How to measure speed without an encoder

[bdoutney]

AB ML 1100
I have a machine with a gripper conveyor coming off it with a prox giving gripper pulses. I measure the time between pulses to preset a value in a timer that varies the amount dwell time a gripper release cylinder waits to fire after a gripper pulse at different speeds. this is working fine.
Now they have a new process that needs to turn off when above say 6500 grippers/hour. So I need to have some sort of speed measurement to work with that I can scale to the true machine speed and I only have the time between grippers value. I have done this with encoders but cannot get this to work. The problem is I am not getting a workable dwell time between grippers value

 

[AlFo]

Can you explain your problem again?

"Now they have a new process that needs to turn off when above say 6500 grippers/hour. So I need to have some sort of speed measurement to work with that I can scale to the true machine speed and I only have the time between grippers value. I have done this with encoders but cannot get this to work. The problem is I am not getting a workable dwell time between grippers value"

Not sure what exactly you mean by this^^?

-
Al

http://theplchub.com

 

[iant]

by calculation the 6500/H is 0.55 seconds per gripper
when you say turn off you can simply put in this value that
will stop the gripper if that 'Watchdog' time is ever faster than that value.
If, however, you are looking at an overall hour -
you need to count total pulses over a timed period and ensure it is a greater value.
How crucial is the hour time though. personally I would use a 0.5 sec watchdog timer this translated to 7200 but allows for speed variation of the gripper. If this is your problem.

 

[bdoutney]

I am trying to get a value that tracks the changing speed of the machine. All I have right now is the gripper pulse inputs. I was measuring the time between pulses and moving that to N7:0 . I tried to use N7:0 in a LES instruction but it did not work Even though n7:0=600 at the time and was more than source B=100 it still turned on the output bit.

 

[iant]

you may have trouble with the scan time of the PLC.
I can not open your program at the moment.
you need a delay to do the check - probably time one cycle
then check the value during the next cycle - then time the next cycle
(every second cycle)
or
count 10 cycles and check that value during the 11th cycle

 

[bdoutney]

The grippers/hour time is not crucial. So by watchdog do you mean an EQU instruction with my gripper time value=A and .5 sec =B ? I will try that.

 

[iant]

Time the value from pulse one to two take a snapshot of the time value
(ONS MOV to register) then compare that value to be less than 0.55 or 550
start again after that check is complete (a small delay time)
do this over three pulses
Pulse 1 - time
Pulse 2 - snapshot - measure
Pulse 3 - return to - time

 

[bdoutney]

Thanks, it worked great... And then I did not need to use it. This was to stop the timers that were being preset by the pulse speed because at higher speeds there always seemed to be a problem and I needed to constantly be readjusting the the values. Well our mechanics were up there and they noticed the whole actuator assembly was loose and was vibrating down the rail at higher speeds.

 

[iant]

that would be right
I have seen that many times
- It is always the program LOL