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| Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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CNC software highest step rate
Hi,
What's the highest step frequency one can expect from popular CNC software these days? 100kHz? 200kHz? |
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#2
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What do you mean by step rate? Instructions/sec?
wrote: Hi, What's the highest step frequency one can expect from popular CNC software these days? 100kHz? 200kHz? |
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#3
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wrote in message
oups.com... Hi, What's the highest step frequency one can expect from popular CNC software these days? 100kHz? 200kHz? Mach3 will go as high as 45 kHz, although I can't imagine why. Mechanical resonance limits practical speeds to below 25 kHz. |
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#4
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Mach3 will go as high as 45 kHz
That's low IMO, you mean there isn't anything out there approaching 100kHz? although I can't imagine why. Mechanical resonance limits practical speeds to below 25 kHz. Typically, the same software is used by step compatible servo drives. Example, a 4X only servo drive (Gecko 320 for example) driving a 2600 RPM DC motor with a 500 CPR encoder mounted on the motor shaft needs a step rate of 88kHz in order for the motor to reach maximum speed. Resonance is a non-issue. |
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wrote in message
oups.com... Mach3 will go as high as 45 kHz That's low IMO, you mean there isn't anything out there approaching 100kHz? To clarify, I am not aware of faster speeds. Available step rate is so not the limiting factor on my setup. Unless you're talking about something else entirely, of course. although I can't imagine why. Mechanical resonance limits practical speeds to below 25 kHz. Typically, the same software is used by step compatible servo drives. Example, a 4X only servo drive (Gecko 320 for example) driving a 2600 RPM DC motor with a 500 CPR encoder mounted on the motor shaft needs a step rate of 88kHz in order for the motor to reach maximum speed. Resonance is a non-issue. Sure, if you can get 2600 rpm into your system. Mechanical resonance overwhelms steppers at about 1600 rpm. What are you looking at that it becomes a "non-issue"? I had heard, I think, about the 320's adaptive micro-stepping. If it's that effective, I might give it a shot. 300 ipm seems plenty, but faster is always gooderer. As for step rate on a PC, EMC runs on RTLinux. Dunno why it thinks it needs realtime to run a few simple steppers, but maybe worth looking at for your application. |
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#6
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"Mike Young" wrote in message .. . wrote in message oups.com... Mach3 will go as high as 45 kHz That's low IMO, you mean there isn't anything out there approaching 100kHz? To clarify, I am not aware of faster speeds. Available step rate is so not the limiting factor on my setup. Unless you're talking about something else entirely, of course. [If you really want a higher step pulse rate, you could get a G-101 "G-rex" board from Geckodrive. They claim to put out over 4 million step pulses per second: http://www.geckodrive.com/manuals/G101%20Features.pdf ] although I can't imagine why. Mechanical resonance limits practical speeds to below 25 kHz. Typically, the same software is used by step compatible servo drives. Example, a 4X only servo drive (Gecko 320 for example) driving a 2600 RPM DC motor with a 500 CPR encoder mounted on the motor shaft needs a step rate of 88kHz in order for the motor to reach maximum speed. Resonance is a non-issue. Sure, if you can get 2600 rpm into your system. Mechanical resonance overwhelms steppers at about 1600 rpm. What are you looking at that it becomes a "non-issue"? I had heard, I think, about the 320's adaptive micro-stepping. If it's that effective, I might give it a shot. 300 ipm seems plenty, but faster is always gooderer. [It's a non-issue because it's driving a servo motor; they don't have the same resonance problems as steppers. While the control can send the same step/direction signals that control steppers, the 320 drives will accept those and convert them to something servos can run with. 2600 rpm motors usually require some belt reduction to be useful in a CNC system, though. ] As for step rate on a PC, EMC runs on RTLinux. Dunno why it thinks it needs realtime to run a few simple steppers, but maybe worth looking at for your application. [Realtime is necessary to avoid having your milling job disrupted when the OS decides to reshuffle its memory, or do other "housekeeping" functions. Mach2 gets around this by infecting Windows like a virus, to grab first priority for step pulsing. Most other Windows-based systems, like Flashcut's, use a dedicated pulse generator of some sort, instead of forcing the computer to do it.] Andrew Werby www.computersculpture.com |
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#8
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How about inches / sec
Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH, NRA Life NRA Second Amendment Task Force Charter Founder Tim Killian wrote: What do you mean by step rate? Instructions/sec? wrote: Hi, What's the highest step frequency one can expect from popular CNC software these days? 100kHz? 200kHz? ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
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#9
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wrote:
What's the highest step frequency one can expect from popular CNC software these days? 100kHz? 200kHz? If you want to look at a complete system with high performance, check out http://www.cncteknix.com They have controllers which can handle 40,000 steps/sec per channel (max 4 channels), with high current capability, and G-code interpreter all included. Regards, Ian Kirby. |
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According to Mike Young :
wrote in message oups.com... [ ... ] although I can't imagine why. Mechanical resonance limits practical speeds to below 25 kHz. Typically, the same software is used by step compatible servo drives. Example, a 4X only servo drive (Gecko 320 for example) driving a 2600 RPM DC motor with a 500 CPR encoder mounted on the motor shaft needs a step rate of 88kHz in order for the motor to reach maximum speed. Resonance is a non-issue. Sure, if you can get 2600 rpm into your system. Mechanical resonance overwhelms steppers at about 1600 rpm. What are you looking at that it becomes a "non-issue"? Apparently -- a *non*-stepper fed from something which thinks that it is driving a stepper. Feed the steps that fast and the servo will be moving smoothly, instead of jumping and stopping to excite resonances. I had heard, I think, about the 320's adaptive micro-stepping. If it's that effective, I might give it a shot. 300 ipm seems plenty, but faster is always gooderer. As for step rate on a PC, EMC runs on RTLinux. Dunno why it thinks it needs realtime to run a few simple steppers, but maybe worth looking at for your application. EMC runs on a real-time kernel because it can *also* talk to analog servo motors, with encoders giving feedback as to the actual position, which I think is more interrupt intensive than driving a stepper. You've got three axes all reporting change of position at the same time (sometimes), and updates to the D/A converters to reset the servo speed to compensate for load or whatever, and possible interrupts from the limit switches as well. You want to give those limit switches very high priority so it can stop things before you hit a hard stop and damage the ballscrews and nuts or something else. Enjoy, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#11
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"DoN. Nichols" wrote in message
... EMC runs on a real-time kernel because it can *also* talk to analog servo motors, with encoders giving feedback as to the actual position, which I think is more interrupt intensive than driving a stepper. You've got three axes all reporting change of position at the same time (sometimes), and updates to the D/A converters to reset the servo speed to compensate for load or whatever, and possible interrupts from the limit switches as well. You want to give those limit switches very high priority so it can stop things before you hit a hard stop and damage the ballscrews and nuts or something else. I'd been curious about that. Why are the limit switches not typically hardwired to disable the drive, but rather simply reports impending crisis to the software? It seems something already has gone wrong by that time. So, EMC can run closed loop directly without additional hardware support? (Beyond power drivers for the stepper, that is. ?) That's something, given that old PC's are essentially freebies. It would seem a perfect waste of computational resources otherwise. Replacing Gecko 320's, for example, with simple drivers would be a pretty big savings for the hobbyist. In general, are servos considerably different from a stepper with an encoder? |
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#12
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If you want to look at a complete system with high performance
Not really. Just wanted to know the highest step frequency one can expect from popular CNC software these days. Looks as though it's DeskCNC at 125 kHz because it uses the serial instead of parallel port. You'll need a compatible controller in order to get the needed step and direction outputs so it's really a software/hardware combo like other pulse generation schemes mentioned except probably more costly with even lower step rate. |
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#13
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Replacing Gecko 320's, for example, with simple drivers would be a pretty
big savings for the hobbyist. Let's make certain we're on the same page first.....The Gecko 320 is a DC servo drive. Besides cost, its 4X only support is another limitation for the hobbyist who is likely to be looking at surplus DC motors. I bought one for evaluation and concluded it was not the drive for me. Instead, I went with simple microcontroller based drives with both 1X and 4X support at about half the price. So if we 're talking about the same Gecko drive then simpler and cheaper drives already exist. "Mike Young" wrote in message t... |
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#14
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On Thu, 22 Sep 2005 23:46:13 GMT, "oparr" wrote:
Replacing Gecko 320's, for example, with simple drivers would be a pretty big savings for the hobbyist. Let's make certain we're on the same page first.....The Gecko 320 is a DC servo drive. Besides cost, its 4X only support is another limitation for the hobbyist who is likely to be looking at surplus DC motors. I bought one for evaluation and concluded it was not the drive for me. Instead, I went with simple microcontroller based drives with both 1X and 4X support at about half the price. So if we 're talking about the same Gecko drive then simpler and cheaper drives already exist. "Mike Young" wrote in message et... Greetings Oparr, What do you mean by only 4X support. What are 1X and 4X? And why would 4X only be a problem with surplus motors? I have bought 3 of these drivers and the specs seem to me like they will work with the servo motors & encoders I already have and the step and direction software I have. Eric |
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#15
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On Thu, 22 Sep 2005 23:46:13 GMT, "oparr" wrote:
Oparr, I forgot to ask what the cheaper drives are that you have found. I will be building a positioning table for plasma cutting this winter if time permits. Less expensive drives would be nice. Eric Replacing Gecko 320's, for example, with simple drivers would be a pretty big savings for the hobbyist. Let's make certain we're on the same page first.....The Gecko 320 is a DC servo drive. Besides cost, its 4X only support is another limitation for the hobbyist who is likely to be looking at surplus DC motors. I bought one for evaluation and concluded it was not the drive for me. Instead, I went with simple microcontroller based drives with both 1X and 4X support at about half the price. So if we 're talking about the same Gecko drive then simpler and cheaper drives already exist. "Mike Young" wrote in message et... |
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#16
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According to Mike Young :
"DoN. Nichols" wrote in message ... EMC runs on a real-time kernel because it can *also* talk to analog servo motors, with encoders giving feedback as to the actual position, which I think is more interrupt intensive than driving a stepper. You've got three axes all reporting change of position at the same time (sometimes), and updates to the D/A converters to reset the servo speed to compensate for load or whatever, and possible interrupts from the limit switches as well. You want to give those limit switches very high priority so it can stop things before you hit a hard stop and damage the ballscrews and nuts or something else. I'd been curious about that. Why are the limit switches not typically hardwired to disable the drive, but rather simply reports impending crisis to the software? It seems something already has gone wrong by that time. I agree in general -- the axis should be disabled in hardware -- at least in the direction towards that stop -- but it is also possible that other moves are happening at the same time, and you want the controller told at the earliest opportunity to *stop* right now!. :-) As for having the hardware limit switches disable *all* motion at once, this is a bit of a problem with a machine designed from scratch as CNC. I used an Anilam retrofit on a Bridgeport clone mill at work some years ago, and whenever someone managed to hit a limit, the whole controller locked up until you grabbed the appropriate handwheel (and there was not even an indication of which one hit the stop, so if more than one axis was close to the stops you had to tweak them all), and then you hit the reset button to regain control. However, replace that with a machine designed from the ground up for CNC (e.g. my old Bridgeport BOSS-3 machine -- and I believe all past that at least up to the BOSS-6, and probably to the BOSS-8), and you have *no* handwheels. The X, Y, and Z axis steppers (or servos for past the BOSS-6) are the *only* thing which controls the axes. The Z-axis you could grip the end of the motor spindle and back it off, but everything else was buried under belt guards, and the X-axis was also deep under the table, as instead of turning the leadscrew (which can have whip at high speeds), it mounted the leadscrew rigidly, and turned the ball nut in opposed bearings. So -- with all electronics switched off by hitting a stop, you wound up with needing to partially disassemble the mill to gain access to something which could allow you to back off of the stop in question. Now -- servo amplifiers come with sets of contacts to inhibit motion in a single direction, so you can lock out the drive in the direction which created the problem, but still allow backing away from the stop using jog controls. So, EMC can run closed loop directly without additional hardware support? Indeed so. It is the only way to work when using real servos as they are designed. (Beyond power drivers for the stepper, that is. ?) Stepper? This is with real servo motors, which need an amplifier which sums the speed command voltage from the controller (a Servo-to-go card in the PC in the case of EMC) with the tach generator feedback from the motor to get a precise speed from the motor (and thus from the axis), and the encoder is used to tell the controller exactly where you are at the moment. The Servo-to-go card also handles that, with a counter for each axis to deal with storing encoder pulses which arrive when the controller software can't catch them, so it can update when it can get back to things. That's something, given that old PC's are essentially freebies. It would seem a perfect waste of computational resources otherwise. Replacing Gecko 320's, for example, with simple drivers would be a pretty big savings for the hobbyist. Well ... it depends on what you call "simple drivers". You need an amplifier capable of producing say +/- 40 VDC at at +/- 7A to a typical servo motor. And you need one per axis. I've gotten such servo amplifiers at hamfests and from eBay auctions for quite reasonable prices. But I consider it a pity that Gecko does not make a real servo amplifier, with command voltage input for speed, and with inputs from a tach generator for verification of the speed. They are more energy efficient, since they are running as switching mode regulators, instead of the analog pass transistors used in the servo amps which I have gotten. Each amp has its own Rotron muffin fan and big heatsink to keep the output transistors cool. (And its own transformer and power supply.) There are switching mode servo amps available which run from a common power supply, but they are still quite a bit more expensive than the servo amps which I have. The fact that you can set a speed command voltage to the servo amp and get a steady speed from the servo motor is why resonance is not a problem at high speeds with servo motors used as intended. If the Gecko 320 is the one which drives servo motors (but pretends that they are steppers), then it does nothing with the tach feedback from the servo motor, and uses the encoder to tell which "stepper motor step" it is at. Feeding it several steps very quickly will increment a counter which is compared to another counter run from the encoder which produces a voltage to drive the servo motor in the intended direction until the count from the encoder catches up. At certain speeds, you have the servo motor moving at pulsing speeds, just like the stepper that it is pretending to be, and thus have the resonance problem again. If the Gecko were to honor the tach feedback, it could run at a smooth speed even with step pulses as input. One pluse behind gets a command voltage of 1mV. Two pulses gets 4mV, three pulses gets 16mV and so on, so the motor would speed up as needed to catch up, instead of having to try to run at full speed for a single pulse of offset. In general, are servos considerably different from a stepper with an encoder? Considerably so. I think that I have covered most of it above, in answering the other points. Note that I've taken a DC servo motor and amplifier, and set it up so 10VDC input is full speed from the motor, and then set the voltage down to 0.0001 V (the smallest that I could repeatably produce from the power supply in question), and had to stick some tape to the output shaft of the motor (sort of like a flag), and spend some time watching before I could even be sure that it was moving. So -- with the speed command, you can get a much smoother surface when cutting a shallow angle than you can with steppers. Well -- micro-stepping might come close, but it takes a lot more work from the controller -- each micro-step has to be generated, instead of simply outputting a voltage proportional to the desired feed in that axis, and checking every so often that it was where it should be. If not , tweak the command voltage a bit until it is running at precisely the speed you want. The other axes will be running at their own commanded speeds to give you a very smooth cut. (Yes, encoders will limit your resolution for precise positions, but the slow motor motion will carry you through the intervening spaces to eliminate major steps.) (In contrast, when I tried to turn a Morse taper with my Compact-5/CNC lathe, I could see the steps (which were 0.002" diameter). So -- someday, I intend to retrofit the Compact-5 with servos and an EMC controller. Note that the Gecko drive, with a servo motor, will still be producing the steps, because it does not try for a steady-state speed. I hope that this helps, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#17
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What do you mean by only 4X support.
Typically, the incremental quadrature encoder has A and B channels with pulses 90 degrees out of phase when rotating. Not only does this allow for the decoding of direction information but also allows for a pulse rate four times that of either channel. Hence the 4X. If a servo drive only supports 4X then the step frequency required from software to support any encoder RPM is; (RPM/60)*CPR*4 If the encoder is mounted on the motor shaft as opposed to a driven shaft geared down from the motor shaft then you have a worst case scenario in terms of required software step rate. If the highest step rate from software is less than the above when RPM is the maximum RPM of the motor then the motor will never be able to reach top speed. 1X support will reduce the above requirement by a factor of four. "Eric R Snow" wrote in message ... |
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#18
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I forgot to ask what the cheaper drives are that you have found.
Follow the link below. Note the voltage and current limitations when compared to the Gecko 320, they may not meet your requirements; http://www.cadcamcadcam.com/index.as...PROD&ProdID=11 "Eric R Snow" wrote in message ... |
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#19
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At certain speeds, you have the servo motor moving at pulsing
speeds, just like the stepper that it is pretending to be, and thus have the resonance problem again. This is nonsense. At no time is the DC motor pretending to be anything. It is being itself driven by a PWM voltage always. Anything resembling resonance issues in steppers is due to either improper tuning or exceeding the specifications of the system. "DoN. Nichols" wrote in message ... God is dividing by zero --- |
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#21
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According to oparr :
At certain speeds, you have the servo motor moving at pulsing speeds, just like the stepper that it is pretending to be, and thus have the resonance problem again. This is nonsense. At no time is the DC motor pretending to be anything. It is being itself driven by a PWM voltage always. The DC servo motor *in combination with the Gecko drive* is pretending to be a stepper motor. It accepts step and direction pulses like a stepper driver does, and for each step, it puts out torque until the encoder count matches the input pulse count. Thus, it has pulses of torque, just as a stepper motor does. Anything resembling resonance issues in steppers is due to either improper tuning or exceeding the specifications of the system. How do you tune a servo whose drive electronics (the Gecko) pay no attention to the tach generator? There is not even anywhere to connect the tach generator. It (the Gecko) is a package of electronics intended to make a servo motor behave like a stepper motor, to allow using it with a simpler controller. This is not what I need, as I intend to use the servo motors as designed, with tach feedback and encoders, not the way Gecko mis-uses them. Enjoy, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#22
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"oparr" wrote in message
news:9nHYe.10891$i86.2765@trndny01... Replacing Gecko 320's, for example, with simple drivers would be a pretty big savings for the hobbyist. Let's make certain we're on the same page first.....The Gecko 320 is a DC servo drive. Besides cost, its 4X only support is another limitation for the hobbyist who is likely to be looking at surplus DC motors. I bought one for evaluation and concluded it was not the drive for me. Instead, I went with simple microcontroller based drives with both 1X and 4X support at about half the price. So if we 're talking about the same Gecko drive then simpler and cheaper drives already exist. Like Eric, I don't know what 4X and 1X are. Is that 4 microsteps vs. full step? Something else? Let's back all the way up. The original context was about EMC and its ability to drive servos on the PC's parallel port. That's where the 320 came in. Most PC software operate steppers in open-loop only: no encoder input, and sends step and direction lines going out. The 320 reads the encoder quadrature input, closing the loop. The encoder resolution has to match the step size. That is, one encoder tick for each step or microstep. When they get too far out of sync, the 320 signals a fault and then resets. The original quote refers to replacing the 320's functionality with a freebie, otherwise unused old PC running EMC, to run steppers in closed loop. I know nothing about servos. Simplistically, I think of them as steppers with a matched encoder. Probably too simplistic, but workable, since I don't expect to ever want to pay the extra for whatever they bring. I'm aware of other and cheaper drives, ranging from HobbyCNC's u-solder-it 4-axis $99 special, and up. I'm not sure what it was you were trying to say. |
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#23
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"DoN. Nichols" wrote in message
... Considerably so. I think that I have covered most of it above, in answering the other points. Note that I've taken a DC servo motor and amplifier, and set it up so 10VDC input is full speed from the motor, and then set the voltage down to 0.0001 V (the smallest that I could repeatably produce from the power supply in question), and had to stick some tape to the output shaft of the motor (sort of like a flag), and spend some time watching before I could even be sure that it was moving. Got it now. Thanks. |
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#24
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oparr writes:
http://www.cadcamcadcam.com/index.as...PROD&ProdID=11 30 volts at 5 amps? Hopelessly underrated for most CNC applications. |
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#25
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DoN. Nichols writes:
If the Gecko 320 is the one which drives servo motors (but pretends that they are steppers), then it does nothing with the tach feedback from the servo motor, and uses the encoder to tell which "stepper motor step" it is at. Feeding it several steps very quickly will increment a counter which is compared to another counter run from the encoder which produces a voltage to drive the servo motor in the intended direction until the count from the encoder catches up. At certain speeds, you have the servo motor moving at pulsing speeds, just like the stepper that it is pretending to be, and thus have the resonance problem again. This is speculative fantasy. Gecko controlled servos do not behave this way. PID digital feedback yields smooth motion. Note that the Gecko drive, with a servo motor, will still be producing the steps, because it does not try for a steady-state speed. More silliness. That's not how PID digital controls work. The step/direction signals are simply a method of *communication* between PC and controller. The motion does not exhibit stepping. Your efficiency notion is also wrong. Geckos use PWM MOSFETs which are very efficient. Heatsinks are hardly necessary for many high-power applications. |
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#26
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DoN. Nichols writes:
The DC servo motor *in combination with the Gecko drive* is pretending to be a stepper motor. It accepts step and direction pulses like a stepper driver does, and for each step, it puts out torque until the encoder count matches the input pulse count. Thus, it has pulses of torque, just as a stepper motor does. Nonsense. You don't understand this device, and obviously have never used one. |
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#27
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According to Richard J Kinch :
DoN. Nichols writes: The DC servo motor *in combination with the Gecko drive* is pretending to be a stepper motor. It accepts step and direction pulses like a stepper driver does, and for each step, it puts out torque until the encoder count matches the input pulse count. Thus, it has pulses of torque, just as a stepper motor does. Nonsense. You don't understand this device, and obviously have never used one. O.K. I can't get through to the Gecko web page -- has the URL changed, or have they gone out of business? However, somewhere around here I have the saved PDF files of the manuals for them. I could find no place for the connection of the tachometer feedback wires from the servo motor, so it can't be paying attention to the motor's velocity. It just moves it until the encoder says that it has moved far enough, and then stops -- suddenly. I've used servo amps, and know how they work. The Gecko is *not* one of these. Not even the one of the four models which is designed to work with servo motors. In any case -- it is *not* what *I* need. If you have more information to tell me otherwise, please post it. Enjoy, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#28
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According to Richard J Kinch :
DoN. Nichols writes: If the Gecko 320 is the one which drives servo motors (but pretends that they are steppers), then it does nothing with the tach feedback from the servo motor, and uses the encoder to tell which "stepper motor step" it is at. Feeding it several steps very quickly will increment a counter which is compared to another counter run from the encoder which produces a voltage to drive the servo motor in the intended direction until the count from the encoder catches up. At certain speeds, you have the servo motor moving at pulsing speeds, just like the stepper that it is pretending to be, and thus have the resonance problem again. This is speculative fantasy. Gecko controlled servos do not behave this way. PID digital feedback yields smooth motion. Note that the Gecko drive, with a servo motor, will still be producing the steps, because it does not try for a steady-state speed. More silliness. That's not how PID digital controls work. The step/direction signals are simply a method of *communication* between PC and controller. The motion does not exhibit stepping. So -- what happens if you feed it a pulse, wait five seconds and then feed it another pulse? Are you saying that it is going to *predict* exactly when that second pulse will come, and will move at a steady speed just right so when the second pulse comes it will be in the right place? If so, it must have some rather impressive CPU power built into it -- and I see no provisions for that. Your efficiency notion is also wrong. Geckos use PWM MOSFETs which are very efficient. Heatsinks are hardly necessary for many high-power applications. Here -- you are obviously misreading me. I did not claim that the Gecko was energy inefficient. Instead, I was saying that the *analog* servo amplifiers which *I* have and use are energy inefficient, and I had hoped that Gecko would have come out with a *real* servo amplifier using PWM drivers. They have so far disappointed me in that hope. DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#29
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According to Mike Young :
"oparr" wrote in message news:9nHYe.10891$i86.2765@trndny01... Replacing Gecko 320's, for example, with simple drivers would be a pretty big savings for the hobbyist. Let's make certain we're on the same page first.....The Gecko 320 is a DC servo drive. Besides cost, its 4X only support is another limitation for the hobbyist who is likely to be looking at surplus DC motors. I bought one for evaluation and concluded it was not the drive for me. Instead, I went with simple microcontroller based drives with both 1X and 4X support at about half the price. So if we 're talking about the same Gecko drive then simpler and cheaper drives already exist. Like Eric, I don't know what 4X and 1X are. Is that 4 microsteps vs. full step? Something else? Let's back all the way up. The original context was about EMC and its ability to drive servos on the PC's parallel port. While the EMC can drive servos through the PC's parallel port, with the help of the Gecko 320, that was not what I was talking about. The use of the servos and amplifiers which I was talking about involves the Servo-to-go card, which was priced at $888.00 when I got it, for an ICA bus socket. That contains lots of buffered I/O connections of various sorts, plus one D/A converter per axis (and the $888.00 version can handle up to eight axes). To set the speed, the computer writes a value to the D/A which converts it from the computer's "D"igtal numbers to an "A"nalog voltage, which commands the servo amplifier to run the servo motor at a selected speed. The Servo-to-go keeps track of the encoder position to let the computer tell whether the speed that it set was correct. If not, the computer can make corrections in time to avoid problems. That's where the 320 came in. Most PC software operate steppers in open-loop only: no encoder input, and sends step and direction lines going out. The 320 reads the encoder quadrature input, closing the loop. The encoder resolution has to match the step size. That is, one encoder tick for each step or microstep. When they get too far out of sync, the 320 signals a fault and then resets. The original quote refers to replacing the 320's functionality with a freebie, otherwise unused old PC running EMC, to run steppers in closed loop. The loop closing with the EMC card and servo motors (*not* steppers) closes the loop through the computer, so the computer need not take it on faith that just because it told it to go so far, that it has actually accomplished that -- unlike with steppers, where trying to run too fast in the face of a load will cause it to miss steps. And trying to run even slow in the face of a serious load will still cause it to miss steps. I know nothing about servos. Simplistically, I think of them as steppers with a matched encoder. Probably too simplistic, but workable, since I don't expect to ever want to pay the extra for whatever they bring. Think of them as a motor, with an encoder (though that may be on the machine axis, telling where it *really* is), *and* a tachometer generator which produces a voltage proportional to the motor's speed. Thus, when the servo amp receives a voltage saying to go so-and-so fast, it has a way of telling whether the motor is doing that. The servo amp combines the speed command voltage with the feedback voltage from the tach generator, and amplifies the *difference* between them to produce a voltage to the motor's armature. The amplifier has a lot of gain, so it does not take much difference to produce a lot of output. The amplifier is a high-power version of an operational amplifier -- designed for summing signals. To my mind, a motor with an encoder, but *not* a tach generator is not a servo motor. (Though the Gecko drive can work as well with it as it does with one with a tach generator, as it ignores the tach generator -- it does not even have terminals to connect them to.) Since it is PWM, it can sort of work around that, by measuring the voltage during the intervals when it is not actually pumping current into the motor, but this is not as precise a means of control. I'm aware of other and cheaper drives, ranging from HobbyCNC's u-solder-it 4-axis $99 special, and up. I'm not sure what it was you were trying to say. Well ... at least you have what *I* was trying to say above. Enjoy, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#31
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On Fri, 23 Sep 2005 01:20:18 GMT, "oparr" wrote:
I forgot to ask what the cheaper drives are that you have found. Follow the link below. Note the voltage and current limitations when compared to the Gecko 320, they may not meet your requirements; http://www.cadcamcadcam.com/index.as...PROD&ProdID=11 "Eric R Snow" wrote in message .. . Thanks Oparr for both replies. Eric |
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#32
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Hopelessly underrated for most CNC applications.
Correct! Hobbyist applications are only a small fraction of all CNC applications. |
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#33
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I'm not sure what it was you were trying to say.
Based on your last post, I think it suffices to say that there is no simple way under the sun you can use a Gecko 320 to drive a stepper motor even if it was equipped with an encoder. Repeating....It is a DC servo drive. |
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#34
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So -- what happens if you feed it a pulse, wait five seconds and
then feed it another pulse? Are you saying that it is going to *predict* exactly when that second pulse will come, and will move at a steady speed just right so when the second pulse comes it will be in the right place? If so, it must have some rather impressive CPU power built into it -- and I see no provisions for that. Just about any motion control scheme should cause a jump in linear postion based on a single step. I see where you are trying to go with this so I'll save you the trouble. It is the **frequency** of the step input that excites resonance in a stepper motor. The DC motor is isolated from the **frequency** of the step input. The only **frequency** the DC motor sees is the frequency of the PWM signal applied to the controlling H-bridge. It is constant and bears no relationship whatsoever to the step input **frequency**. It is the duty cycle of the PWM signal that is varied in order to provide a moving equilibrium between step input and encoder counts in response to changes in step input **frequency**. The tuning of the DC servo system will determine the behaviour of the movement mentioned. If the **frequency** of the step input suddenly changes then the P & D settings (aka gain and damping) are supposed to critically damp the movement in a properly tuned system. An improperly tuned system can break out into violent oscillations similar to a stepper in resonance. Perhaps this is what you've seen and confused it with resonance. |
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#36
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Don,
I've looked at the Gecko 320, and it is a full servo, just with some severe limitations. It uses a stepper like interface in that you give it a direction and pulse signal, which is actually a pretty reasonable way to comunicate with a controller. As long as you have a controller that can run the pulse ramps (and keep track of them) it's pretty straight forward. If fact there are many low cost controllers that control in just this way. The Gecko 320 is a full PID controller. The tach signal is derived by looking at the rate the encoder signal moves. This is reasonable IF the encoder is mounted on the motor, but really sucks if encoder is on the leadscrew, which is driven from a belt. The extra element in the servo path limits the optimal tuning that you can do. I would like to upgrade my mill to a newer controller and was looking at these. I couldn't get near the stiffness out of the gecko's as my current system. Gecko has the block diagram for the 320 on their web site. If you are familiar with servo's it's pretty straight forward. As far as having "stepping", its not quite what you think. The Gecko 320 method has no worse "stepping" as a servo done with EMC. Both are limited by the discrete positions of the encoder. If anyone want to try out a 320 (I think that's the one I have), let me know. Since it won't work for me I need to move on to something else. Terry "DoN. Nichols" wrote in message ... According to Mike Young : "oparr" wrote in message news:9nHYe.10891$i86.2765@trndny01... Replacing Gecko 320's, for example, with simple drivers would be a pretty big savings for the hobbyist. Let's make certain we're on the same page first.....The Gecko 320 is a DC servo drive. Besides cost, its 4X only support is another limitation for the hobbyist who is likely to be looking at surplus DC motors. I bought one for evaluation and concluded it was not the drive for me. Instead, I went with simple microcontroller based drives with both 1X and 4X support at about half the price. So if we 're talking about the same Gecko drive then simpler and cheaper drives already exist. Like Eric, I don't know what 4X and 1X are. Is that 4 microsteps vs. full step? Something else? Let's back all the way up. The original context was about EMC and its ability to drive servos on the PC's parallel port. While the EMC can drive servos through the PC's parallel port, with the help of the Gecko 320, that was not what I was talking about. The use of the servos and amplifiers which I was talking about involves the Servo-to-go card, which was priced at $888.00 when I got it, for an ICA bus socket. That contains lots of buffered I/O connections of various sorts, plus one D/A converter per axis (and the $888.00 version can handle up to eight axes). To set the speed, the computer writes a value to the D/A which converts it from the computer's "D"igtal numbers to an "A"nalog voltage, which commands the servo amplifier to run the servo motor at a selected speed. The Servo-to-go keeps track of the encoder position to let the computer tell whether the speed that it set was correct. If not, the computer can make corrections in time to avoid problems. That's where the 320 came in. Most PC software operate steppers in open-loop only: no encoder input, and sends step and direction lines going out. The 320 reads the encoder quadrature input, closing the loop. The encoder resolution has to match the step size. That is, one encoder tick for each step or microstep. When they get too far out of sync, the 320 signals a fault and then resets. The original quote refers to replacing the 320's functionality with a freebie, otherwise unused old PC running EMC, to run steppers in closed loop. The loop closing with the EMC card and servo motors (*not* steppers) closes the loop through the computer, so the computer need not take it on faith that just because it told it to go so far, that it has actually accomplished that -- unlike with steppers, where trying to run too fast in the face of a load will cause it to miss steps. And trying to run even slow in the face of a serious load will still cause it to miss steps. I know nothing about servos. Simplistically, I think of them as steppers with a matched encoder. Probably too simplistic, but workable, since I don't expect to ever want to pay the extra for whatever they bring. Think of them as a motor, with an encoder (though that may be on the machine axis, telling where it *really* is), *and* a tachometer generator which produces a voltage proportional to the motor's speed. Thus, when the servo amp receives a voltage saying to go so-and-so fast, it has a way of telling whether the motor is doing that. The servo amp combines the speed command voltage with the feedback voltage from the tach generator, and amplifies the *difference* between them to produce a voltage to the motor's armature. The amplifier has a lot of gain, so it does not take much difference to produce a lot of output. The amplifier is a high-power version of an operational amplifier -- designed for summing signals. To my mind, a motor with an encoder, but *not* a tach generator is not a servo motor. (Though the Gecko drive can work as well with it as it does with one with a tach generator, as it ignores the tach generator -- it does not even have terminals to connect them to.) Since it is PWM, it can sort of work around that, by measuring the voltage during the intervals when it is not actually pumping current into the motor, but this is not as precise a means of control. I'm aware of other and cheaper drives, ranging from HobbyCNC's u-solder-it 4-axis $99 special, and up. I'm not sure what it was you were trying to say. Well ... at least you have what *I* was trying to say above. Enjoy, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#37
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#38
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Correct, It's been a while since I looked at it.
Terry wrote in message oups.com... The Gecko 320 is a full PID controller There is no integral component. |
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#39
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DoN. Nichols writes:
O.K. I can't get through to the Gecko web page -- has the URL changed, or have they gone out of business? Lately they have hosting problems. But from what I hear, their business is galloping along. I could find no place for the connection of the tachometer feedback wires from the servo motor, so it can't be paying attention to the motor's velocity. It just moves it until the encoder says that it has moved far enough, and then stops -- suddenly. No, not at all. You're speculating, and wrongly so. You're confusing encoder quantization with stepper motor poles. You're confusing step/dir communication with servo control. You're don't appreciate that a digital position encoder can be digitally differentiated to effect a digital tachometer or accelerometer. Do some reading on digital servo feedback loops using PID control. http://www.embedded.com/2000/0010/0010feat3.htm |
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#40
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Hopelessly underrated for most CNC applications.
Correct! Hobbyist applications are only a small fraction of all CNC applications. I don't know of *any* metalworking machines that will scoot with such tiny amounts power, unless you're gearing down to nothing. Maybe you're suggesting PCB drills or wood routers that don't require much torque. 150 watts for $80, vs Gecko's 1600 watts for $120, I'd choose the latter. The former is no bargain, per watt. It *is* all about the oomph, you know. Why would a hobbyist hand-build a car, and then put a lawnmower engine in it? |
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