation: 


The important functionality for safety is that the COUT stays off until the command has been zero.

This is for avoiding unexpected restart after intermittent faults.

Then there are differences in how the controller detects the fault. There is open load and under current, both reported as status open load in IQANdesign. For hardware diagnostics that only run when the output is on (as with this question on COUT undercurrent), the controller will make an attempt at clearing the error and activating when the command changed from zero to a nonzero value.

MC2 is quite different, it only has one way of checking for open load; by checking undercurrent when activating.