Force Control with 1771-QB Module

[Steve Wiebe]

I have a gang edger for sawing up cants. The cants need to stay up against a line bar in order to be cut properly. There is a roll on a hydraulic cylinder pressing against the side of the cant for this. The roll is currently controlled by giving it a target position slightly less than the width of the cant.
There are problems with this:
- tapered cants don't get pressed at all (extra programming would help with this)
- small cants can be bent out of shape
- cants can get pressed against the line bar so hard that they stop traveling

I would like to try using force, not position, to control this roll when it is pressing against a cant (position mode between cants). I'm thinking of doing this by adding a pressure sensor to each end of the hydraulic cylinder and then regulate position based on the pressure difference to regulate force.

My gut instinct is that this will not work with beans. The system will be too slow and imprecise. What I would like to use is an RMC101 controller from Delta Computer Systems but money is very tight right now. I'm looking for ideas and feedback I could take to my bosses. Any help is greatly appreciated.

PLC is an A-B PLC-5 with a 1771-QB module controlling the roll.
Feed speed is ~400 Feet per minute.

Thanks,
Steve.

 

[leitmotif]

Steve
The one think I do know here is what a cant is. OK so you are sawing different thickness cants. Need to keep them against a guide to get straight cut material.

Suggest one (two??) cylinders on saw infeed to press the cant against guide. Air or hydraulic (air may be better because it is not as "rigid" (incompressible) as hydraulic and will allow for a little variation and reduce jams.

Rollers on piston would press cant against guide and with moderate ie just enough pressure.
Suggest lowest pressure for smaller size, medium for medium size and high for big. That would eliminate any problems with trying to find right position. Pressures could be set by operator with a regulator or by PLC if you want. This would give you the simplest system with least folderol and maintenance.

Dan Bentler

 

[Plastic]

Off the shelf force controller

Bosch-Rexroth HACD card.

http://www.boschrexroth-us.com/coun...CD/index.jsp;jsessionid=abcCKuB1rOXLUgZsEwkbs

 

[Peter Nachtwey]

I was going to mention the air over hydraulics too

However, I think your biggest problem is that you have only one roll to hold the wood against the guide bar. Even your current method should work if you had multiple rolls that could go to different locations when there is taper.

I don't think the pressure control or limiting is near as important as having the multiple axes to hold the wood so you can adjust for the taper.

 

[Steve Wiebe]

Wow, Thanks for all the quick replies

We had an air cylinder here originally. It never had enough power, was too slow, and was difficult to position. They'll never go back to it.

Air over hydraulics would be a big improvement (plus simple, reliable, cheap, ... ie: perfect) but there isn't any room. I'll suggest it in case the mechanical brains can jury rig it somehow.

The Bosch-Rexroth card looks very interesting but no one here has any experience with it. I'll check it out though.

We already have a couple of RMC101 controllers. They work great and we have spare parts which is why I was thinking of using one here.
Peter, I was hoping you would say the RMC101 would be perfect so I could use your opinion as leverage with the bean counters Oh well

Thank you everyone for your time
Steve.

 

[Peter Nachtwey]

We had an air cylinder here originally. It never had enough power, was too slow, and was difficult to position. They'll never go back to it.

Obvious air alone is not the way to go. A small air cylinder at the end of the hydraulic cylinder would work. If you can get close within 1/16th of an inch you can try rubber dampers that can give about 1/8 of an inch on the end of you hydraulic actuators but they probably wouldn't support the weight of the roll.

Peter, I was hoping you would say the RMC101 would be perfect so I could use your opinion as leverage with the bean counters Oh well

Yes but they would know I am trying to sell something.

This is a mechanical design problem. I like the way USNR designs their new edgers. There may be others that do something similar.
USNR does just the opposite of what you are doing. They have set pins and there are multiple set pins for different length boards. The set pins push the board up against the air cushions instead of the other way around. This has THREE BIG advantages over the way you are doing it.
1. The set pins are light and fast as opposed to your rolls on the end of cylinders.
2. The set pins don't give. The go to the precise location. The air cushions do all the giving but the position is not done with air.
3. There are multiple set pins so a short cant may use pins 1 and 2. A longer cant uses pins 1 and 3 etc. This way the taper can be taken into account.

Another big advantage is that if you are cutting studs then the cycle time must be short. It is possible to have TWO sets of set pins. Each lug chain has actuator on either side of the chain guide. Now there is pin 1a pin 1b pin 2a and pin 2b. So now the 'a' actuators can be pushing the board forward while the 'b' actuators are coming back.

Unfortunately what I am telling you would cost big $$$ but if you think about it you know what I am saying is correct. Maybe the next time you buy a machine or do a retrofit you will think about these points.

Back to reality. Does the scan data from your optimizer provide taper data to your PLC? That would help. If not then you will may not need force control but you do need some sort of feedback that lets you know when you hit something. A pressure sensor on the cap side of the piston will have a pressure spike when it contacts the wood. Here is another question. How fast are these actuators with rolls move? If they did detect a pressure spike then how much time will it take then to stop? How much will the wood by compressed in that time? At 10 inches per second the travel distance is 0.01 inches but because the actuator is slowing down the travel distance is half that but I bet the speed is 30-40 inches per second at least. I don't think any pressure system is going to keep the actuator from crushing boards because the hydraulics will not react fast enough if you are making contact at high speeds. You need scan data from the optimizer to tell the actuator where to go and when to slow down to so contact with the board is made at almost 0 speed.

Your comment about the system originally using air cylinders has me shaking my head in wonder.

 

[Steve Wiebe]

Hi Peter,

Thanks for your reply.

Yes, I tried to pull a fast one on the bean counters. Shame on me

What you described sounds like our board edgers. Much different setup than this gang edger. Sorry to mislead you.

Quote: I don't think any pressure system is going to keep the actuator from crushing boards because the hydraulics will not react fast enough if you are making contact at high speeds.

Well... we have 6 feedrolls on a planer that go up to a set position when there is no lumber and come down with a predefined force when there is lumber. Speeds are 2400+ FPM. Works great. Uses RMC controllers with analog pressure sensors. This is what made me think to try it in this application.

The Maintenance Super says he'll try to fit an air cylinder on the end of the hydraulic cylinder. I'll believe it when I see it.

Thanks,
Steve.

PS. I see you've moved to the USSA. I smell an off topic thread coming up.

 

[Bruce99]

Edger infeed

First, I understand your problem. Been there. My first question is what is scanning the cant? There is a real problem with the scanner if it can not deal with small or tapered cants. Second, is the PLC getting the right position data from the optimizer? The shape and size of the wood is not an issue here, it is the scanning and PLC positioning. RMCs are the best solution but before you get any further, is the scanning and PLC data exchange working in sequence? Or is the PLC positioning for the wrong cant? Something here just does not sound right. What probes are you using and where do they feed into the solution from the optimizer? 95% of the positioning problems on hydraulic systems can be traced to one of three things.. Probe, calibration, scanner input. By your description of the problem the PLC has no idea where the cant is and can not position the cylinder where it should be. If this system worked at one time, then you have to look at the changes made and see if there are position parameters that need to be changed. A simple calibration test by telling the roll to go to X and going to the infeed and seeing if it is where it should be would tell if it is moving right. Next force the PLC to move the roll to a position X and see if it is close. Work your way back into the PLC program. Ive seen the program set scale values that change in the program. Once you eliminate the PLC to positioner as a problem then check the scanner and its data transfer to the PLC.
This is a start, hope it helps

 

[Peter Nachtwey]

What you described sounds like our board edgers. Much different setup than this gang edger. Sorry to mislead you.

You mentioned crushing boards and I went astray. Wrong equipment but the points are still valid. A good mechanical design will move less mass, shorter distances at slower speeds.

Well... we have 6 feedrolls on a planer that go up to a set position when there is no lumber and come down with a predefined force when there is lumber. Speeds are 2400+ FPM. Works great. Uses RMC controllers with analog pressure sensors. This is what made me think to try it in this application.

Yes, the RMCs are good but not good enough to do it without some good mechanics and hydraulics. It is important to study why the planer applications it works so well.
1. as you pointed out, the actuators go to a set point but that is just above the wood so that the motion till contact is small and the actuator isn't moving fast. How fast does the holding roll on the gang edger move?
2. The planer wood is already cut and relatively smooth compared to an uncut cant. Even so a planer press roll will see pressure spikes when a roll rolls over a wood chip.
Warped boards will also cause pressure spikes. The edges of a cant on a gang edger are not smooth. Our controller has been successfully used with air over hydraulic system for holding wood and even centering logs. Air cylinder is necessary when the word is uneven. An 'pressure relief' accumulator may work too.
3. The hydraulics and mecahnics are not fast enough to stop the pressure spikes. Planer systems have a small 'pressure relief' accumulator between the valve and the piston on the pressure side that absorbs pressure spikes. This 'pressure relief' accumulator must be pre charged to above the system pressure so that during normal operation the accumulator doesn't let no oil flow in. That way it doesn't affect the performance of the RMC, valve and actuator UNLESS there is a big pressure spike. When the roll is pushed back only a 1/8 of an inch there will be a huge pressure spike. The RMC may miss most of the first millisecond and the valve and mass definitely can't responde that fast. This little bit of oil backs up into the 'pressure relief' accumulator. It is usually less that 1 cubic inch of oil. The 'pressure relief' accumulator limits the pressure spike and protects the pressure sensors. All the energy get put back into the system when the spike has passed. Think about this trick.
4. You mention that there are 6 of these press rolls. You mentioned only one for your gang edger. You may need many more to hold a tapered cant.

There was a little learning required before the planner rolls were done right. Fortunatlely for me/us Coe, Coastal Machine and UNSR are/were all with 30 miles of us so we had no trouble getting experience on how to do it right in the shop.

Bruce99, is on the money about getting information from the optimizer so the holding roll doesn't need to move any farther than it needs to. I think you need more than just one roll to be able to hanlde the taper.

The RMC can't defy physics. If the your cant edger's holding roll is moving quickly when it makes contact then kinetic energy must be dissappated somewhere. There will be a pressure spike. The roll may crush the wood. etc

Bruce99 and I both asked about the availabilty of scan data so you can prespot the rolls and then move them the minimum amount at minimum speed to get the job done.

PS. I see you've moved to the USSA. I smell an off topic thread coming up.

That is my protest address. Delta will shortly have a new address for real. That deserves a thread on a slow weekend.

 

[Steve Wiebe]

Hi guys,

This roll has never worked properly. It is an in-house add-on from before my time with the company. It is merely a guide roll to keep the cant up against a line bar. No fancy positioning with multiple rolls/pins/clamps... like a board edger. Scanning is a low resolution Banner Beam-Array (likely set to 3/8" resolution, I'd have to check). A one-shot took a reading and used this for the entire cant. I'm currently building a word shift array to constantly carry size data from the scanner to the roll (about 6 feet away). This will be a big improvement for dealing with pre-positioning the roll and handling taper. I'd take a picture and post it to help clear up any confusion but boy would I be in trouble. I programmed some speed routines for controlling the amount of lumber backlog going to a lug loader. A co-worker took a picture that ended up on the cover of a lumber trade magazine and the sky fell over it. Opps.

The fundamental design problem is oil is obviously not compressible. Between this and the low resolution scanner, I'm back to thinking air over hydraulics would be best but it will be tough to install without big changes.

I've noticed the 'pressure relief' accumulators at the planer. I assumed they were just regular accumulators. Clever idea. I've learned something useful today, thanks

Hey Bruce99, I work about 4 hours west in Smithers. Small world.

Steve.

 

[Peter Nachtwey]

Scanning is a low resolution Banner Beam-Array (likely set to 3/8" resolution, I'd have to check). A one-shot took a reading and used this for the entire cant.

Are you sure you will not get in trouble for admitting this?

I'm currently building a word shift array to constantly carry size data from the scanner to the roll (about 6 feet away). This will be a big improvement for dealing with pre-positioning the roll and handling taper.

Yes, I don't think you can do much until you have that scan data but I don't know if it is good enough to help.

I'd take a picture and post it to help clear up any confusion but boy would I be in trouble.

Why, a picture wouldn't reveal any trade secrets and it would help. The technology you described is ancient and unless the saw mill developed the technology there isn't any point in keeping a secret because the OEMs have and control the technology.

I programmed some speed routines for controlling the amount of lumber backlog going to a lug loader. A co-worker took a picture that ended up on the cover of a lumber trade magazine and the sky fell over it. Opps.

Some places are funny that way. Other don't care and are proud to show off what they have because it is the latest and greatest. In the end I don't think it is the machine itself but the type of machine and material flow that is important. Everyone can buy the same machines.

We get pictures, videos and schematics all the time.

Hey Bruce99, I work about 4 hours west in Smithers. Small world.

Steve.

I have never been to Smithers. I just looked up Smithers on Google Earth. I bet the view of the glacier and mountain is fantastic in person.

BTW, I will be at the New Orleans "saw dust festival" AKA
http://www.sfpaexpo.com/
in June. The show looks pitifully small which is too bad but we have paid for the booth.

 

[Bruce99]

cant scans

Smithers? hey right on! Bad news is you can not scan a cant this way. I know where you can get a real cant scanner. The problem is it takes a lot of work and TLC to keep it working right. The array you are using just can not do what is required to do for positioning. You need input points for the full length of the cant. The closer the points the better. But you only have one roll, so you need to pass the input data to output of this roll and this means passing the array data to a table, then incrementing the output to the roll based on the feed speed. A bit shift can do this with a prox type encoder on the infeed. Pass the table values with the prox pulse and shift the array data with the same pulse. Problem is, when the cant slips or gets stuck in the saw box. By the way it is good to see someone still cutting wood. Good luck.

 

[Peter Nachtwey]

Cant scanning

http://www.deltamotion.com/peter/Pictures/JoeScan/
Look at the the pictures in this directory.
http://joescan.com/
Joe Nelson is young smart engineer that I should have hired when I had the chance. Now he has his own company in downtown Vancouver, WA.
In the Portland area we have 'engineering meetings' where the guys in the saw mill biz talk and exchange ideas, an drink beer. It is funny how the Joe scan scanner use the same processor as the Delta RMC 75E and RMC150. It isn't coincidence.

BTW, if anybody is going to the New Orleans saw dust festival there is going to be an 'engineering meeting' at a bar in the French quarter. I need to look it up while at work but contact me if you are inteested.