Inverse Relation

ielkhatib

Member
Join Date
Apr 2020
Location
Egypt
Posts
34
Hi Guys,

I'm Using Delta DVP-ES200R On a project, I wanna Establish an Inverse Relationship Between Two Variables (Speed, Motor AMPS)...
I Have the motor current ... What I want to do is:

When the motor amps increases ... the speed of the feeder will decrease automatically and vice versa , keeping the motor amps inside the limited range.
 
A PID with a deadband might work, depending on the dynamics.


Does the Delta have a PID? If not, an equivalent implementation is possible via code.
 
It appears that it has PID function built in, however, you could roll a simple PI controller as the one supplied may be a bit complicated, it appears that these units are loosley based on Mitsubishi & other Japanese makes so it would not surprise me if there are many similarities. Here is a link to the manual.

http://profsite.um.ac.ir/~shoraka/Delta PLC Applications Programmig.pdf

A Quick look at the manual seems to show a number of typical PID applications with good explanations.
 
Last edited:
Yes. But it has been done better elsewhere; I suggest you ask The Google to search for PID control. E.g. see here:

https://literature.rockwellautomation.com/idc/groups/literature/documents/wp/logix-wp008_-en-p.pdf

I suggest the velocity form of the formula, whether you use independent or dependent gains is up to you. There are many other aspects of PID covered elsewhere e.g. update timing (Δt), anti-windup, auto, manual, bumpless transfer, filtering, deadband, tuning, etc. As @parky notes, PI control may be enough, and the final, derivative term of the equations probably does not need to be implemented.

P.S. this is no guarantee that a PID will work for your process. If you are asking how to implement PID, then it may be a stretch for you to implement it correctly and/or tune it. With Integral and Derivative gains set equal to zero, the PID becomes the same as the SCLP instruction @gclshortt suggests, but that may not respond adequately to variation in the process, such as viscosity change in the fed material, to maintain the amps in the target range; increasing the output scaling i.e. K in the SCLP formula

Code:
    (speedMax - speedOutput)   (ampsInput - ampsMin)
K = ------------------------ = ---------------------
    (speedMax -    speedMin)   (ampsMax   - ampsMin)

where
- ampsInput = measured current analog input, known
- ampsMin = low limit of current range, known, constant
- ampsMax = high limit of current range, known, constant
- speedMin = low limit of speed (e.g. 0%), known, constant
- speedMax = high limit of speed (e.g. 100%), known, constant
- speedOutput = speed analog output, calculated
may cause process instability.
 
Last edited:
Whether you use a built-in PID function or a roll your own solution, be sure to not make the adjustments to the speed command too rapidly.
When you increase speed to compensate for a low current condition, the current draw during acceleration to the new speed will be higher than the draw once the conveyor reaches the speed. You don't want to make another correction to the speed command while accelerating. Similarly, the current draw during deceleration will be lower than the draw once the conveyor reaches the lower speed.
 
I've done similar limiting control with blowers. The easiest way to do it is to have a dead band controller. When the amps exceed the upper limit step down the speed and start a time delay. When the delay times out sep down again and delay. Repeat until the current is below the lower limit.

Upper and lower limits, the size of the step, and the delay between steps are all adjustable. The upper limit should be a little below the current that represents an actual problem.

One more hint: most VFDs have a current limiting or rollback function built into the drive that will accomplish your objective without programming and even if the dive is in manual speed control. Check it out.
 
I've done similar limiting control with blowers. The easiest way to do it is to have a dead band controller. When the amps exceed the upper limit step down the speed and start a time delay. When the delay times out sep down again and delay. Repeat until the current is below the lower limit.

Upper and lower limits, the size of the step, and the delay between steps are all adjustable. The upper limit should be a little below the current that represents an actual problem.

One more hint: most VFDs have a current limiting or rollback function built into the drive that will accomplish your objective without programming and even if the dive is in manual speed control. Check it out.
Great tips Tom! This would be my suggestion as well.
 

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