High Speed Counter Missing Pulses?

Thanks in advance for any help..

We just built a new machine utilizing a Delta SS PLC. There is an arm on a chain that is being pulled by a 3/4 HP DC motor. connected to the shaft is a 60 pulse encoder with a 6:1 gear ratio. every rotation of the shaft takes 3 seconds and generates 360 pulses.

When the high speed counter on the Delta reaches the setpoint, the motor/arm is supposed to stop. but we are having horrible inconsistency with the stopping point.

the motor is connected to a pot and when the speed of the motor is increased the counter is even more inconsistent.

example:

motor speed @ 50% - 400 pulses = 24" of travel
motor speed @ 75% - 400 pulses = 40" of travel

i can only assume that our high speed counter is not seeing all the pulses of the encoder. but i'm open to suggestions!

any help anyone can give is greatly appreciated!

i guess you are not using high speed counter inputs (or they are not configured correclty). even the smallest PLCs will have high speed counter able to run at least few kHz. 3 seconds for 360 pulses is some 120Hz (give or take, i don't expect this to run longer time at or below 100Hz and for a moment spins at incredible rate).

you can also have mechanical slip (encoder does not turn as much as expected as it's loosely coupled).

you can have high vibrations that throw off encoder.

you can have wiring problem (loose contacts for example).

you can have shielding problem (shiled has to cover whole cable from begin to end with exception of 1" or so where you terminate.

you can have grounding problem (ground loops causing false signals, check if the encoder shield is connected to ground in one place only).

you can have damaged encoder (scratched or cracked or dirty disc).

you can have power problem with encoder (is the power steady and without transients).

maybe the cable is just too long and routed too close to noise source (VFD or Servo output for example).

etc.

wakeet said:

every rotation of the shaft takes 3 seconds and generates 360 pulses.

Click to expand...

Actually this sounds somewhat slow and should not be a PLC problem.

wakeet said:

When the high speed counter on the Delta reaches the setpoint, the motor/arm is supposed to stop. but we are having horrible inconsistency with the stopping point.

the motor is connected to a pot and when the speed of the motor is increased the counter is even more inconsistent.

example:

motor speed @ 50% - 400 pulses = 24" of travel
motor speed @ 75% - 400 pulses = 40" of travel

Click to expand...

This sounds more like an improperly applied motor to me.

Tell us more about the motor and controller what stopping/holding mode does it have?

wakeet said:

connected to the shaft a 60 pulse encoder with a 6:1 gear ratio. every rotation of the shaft takes 3 seconds and generates 360 pulses.

Click to expand...

Also, please tell us more about this 6 to 1 ratio. Does the shaft rotate once and the encoder rotate 6 times? Or does the shaft have a 6 to 1 ratio to the motor?

Thanks for the quick reply!

regarding the 6:1 ratio, the shaft rotates once and the encoder rotates 6 times.

I'll find out more about the motor and controller and post back.

i guess you are not using high speed counter inputs (or they are not configured correclty). even the smallest PLCs will have high speed counter able to run at least few kHz. 3 seconds for 360 pulses is some 120Hz (give or take, i don't expect this to run longer time at or below 100Hz and for a moment spins at incredible rate).

Click to expand...

this was one of my concerns but as far as i can tell it is configured correctly. i'm using the DCNT C235 and input X0 as instructed in the manual.

maybe the cable is just too long and routed too close to noise source (VFD or Servo output for example).

Click to expand...

the cable is approx 6 feet, although we could probably reduce that to 2 feet if necessary. we also changed the path to remove it from any other cables to prevent interference.

update...

the scan time of my program was 10.2 ms. i removed about half of the instructions in an attempt to clear all the fat. (this additional functionality is not a priority at this moment) the scan time is now 6.4 ms.

without changing anything else the arm moved farther than before. this tells me that the plc is now seeing more pulses than it did previously.

does this mean that there is a problem with the high speed counter?

This sounds more like an improperly applied motor to me.

Tell us more about the motor and controller what stopping/holding mode does it have?

Click to expand...

The motor is a Dayton 1/2 hp 1725 rpm general purpose DC motor. it is on a 50:1 speed reducer.

The drive is a Fincor Single-Phase Adjustable-Speed DC Motor Controller (Series 2330MKII)

I'm not sure what kind of stopping/holding mode it has.

thanks!

I don't know anything about your plc but this is what the manual says

3. System structure of hardware high-speed counter:
A. There are Reset signal and Start signal of external inputs in HHSC0~3. It also can be Reset signal
by setting special M, M1272 (HHSC0), M1274 (HHSC1), M1276 (HHSC2) and M1278 (HHSC3).
And it can be Start signal by setting special M, M1273 (HHSC0), M1275 (HHSC1), M1277
(HHSC2) and M1279 (HHSC3).
B. If input external control signals of R and S aren’t used when using high-speed counter, the
function of input signal can be closed by setting M1264/ M1266/ M1268/ M1270 and M1265 /
M1267/ M1269/ M1271 to True. The corresponding external input can be used as general inputs.
Please refer following for using.
C. When using special M to be high-speed counter, control input with START and TRSET and the​

action will be affected with scan time.

Click to expand...

Mechanically disconnect the encoder. Reset and start the counter. Monitor the counter value. Mark the zero position of the encoder. Rotate the encoder shaft at or about the speeds expected for 10... 20... 30 revs forwards noting the count value at each multiple of 10 revs. Now rotate the shaft backwards the same number of revs. Verify the counter value returns to 0

wakeet
IMO step one should be to check the value of the PLC's encoder counter when the motor has stopped.

motor speed @ 50% - 400 pulses = 24" of travel
motor speed @ 75% - 400 pulses = 40" of travel

Click to expand...

Motor speed 50%, set point 400 counts = 24" of travel = PLC's counter value = 400 counts when stopped?
Motor speed 75%, set point 400 counts = 40" of travel = PLC's counter value = 400 counts when stopped?

if both of above are true then the problem is in your feedback loop.

Motor speed 50%, set point 400 counts = 24" of travel = PLC's counter value = 400 counts when stopped?
Motor speed 75%, set point 400 counts = 40" of travel = PLC's counter value = 640 counts when stopped?

If these are true then the problem would be in your motor control loop.

Tommy

i guess you are not using high speed counter inputs (or they are not configured correclty). even the smallest PLCs will have high speed counter able to run at least few kHz. 3 seconds for 360 pulses is some 120Hz (give or take, i don't expect this to run longer time at or below 100Hz and for a moment spins at incredible rate).

Click to expand...

problem solved! as it turns out the first suggestion was the correct one.

i called the tech support for the Delta PLC. (i'm a total newb at this, only been working with PLC's for a few days! the programmer was out of the country so i was the internal guy with the task of troubleshooting...)

The programmer didn't utilize high speed counters in his original program. after my research i felt that a high speed counter was necessary for our application. i reporgrammed it utilizing the high speed counter.

X0 was the encoder input (correclty)
DCNT and C235 was the instruction (correctly)

my problem was i had X0 as the trigger to start the counter. once i removed the X0 from that line and used another trigger the counter started seeing every pulse.

it entailed rewriting a few sections of the program and utilizing the counter differently but all is well!

instead of my previous location tolerance of +/- 1" i am now +/- 1/32"!

thanks so much for everyone's overwhelming support.

Explanation required

I am trying to program a PLC (Programmable Logic Controller), Model: DELTA, DVP 28SV series, in Ladder Programing mode.
I need to use a High speed counter, (C235, using X0 pulses) but could not find a clear explanation in the DELTA Manual or their software program, WPLSoft ver 2.20.11
Can anyone write a small routine, that uses the above C235, that will definitely work as a trial?
Can I use the simulator to try it?
Thanks