Shift Register Not Tracking

[drbitboy]

In the attached image, encoder pulses are scaled to constant relative time at three different line speeds: 16Hz; 8Hz; 4Hz; Hz ≡ pulse/s.

Assumptions

• BSR (Bit Shift Right); reject pusher touches bottle at bit position 0
• 16 encoder pulses model distance from camera to reject
  • PLC inserts fail bit at bit position 16
• ΔTreject = 0.25s: pusher delay is 250ms and constant for all, i.e. independent of, line speeds
  • from PLC writing the output bit to command fully retracted pusher to extend, to pusher (or air) touching bottle
  • PLC needs to issue reject trigger signal 0.25sbefore fail bit reaches bit position 0
    • i.e. before modeled reject bottle reaches modeled reject pusher
• Camera inspection has no delay; fail bit is written at bit position 16 i.e. at camera position*

Result

• Fail bit position at which PLC issues reject trigger is linear with line speed:

Trigger Position = ΔTreject * speed​

This is what @Steve Bailey said in Post #2.

* Adding an inspection delay, e.g. ΔTinspect = 25ms between camera inspection trigger at bit position 16 and fail signal received by PLC, when failed bottle is "at" a downstream bit position, simply adds that delay to the Trigger Position calculation:

Trigger Position = (ΔTinspect + ΔTreject) * speed​

 

[tarik1978]

One idea is to slow down to a low constant speed for reject, and then after the reject is done resume to normal speed.

 

[Steve Bailey]

One idea is to slow down to a low constant speed for reject, and then after the reject is done resume to normal speed.

You won't make any friends in production with a suggestion like that.
And the OP stated that the reject timing was fine at normal line speed, but too early at low speed.

 

[drbitboy]

One idea is to slow down to a low constant speed for reject, and then after the reject is done resume to normal speed.

In addition to what @Steve Bailey said, this does not correct if the inspection delay is significant.

 

[ASF]

Boy, I should learn not to post after 18 hour days...lol. I explained to myself how it's an inverse proportional function and then called it linear. 100 lashes for me.

Talk about not seeing the forest for the trees.

I will say in "practical" terms, most production lines run within a relative narrow confine of speed range and therefore linear approximation works 99% of the time. But in theory from 0 to infinity, ASF and Bitboy are completely accurate that across that range, it's a decay function.

Mea culpa all around

We've all been there. I still have vivid memories of that day realising that dwell time and speed were not linear, and it completely broke my brain. I couldn't focus on anything else until I'd rewired my brain to make sense of it

 

[drbitboy]

We've all been there. I still have vivid memories of that day realising that dwell time and speed were not linear, and it completely broke my brain. I couldn't focus on anything else until I'd rewired my brain to make sense of it

heh. Multiplying by unity is one thing; dividing by unity is a whole 'nuther ball o' wax.