Encoder or pulse transmitter...

[Borte]

Hello everybody!

I have a conveyor that is working in the following order:

1. Part loaded onto conveyor
2. Conveyor moves / "steps" forward (moves a fixed distance; motor runs until it receives a signal from a proximity switch which are mounted on a plate that rotates with the conveyor. This plate will give a total of 5 pulses pr. rotation. 1 pulse = 1 step)
3. Conveyor waits a certain time
4. Sequence repeats...

The thing is that I now want to be able to vary the distance the conveyor are moving from the plc. So I was thinking along the lines of using a encoder (absolute, since I'm not able to move to a referance) or using some kind of pulse transmitter. Or maybe it's just as simple as adding more "pins" to the rotating plate so I'll get more pulses pr. revolution. I think this should cover the resolution thats needed.

Does anyone in here have any experince with which solution I should use? Encoder / pulse transmitter?

Thanks in advance!

Cheers
Borte

 

[RheinhardtP]

Problem with pulse speed

I had a simular application recently using a shuttle car that moves between a feed position Px1 a line 1 and 2 Px1 and Px2.

After getting a pulse tranmitter that works on a slotted fixed beam. the transmitter is connected to the moving shuttle car.

The problem i had that was my cycle time was too high and i missed pulses because of the speed the shuttle had to travel over the distance.

Solution was to get a high speed counter card.

Even with the additional card the prize was still cheaper than most encoding devices we have used in the past.

It was also a better option because of the simplicity of the counting device, the techies in Swaziland hav no experience with encoder type devices....

Regards

 

[Borte]

i dont think scan time is a issue for me here since we're already able to stop the conveyor on a signal that's comming from a sensor connected to a plate that rotates with the conveyor. I think the actual pulse that I'm receving today is close to one second long.

So you think that using a pulse transmitter and counting the pulses with the plc would work?

Cheers
Borte

 

[RheinhardtP]

Willl work

Yes i cant see any reason why not, as long as your differences between your pulses are long enough.

I actually did the Factory Acceptance of this shuttle car running the unit at a slow pace. Worked nicely.

I'm assuming the 5 pick-ups are at this point your only 5 references on stopping the motor.

Are you considering adding extra reference flags for additional positioning??

It will be alot chepaer that an encoder....

Regards

 

[Borte]

I have been thinking about adding more pick-ups to increase the resolution. I will only use this as a base for the pulses though since, I sometimes have to stop between two of the pick-ups.

Cheers
Borte

 

[OkiePC]

IF your conveyor speed is available to the PLC, and the 5 pulses are equally spaced, you may be able to interpolate points between the existing pulses via the formula D=RT.

 

[Bruce99]

conveyor distance

There are many ways. Cut an 8" circle out of steel and around the edge, cut out 10 equal distant sections. Mount on the non drive shaft and use a 110v prox mounted looking up (or down) at the wheel. The number of pules equals the distance. Use the input to pulse a counter. If the counter Acc equals 200 stop motor. If the conveyor runs at a high speed, use a DC prox and a high speed dc input card. I have even seen bolts welded to a piece of round stock welded on the take-up shaft. A larger chain sprocket also works. The prox has to be mounted very close to work though. Any shaft movment will knock the prox off its mount...) This sounds like a fun project.

 

[Borte]

Bruce99: Basically you have descriped the setup of the oven the way it is today. The only difference is that we only have 5 pulses not 10.

OkiePC: I don't have any feedback of the speed. It's running at a "fixed" speed over a contactor only.

I think based on the feedbacks here that adding more puckups so I get more pulses pr. revolution would be the best solution. Then I'll just connect this signal to a counter since this is not running at a high speed.

Cheers
Borte

 

[Lancie1]

Borte,
You kind of missed Bruce and Okie's points. If you have 5 pulses, (or 10 pulses, or 20, or whatever number) occurring over some time period, you also have the average speed, as Speed = (Distance that conveyor travels per pulse) multiplied by (number of Pulses) divided by (amount of Time in which they occurred).

More importantly, if you have a plate with prox switches that are triggered proportionaly to the movement of the conveyor, then you should be able to calculate how many feet (meters) of travel each pulse represents. By adding extra pulses, either with plate or preferrable a rotating wheel on the shaft, you can control the conveyor to stop at any given point (within the tolereances of your motor braking and other errors, such as belt stretch, switch drift), by counting the pulses and setting equal to the distance traveled.

 

[Borte]

I know I can calculate the speed but I'm not interested in the speed. I want to stop the conveyor on certain positions that's between the 5 positions thats available to me.

I can however use the speed to calculate how long I have to run the conveyor to get to the postion I want, but I think this adds another (not needed) level of calculation.

Cheers
Borte