Messages

1051

General Mach Discussion / Re: Watch the shop-vac gremlin

« on: September 07, 2007, 10:08:12 AM »

Shop-vacs or any sort of dust collection will produce A LOT of static electricity.  I would guess that is what was causing you troubles. 

1052

General Mach Discussion / Re: PWM inverter for 12 VDC motor

« on: August 29, 2007, 04:27:57 PM »

The Homan Digi-Speed board converts the Mach PWM signal into an analog reference voltage, you still need a speed controller.  A quick search turned up this: http://www.centent.com/cn0105.htm

Never used on of them though, but it might give you an idea of what to look for.

1053

General Mach Discussion / Re: PWM inverter for 12 VDC motor

« on: August 29, 2007, 10:29:28 AM »

If you have a 12VDC motor you don't need an inverter (which converts DC to AC).  You need to look for a speed controller that will drive a 12VDC motor. 

1054

General Mach Discussion / Re: Accurate homing

« on: August 23, 2007, 01:46:03 PM »

This thread really hit a sensitive spot with me.  After working for years on many types of equipment, seeing how large machinery manufactures can make a really fantastic machine and then REALLY screw up the safety related aspects of the design.  I've worked on a $300K seam welder that the mfg fitted with N.O. mechanical home/limit switches.  When the cabling wore through due to poor cable management (another beef of mine) , the machine would not stop when the head homed and consequently $2,000 (or more) worth of TIG and plasma torches were ripped off the machine.  This situation was not only costly but dangerous for the operator.  When I asked the mfgs. engineers about reprogramming their controller to work with N.C. switches I was told that their controller DID NOT have the ability to work with N.C. switches, and they looked at me like I had two heads.  My point is that safety of the machines operator (or bystanders) and secondly the machine should be paramount.  It's important to understand what the different safety system are for and what they should do (and not do).  Here are a few links to that effect:

http://www.designnews.com/article/CA6250678.html
This article talks about a drive system that allows the drives to remain powered in a safe stop condition.  There are two very relevant paragraphs toward the end:

Peabody distinguishes between a safe stop and an e-stop. A safe stop allows a machine to return to a known position from which it can commence working without an elaborate reset process. An e-stop removes power to the machine immediately, stopping it but possibly scrambling programs. After an e-stop, a machine needs time to gather its wits.

With robots, an operator can push a safe stop button and a locked perimeter guarding will remain that way until the robot has come to a controlled stop. Only then will the guard unlock.

Here is a small PDF from a E-Stop mfg.  This paragraph titled "How shall an E-stop stop the?
http://www.baumerelectric.com.au/pdf/ENG_Chapter_E_Stop.pdf

Now there are different regulations depending on your location and/or the machines function.  Different industries have different requirements.  The gist of it is that an E-stop should safely stop the machine as quickly as possible and remove power to the drives.  With our small table top machines we can be a bit lax and not spend much time worrying about this.  When we build bigger and faster machines we need to take it seriously.  Stopping a machine (through feed hold or similar functions) is not the same as an emergency stop (limit).  Other features in machine controls like feed hold, homing sensors and soft limits are there to keep us from hitting the limit switches or the need to hit E-stop.  I guess my big point is, we should not try and design around the intended function of an emergency stop circuit.   Use it for only the correct purpose and be safe.  OK, I'll get off my soap box now...

1055

General Mach Discussion / Re: Accurate homing

« on: August 20, 2007, 09:01:56 PM »

However, the Trak system at one of my clients never requires homing, and both a friend’s AHHA system as well as his modern Akira-Seki VMC don't require homing after e-stop or limit events.

Naturally, Improved confidence in my homing repeatability will help for safety as I'd rather not second guess the ramifications of hitting the e-stop.

I think the key word here is 'require', what's required and what the best operational procedure is are sometimes two different things, and to some extent its also a subjective thing.

And, yes. I do occasionally jog in the wrong direction and hit the limits as my tool change position is at machine 0,0,0

You should try using soft-limits.  Mach will decelerate as you approach the soft limit when jogging.  Works great.

Also, for servo systems, an uncontrolled e-stop is downright unsafe. I made a custom 1800IPM gantry system for a client this year using a National Instruments motion card, servomotors and Gecko G340 drivers. Controlled motion on e-stop and soft limit events were a requirement. Suddenly stopping the pulse stream resulted in a drive error/shut-down allowing the carriage to freewheel and do (more) damage. (I wrote my own g-code CAM software in LabView for this project - - a far cry from Mach in features but not bad, I think).

You right, safety is the key issue.  Many machines also employ axis brakes as a safety feature.  What if the control or drive is failing and when you hit e-stop you may not be able to safely decelerate.    The old Bridgeport VMC I'm working on now, has axis brakes and the Galil motion control card will allow you to decelerate on E-stop or to go dead stop.  I'm considering trying to employ a rapid deceleration with the Galil , flowed by a short time delay before the braking relay (which also removed power to drives) kicks in.  If I have to choose, I'll go with the brakes.

Why can’t the index pulse of an encoder be fed in as a Mach home switch?

Right, the roigersmachine error detection board is a great idea for a stepper system with encoder or glass scale feedback.

Why can’t the index pulse of an encoder be fed in as a Mach home switch?

Does anyone know if Mach's encoder DROs are consistent across e-stops and limit events?

To be accurate the the controller needs to see the edge of the index pulse, (edge triggerd).  There is no guarantee that all parallel port inputs will latch at the same time so trying to edge trigger on a fast signal, w.r.t another input would be at best, tough.  Also the parallel port can start out in an unknown state, if you are making use of the 'charge-pump' you will have the ability to disable the drives when Mach is not actively driving the parallel port.  On my little DynaMill I can E-Stop from a stop and restart Mach and never notice the difference.  If it was moving I would not take the chance.  On the Bridgeport conversion I have the luxury of the Galil with closed loop control.  I still would home after an E-stop though.

Does anyone know if Mach's encoder DROs are consistent across e-stops and limit events?

In my experience, I think so.  Still, the question is IMHO, should you trust it?  Take a look at the G100 and Galil motion control boards if you have needs that go beyond what a few parallel ports will do. 

1056

General Mach Discussion / Re: Accurate homing

« on: August 20, 2007, 01:10:18 PM »

Currently my lack in confidence at maintaining an accurate home is bringing me very close to abandoning Mach. This is because e-stop and Limit events are very common, and after which Mach requires re-homing.

I can understand your frustration, but I don't think you fully understood the problem before trying to develop a solution.  Starting at the beginning, e-stops and running into the limits should not be common events.  Both are there for the safety of the operator and machine.  When these events occur a control should shut down the motors, period.  The motors and driven mechanism WILL continue moving some unknown distance (dependant on velocity at the time of shutdown etc).  Again, an emergency event such as E-Stop or limit switch should not have occurred, even with incremental/absolute encoders in the system the control has no way of good way of knowing exactly where the machine is at.    The homing process ensures that the machine knows where its parts are at.  Even robots, which have very good absolute encoders, will home and look for an index pulse within a certain range of the home position.

Requirement: The only time a CAM system (without absolute positioning) should require homing is at power up, after a motor drive failure, or when the operator decides it's necessary (after a crash).

Again, E-stops and limit switches should not be common events.  If one of these events has occurred the operator may have no way of knowing if something was damaged in the incident or if the machine is still where it should be.  Even absolute encoders do not guarantee that you are really at a certain point, if the encoder or wiring is faulty you can get bogus readings to the control and the control will not know where you are at.  The homing process insures that the machine parts are physically in a known starting position.

So to get you off on the right foot here are two suggestions:

1) Figure out why E-Stops and limit events are common and fix it.  It could be a combination of your process and operator training.
2) Investigate the board from Rogers machine that Brian mentioned.  It combines the index pulse and limit sensor giving you a very accurate home.

1057

General Mach Discussion / Re: Deckel Mach Retrofit

« on: August 15, 2007, 03:57:02 PM »

With Mach, servos and Pixies (or other S/D to analog converters).  The Pixie translates each step (pulse) from Mach into a distance that the servo should move (the relationship has to do with #of encoder lines and the gearing of the drive).  So when Mach is done driving an axis the axis is done moving, period.  If the servo can't keep up a following error is generated (that can/should shut Mach down).  The drive and Mach are always in sync, else and error is generated.  Remember servos don't freewheel, when they are powered are held in position unless commanded to move from that position.

The situation you are talking about (rounded corners) has more to do with physics.  Since the table has mass it cannot immediately stop/start.  It accelerates from one point and decelerates as it gets to the next point.  If you want a nice sharp corner then turn of CV mode (run in exact stop) in Mach and it will come to a dead stop at each corner before proceeding.  With CV mode some rounding will occur but you get a  faster and smoother operation of the machine.  ALL machines /controls do this.

1058

General Mach Discussion / Re: Deckel Mach Retrofit

« on: August 15, 2007, 02:53:00 PM »

Alan said -> The real pickle is that the time for the cut of the line can vary a bit as a result of loading,

Well, sort of, but no.  In an 'open loop' (for our purposes a stepper system), the stepper is pulsed at a rate in accordance with the velocity commanded by the program.  Until you reach the point of overloading the stepper its velocity will be synchronous to the command pulses, that's how they work (one pulse = a fixed angular displacement).  The control (Mach III) knows how far it is from point A to Point B, and how many steps it will take to get there, the time taken will always be the same, based on your programmed feed rate.  Open loop stepper systems are always detuned to ensure that there is adequate torque for any expected situation.

With an open loop motor system, varying load will increase/decrease travel time.  Closed loop systems, be they steppers or servos, do have feedback.  In the case of (servo) motors the feedback is essential, otherwise the control would have no idea how far/fast the motor had traveled.   (A servo after all is just a motor with feedback.)  With this feedback (encoders and/or resolvers) the (servo) control can constantly monitor the angular velocity and rotation of the motor and tweak the commanded voltage to the servo drive to keep it motor going the correct velocity and/or distance.  The commanded velocity is still a function of the feed rate set in your program and at a given feed rate, the time from Point A to Point B will always be the same, despite load.  If the drive is not capable of the required speed/torque then the (servo or stepper) control will raise an error as the commanded and actual position will be too far apart.

Brian Said -> For the people that are hard core and need to have it all one one board because it makes them feel better there is the Galil BUT you need to know that there is a limitation of the accel needing to be the same AND that I have not coded in the probing Sad One thing that it will do VERY well is homing to an index pulse Smiley If there are people out there that think they can do better... I have the plugin code and if you think you can handle the code I would be willing to work with you... You need to have VS2003..

The other consideration is the total number and type of I/O needed in the system and the resulting cost/complexity of the possible ways of getting the required I/O.  In the case of the Bridegport VMC I'm converting, the cost difference between going with a G-Rex or one/two LPT's with a PLC to handle the rest of the I/O was only around $500.  The system complexity is significantly lower though.

1059

General Mach Discussion / Re: Clarification of charge pump frequency

« on: August 01, 2007, 10:10:26 PM »

The frequency is 12.5 kHz.  There is a setting, somewhere, in mach that changes it to 7 kHz as that is what the samll laser cutter/engravers use.  My guess is that you turned that setting on by mistake.

1060

General Mach Discussion / Re: Pull-up Resistors

« on: July 26, 2007, 03:47:16 PM »

Can someone please direct me to information on pull up resistors?

Ken, that's a pretty wide open question.  What are you trying to accomplish, hook up etc?  If you can give us an idea of what you are trying to do, then perhaps someone can tell you how pull-up resistors relate to that subject.