Rs Logix 500

Greetings to all, My scenario is this... I am in the diecasting arena. I have a automatic ingot loader with a conveyor. I need to start and stop the conveyor in approx. 6 inch increments which will place an ingot in the loading position. What is the best way. I have a limit switch that can monitor the position of the conveyor. I was using a timer and a limit switch, but it is not consistent. The conveyor is "home made", so the spacing could be bad. Sometimes the limit swith gets made and the conveyor wont move until I take the L/S off the cam. Thanks in advance

Belt Positioning

Sounds like this could be a good application for a belt
encoder or shaft encoder (Tach, PPI-pulse post indicator).
These devices can be configured to provide a given number
of pulses per inch. Program a counter in the PLC to count
the pulses and at the preset number of pulses stop the conveyor.

Using this method you may need to consider a few other elements.
The speed of the belt, the scan time of the PLC and Mechanical Lag
time of the equipment. Many pulses per inch in a very short time
may not be counted correctly by the PLC. If the conveyor is low
speed and you are using a PPI with a resolution of 2 pulses to 16
pulses per inch a normal PLC input should work OK. For a high speed
belt you may need to install a High Speed Counter input module to
capture all the pulses.

You may also want to install a pneumatic Cluch/Brake betwwen the
motor and the reducer to stop and start the belt for more accurate
belt position as oppossed to using the motor starter for position
control.

I hope you find this information helpful

Thanx Dan

another alternative might be to use a proximty switch to detect your product on the conveyor to shut down the process as needed, you could use a through beam or inductive or capacitive depending on the make up of the sensing material

Try this:

Latch the conveyor Run signal until the switch is made.
Unlatch the Run signal as soon as the swith is made and allow the conveyor Run signal to be Set on your timeout Bit.

Sounds a bit confusing but it should work.. I will do the logic tommorrow and attach the file if I get chance which should help

I have considered those types of solutions, however, I want to be able to do this with ladder logic only. I have some other ingot loaders with are controlled with CP1100 processor by Eagle Signal Controllers, and they operate just fine. I do not have a method of communication with these controllers. The last guy must have taken them home, ha ha. Thanks again. I am open to more suggestions. FYI, I am using micrologix 1000 to do this.

ELarsen said:

Sometimes the limit swith gets made and the conveyor wont move until I take the L/S off the cam

Click to expand...

To address that issue, you might want to use a one-shot from the limit switch to initiate the stopping of the conveyor.

|    L.S.   1-SHOT     STOP REQUEST

|----] [-----] [------------( )

. <--- Side note: Do we still need to do this 'period thing' after closing the ladder tag Phil?
Then it won't matter that the limit switch remains ON. With a one-shot, you're only looking at the transition from OFF to ON.

You said you have other conveyors that work... What type of motor / drive are they using? Are they using a brake as Dan mentioned? Or maybe they're ramping to a slow speed before stopping? You need to investigate how the other conveyors are able to stop precisely...

There's no 'magic' you're gonna do with a PLC if you only have a limit switch and motor contactor to work with...

beerchug

-Eric

PLC scan time and the propagation delay time for signal from the limit switch can be an issue.

What kind of sensor are you using (AC or DC output ?)
AC type sensors are typically much slower than DC types.