Hi,
I interpreted Excessive Error as being the delta between current state & desired state. Why else would the default be so high? My assumption is/was that if the driver is behind in steps, this is not a good thing but probably not fatal as long as it doesn't overcompensate in increasing feed rate in an attempt to catch up
That understanding is close. The servo drive enacts a closed loop trying to reduce the delta between the actual position and the commanded position.
If the servo is lagging the control loop will increase the drive current in order to catch up, if its leading the command the control loop will reduce or even
reverse the current to reduce the error. The position closed loop bandwidth is of the order of 500Hz and so the servo follows the commanded path with
plenty of'get up and go' and very quickly. This correction happens in milliseconds.
Ideally the servo would follow the commanded position exactly but inevitably it will lag a little behind. The Following Error Window, or Excessive Error Condition
as Delta call it, is the limit on the error before we decide that the machine has failed to follow the toolpath and the machine must EStop.
You may recall that when we used open loop steppers that if a stepper lost a step, or several, the machine would carry on assuming that the stepper was
following the correct toolpath despite the fact that it is not, by virtue of those lost steps. We always wished that Mach would stop so we could correct
the lost steps rather than carry on making a bad part. This is the meaning of Following Error....it allows us to program the 'number of steps gained/lost
before Mach Estops'
So to make accurate parts we want the Following Error Window to be a small as possible, and could therefore be assured that the machine is very closely following
the commanded path. If we make the window too small, then the servo will frequently slip just outside of the Following Error Window and fault out.
In order to have the servo follow a toolpath we wish that it have high acceleration, which is synonymous with torque, which is synonymous with current.
We also want the servos to rotate at high speed so that it may track a rapidly moving toolpath and speed in synonymous with applied voltage, given
that the applied voltage must overcome the back EMF.
A good means of measuring how hard a servo is working is to monitor its current. If the current is high its because its trying to accelerate to catch up.
A good means of measuring a servos speed is to monitor its applied voltage. Fortunately Delta has provided the means to monitor those
quantities as an analogue voltage. Thus you could have two meters which would monitor current and voltage in real time without a scope.
Would that be adequate?
The problem about trying to minimize cycle time is as much about good CAM and speeds and feeds as about servos. The idea is that you run the toolpath
but slowly increasing the feedrate (with the override) until the spindle is at some comfortable level short of max current. Having a toolpath that does not have
occasional short bursts of extremely heavy cutting is essential here, otherwise a heavy cutting event will occur on top of a high continuous load and the
spindle will overload and stall. You have to have good Gcode to do it.
Craig