lumber sizing chain

i am working on a railroad tie sizing operation. there are two end saws, one of which is on a screw driven by a motor so you can move the saw for different length ties (6', 8', 10', ect.). i want to use a joy stick to move the saw in and out and some toggle switches (on/off) for the different lengths. i want to use one limit switch on the movable saw and install trips at each length. i was wanting some suggestions on the best way to do this. i was thinking about using a counter for each trip (count 1 6', count 2 8',ect.). if i do this i do i make it count backwards. i hope this make sense.if any more info is needed i will try to provide it.i don't get to do alot of programming so this is probably simple project and i just don't know what to do.thanks for the help. rodney

there are actually going to be 12 lengths. that is the reason for wanting to mount 1 limit switch on the carriage. these ties have not yet been treated so they are not to messy.thanks for replying, rodney

Roddbodd,

I dont think a single limit on the carrage would be very reliable. It's still suspectable to fouling, switch bounce, miss counts etc...

Is this a factory machine or a field tool? If it's a static shop unit to be used indoors, and since it's untreated material, I would tend to go with an encoder based positioning setup. The encoder would be placed static on one end of the track and be driven by a long open ended timing belt. The belt would be housed in an inverted struct and be driven by a small truck or slide mounted in the strut. The truck/slide would have an angle bracket that would attach it to the saw carriage. At each end of the strut/track mount an overtravel/homing limit to prevent overtravel and also be used to home or correct the encoder count. You can have as many stops as your heart desires, and they all can be adjusted by the operator via a small inexpensive HMI. Accuraccy could be to .001" depending on the encoder & belting you choose.

Is this going to be a machine indoors, which makes railroad ties to be shipped out to a job site? Or is this going to be a machine made to be at the job site, cutting them to required length as you lay them?

If it's a stationary machine, in a shop, then it'll be easy to have an encoder based system with an HMI, leaving yourself open for many possible length choice additions in the future. I don't believe mess will be a big problem, because you can easily guard and cover more "Delicate" parts (The encoder, the screw, any cords and wires used, the PLC).

Tharon,

You seem to be a smart guy. I like the way you think!

We have a few saws that use similar methods where I work. Of course, we didn't build ours, we bought them from saw companies (Granco, Elumatec, B&O Saws).

Personally I think it would be quite fun to build something like that from scratch. I always like machines I build/program being finished, working, doing exactly what they were supposed to do.

"I always like machines I build/program being finished, working, doing exactly what they were supposed to do."

That's my big thrill in life also. (well maybe 2nd or third biggest thrill)...

How about this?

If you're going to go with limit switches and you can stand it, mount four switches on the saw and fabricate enough cams (trips?), to be placed at each position, to generate a 4-bit binary code (excluding zero) at each saw position. This would give you fifteen possible codes and eliminate counting.

In the controller you'll need some sort of table to match switch codes to length positions.

.02

this machine is outside at a tie yard. we recieve the raw ties from sawmills and cut them to customer specifications and then treat them. as i said i don't do alot of plc programming so i don't know how to do the encoder setup nor am i sure about the 4-bit binary code. i guess i need something more simple if that is possible. i don't get to do enough of this to be very good. thanks for your replies. i might be in over my head on this one. i would appreciate any suggestions.

multi stop setup

I did a machine with a similar setup.
I used 1 prox switch and a gear that turned as the head moved.
Take the counts from the prox input and scale it to get your inch's/ per rev. then in the PLC you can set up any number of stop points. move in if the next set point is less than current Poisson. Move out if the new set point is larger.
It takes a little programing but works well.
No moving parts to get blocked by saw dust or weir out.
Racer

It's all compromise

Methods

1. Digital Sensor - Field IO tells you where the SAW location
   • Sensor mounted on the Saw
     +ve's small IO count. Easy to add new lengths. By counting from an end of travel can know where you are until power off.
     -ve's Do not always know where the saw is. eg recovery from power off . Sensor wiring must travel with the saw (wire can break due to movement). Position is different if you move In vs Out by the width of the striker. IF Counting and a striker is not sensed will go to the wrong length.
   • Sensor mounted on frame, Striker mounted on Saw
     +ve's Know when you are at a preset length. Recovers well from power off if on a preset length. Can request Move to a particular length. No trailing wires mounted on the saw. One of the simplest methods to program.
     -ve's Do not always know where the saw is. eg recovery from power off between lengths. One sensor required per position. Position is different if you move In vs Out by the width of the striker.
2. Position feedback
   
   • Position feedback Incremental
     typically an encoder mounted on screw (or motor if there is no possible slippage to the screw). A basic encoder is a gear wheel and a prox to sense each gear (ie a pulse counter) {like mracer}
     +ve's can dynamically change lengths. 1000's of different lengths. Can program slowdowns. Can get the Same position if moving In vs Out. Accuracy is much better (+/-1 count possible)
     -ve's Do not always know the saw location (e.g. recovery from power off) must reset the count by moving to a known position
   • Position feedback absolute
     Usually all the benefits of incremental devices plus knowing the saw is when power applied
     Some types are: Laser distance measurement, or Absolute encoders
     -ve's some need Analog inputs, Others require communications. Absolute encoders require High IO counts. Also after device replacement the device requires setting up (e.g. setting the correct absolute position at an endstop)

It all comes down to your application accuracy and operator functionality. Also if you are are comfortable with the installation and programming of the method.


PS don't forget the end of travel limits

Mracer, Similer to an encoder (programming wise).

Roddbodd,

Take the challange and bite on it. The encoder setup would be fairly simple and more reliable then the limits. However, being outside, instead of driving it with a timing belt, it might be best driven by a chain. You can house the encoder in a small NEMA 4 or 4X enclosure (maybe 14" x 14" x 6").

Based on what I know so far. The PLC I would reccomend would be an AD D0-06DR. The encoder would be an AD TRD-GK100-RZD.
(AD = AutomationDirect.com)

Again the drive chain would be housed in an inverted 4" or 6" x 1-5/8" strut (opening facing down). Either end sprocket can be used to drive the encoder.

As far as programming this setup it would be pretty streight forward & you'll get lots of help on this forum if your up front with the important details.