Height Control for Router Table

[Sargon]

I want to use an ultrasonic sensor for height control as my sheets are not always perfectly flat. Blowing compressed air will clear the chip so I get accurate readings from the sensor. The ultrasonic sensor is ideal for this application as it can be used on transparent material and doesn't contact/damage the material.

The sensor output is 0-10V. I can use an op-amp and adjust the gain to condition the signal for THC or similar device. Or I could use it to compare to a set-point to generate THC UP or THC DOWN, but that doesn't solve the problem of no PID loop. I need PID to smooth out the signal so it's not constantly moving up and down. Any recommendations that don't involve electronic design, or at least very simple with minimal development time?

Has anyone developed a PID that works well for this application? If not, can anyone recommend a THC for this application?

Any thoughts?

Someone must have done something like this already!

Thanks,
Chris

[rrc1962]

Any of the affordable THC's out there (hobby level) do not have built in PID loops.  They simply monitor tip voltage and send THC UP/DOWN commands to Mach.  The lack of a PID loop is exactly why you get excessive oscillation if the THC feed rate is set too high.  If you want to spend some money, you can get a stand alone THC that operates outside of Mach3.  These THC's will have built in PID loops and control the Z axis independently of Mach3.

Which way do you want to go?  If you're looking for hobby level THC that can just command Mach3 to make the THC UP/DOWN moves, you can build something easily.  An stand alone industrial THC with PID capability can cost a few grand.  Thermal Dynamics gets just over $10k for theirs.

One thing to consider is that once you move the Z over to an external THC, you will no longer be able to control it using Mach3 directly.  When plasma cutting, other than setting initial torch height and retracting prior to rapids, the THC controls the Z.  There are no Z feedrate moves in the program.

Are you looking to just measure the material height and apply a Z offset to compensate for variations in material height?

[Sargon]

Any of the affordable THC's out there (hobby level) do not have built in PID loops.  They simply monitor tip voltage and send THC UP/DOWN commands to Mach.  The lack of a PID loop is exactly why you get excessive oscillation if the THC feed rate is set too high.

That sucks!

An stand alone industrial THC with PID capability can cost a few grand.  Thermal Dynamics gets just over $10k for theirs.

That is absolutely ridiculous! PID is bordering on ancient technology! We're still in 2011, right? Good. When you said that I thought maybe I had gone back 50 years.... PID has been around almost as long as electronics... Wow... Thanks for the insight. If I need it I'll be happy to develop it myself at those prices. Then sell it for a few hundred and steal the market.... hmmm..... lol

Are you looking to just measure the material height and apply a Z offset to compensate for variations in material height?

That is exactly what I need, but if it does see a chip I don't want it pulling Z up.... I suppose using a slow feedrate for compensation may work alright... Any suggestions?

[BR549]

The low cost THCs DO have the pid loop built in and cost around $500 or less complete . It is a basic function of the system. You can also use a simple PLC and IF you can do the programming build in a PID loop to do it as welll.

THE advantage of using an existing DTHC to do this IS??? They react VERY VERY VERY fast to changing conditions and that is REquired IF you are going to try and keep up with cutting and Z height changes.

Now that brings up another related question HOW FAST is the sonic sensor and HOW accurate is it. A plasma torch is a very course user of the Zheight compared to routing . A plasma torch can have minor changes in Z height and you would probably not ever notice it in the cut quality. Do that too much with routing and you have an ugly mess.

SO are there any specs out there on the Sonic height sensor?? I personelly would not make a sonic sensor my first choice of sensors as it will be effected by swarf conditions and speed of Cut.

ANOTHER thought would be to use a linear scale to monitor the Zheight and slave it to the Z heightaxis  so that the Quad ouput of the sensor  DRIVES the Z axis to react like a CNC control BUT follow the surface directly.

Just some thoughts, (;-) TP

(;-) TP

[BR549]

(;-) This all sounds cool but I bet it would be better in th elong run to just invest in a good vacuum hold down system and get back to cutting(;-)

(;-) TP

[Sargon]

I said the material is not flat, not that I have a problem holding the material. I do indeed use a vacuum system strong enough to pull down 3/4" HDPE. That definitely isn't the problem. The material itself can have variations, and some materials are either too thick or too ridgid to hold with vacuum. That's why I want height control.

Thanks in advance for any help you can offer!

Chris

[BR549]

OK before we go on lets clear somthing up. You seems to want a proven design to do this.

"I would much rather just use an existing, proven design then reinvent the wheel"

BE aware there is no such known function for MACH3 that currently does this. You are treading on unproven ground. Both hardware and software wise.

THe only KNOWN solutions for this ARE machine the surface flat before you work with it OR use a floating Zhead to set Z cut height.THesae are industry standard methods and are proven to work.  THe rest are SWAG in nature and may not give the results you require.

(;-)TP

[Sargon]

Sorry I didn't have time earlier to respond to everything.

I wanted to use something existing just to reduce the time required for the project. I'm not quite sure what you mean by a floating Z... Does it have a depth guide that contacts the material?

The ultrasonic sensor has a response time of 2.5ms.... should be plenty fast for a table with max speed of 130ipm. That translates to a response roughly every 0.0055" at max speed. The swarf should be cleared with compressed air, but may randomly be picked up by the sensor if they happen to move past it at the wrong time, but should be rare. That is one reason for wanting PID.

What do you mean by a linear scale? Please elaborate, I'm not sure what you're referring to.

Thanks for all the help/ideas/insight. It's much appreciated.

I thought surely someone would have done this by now... but maybe not...

Chris

[rrc1962]

"The low cost THCs DO have the pid loop built in and cost around $500 or less complete ."

Which ones?  Anything that interacts with Mach3 through THC UP/DOWN commands doesn't have a PID loop, or if it does, it doesn't do any good.  The idea behind the PID loop is to adjust the feed rate based on the distance of travel and to control acceleration to reduce/eliminate over-travel.  Most of these low cost THC's simply trigger the Mach3 THC commands, which in turn move the Z at the set THC feed rate with no acceleration applied until the input is turned off. when it's turned off, there is always some over-travel, which is dealt with by reducing THC feed rate to where the "bounce" is acceptable, which as you pointed out, is not as big a deal in plasma as it would be in routing. 

[rrc1962]

"That is exactly what I need, but if it does see a chip I don't want it pulling Z up.... I suppose using a slow feedrate for compensation may work alright... Any suggestions?"

Then you don't need a THC.  You want to read the material height, do some math and apply a height offset to the Z to compensate for changes in material thickness.  A THC controls the Z axis.  Sounds like that's not what you're after. 

The only way this could work is if you wanted to take say....a consistent 1/4" off the top of the material.  Then you could adjust the THC to run the Z at a constant - 0.25...Which would be only as accurate as you ultrasonic sensor.

PID loops have been around forever, but there's more to an industrial controller that the PID loop.  It's all relative.  When you drop $200k on a brand new Koike MasterGraph, $30K for a controller and $10K for a THC doesn't sound so bad.