Besides losing step and position (which has a minor irritation drift) , I fail to see how the other axes on a stepper based system will know if one axis stalled completely for example. And here I'm not referring to hitting some extreme where there's a limit switch. Suppose the Z axis on a CNC drill or mill got jammed in the workpiece won't the other axes continue until they too are forcefully disabled if an operator isn't around? Do stepper drives have a "fault bus" like servo drives?
snipped-for-privacy@hotmail.com wrote in news:1127495435.696903.21470 @g14g2000cwa.googlegroups.com:
Totally correct, one axis loosing position will not stop the other axis in the movement. No fault bus at all, but the step/direction servo drives will fault and some software will halt the motion of all axis if an axis faults, some will not. Again this is the cheaper end of the scale and not as comprehensive as the big stuff.
Both scoot and tiny are relatives. Whatever, there's woodworking, robotics and several other fields where lower torque levels are commonplace.
Its more like $195.00 vs $360.00 if you're catering to 3 axes.
No, I don't know. How old are you? You seem like a impressionable teenager who drives a car with one of those 6" tail pipes that's good for nothing but noise.
This is where the MCU based servo drives shine and are destined to be the state of the art IMO. Software is not required to halt operation when any axes faults. In fact the software doesn't even know a fault bus exists. Each drive is connected to the fault bus via open drain inputs. When any drive detects a fault associated with the motor it is driving, it will switch its input to output and pull the bus low causing all other drives to fault. This all takes place in 3 or 4 clock cyles so it's practically instantaneous.
snipped-for-privacy@hotmail.com wrote in news:1127499300.458419.75650 @o13g2000cwo.googlegroups.com:
Yes, agreed. I've tried to do something similar and link each drivers state into the E stop. Crude at best, but while it still might break an endmill it would save the part if you where lucky. Gecko is/has? developed a box that sits between the controller and the drive with an MCU that does most of what you suggest along with other stuff.
Thanks for that. It explains the control problem quite well.
So, taking all that was said before, I take it the 320 solves to the commanded position by controlling the servo's speed. An evenly spaced step pulse train results in smooth movement at the target speed, which settles in quickly if the gains are adjusted correctly. The smooth movement doesn't suffer from a stepper's herky jerky motion, and is thus immune from the related stepper resonance issues. The discrete nature of the step pulses and the encoder stream would seem to be inherently noisy, but is easily averaged out in steady state motion. OTOH, acceleration and deceleration would seem strongly dependent on the controller being able to maintain sync, rather than solely on the motor's available torque as is the case with a stepper. I don't know how big the difference is, or if it's even measurable or objectionable in CNC use.
So, how much can I expect to pay for a 400 oz-in servo?
According to :
Yes -- it allows a DC servo motor to look like a stepper to the controller.
Enjoy, DoN.
Dunno what you mean by noise. There is no noise of any consequence.
Dunno what you mean by this, either. Whatever the motor is capable of doing, the controller and software will actuate.
You can get a new 1/3 or 1/2 HP PMDC motor surplus, and retrofit an encoder, for less than $100. Ready-made DC servos are more expensive.
Measurement noise, as opposed to EMI or audible noise. Not of consequence.
Again, probably of little consequence. I was thinking about the the PID controller characteristics and settling time during accel and decel.
(Thanks for the links. Interesting pages...)
And now we've come full circle. Why servos for hobbyist CNC? What benefits do you gain for the added cost and complexity?
Superior characteristics and performance, less cost.
Steppers made sense only when encoders and digital servo controllers were expensive. Encoders and controllers are cheap now, so steppers are obsolete, except for the smallest and lowest-torque applications.
Hence the question. :) "Hobby" range PMDC motors cost about twice as much as "hobby" steppers of roughly equal speed and torque. Simpler drives can be used with steppers, and the encoder is optional. So what are the superior characteristics? The only that comes to mind is static current draw for steppers; they draw power just sitting still. That seems to be of very little consequence.
Here's just one......One of the advantages of a CNC setup is that one can walk away and leave it to finish what was started. You've probably seem a drill bit get jammed in the workpiece while manually drilling. The same thing can happen while CNC drilling or milling. If this happens with a stepper setup chances are the X and Y axes will do their best to not only destroy the part but destroy themselves and Z axis components as well, behind your back, since there is no switch or feedback to tell the software or the other axes that something went wrong.
Closed loop is undoutedly a Good Thing (TM). It's not exclusive to servos, however. You only need an encoder, and software or controller that reads it.
Why else might I consider discarding the steppers for servos? Motor and drive cost is about $300 per axis; I already have the encoders.
This kind of cost comparison isn't appropriate. Steppers perform dismally. They are only "cheap" when they aren't moving, but the purpose of the system is to move. Steppers are rated with holding torque, but nothing like that torque is available in motion. The "simpler" drive only works up to a few hundred rpms. To get higher speeds you need a sophisticated drive that costs as much or more than a servo drive. And the torque still dribbles off.
Torque is misleading you. Motion components should be compared in terms of power, that is, torque times speed. Steppers start losing their torque at rather low speeds, and that makes them much more expensive for the equivalent performance of servos. The top speed is a fraction of a servo.
Steppers are also absurdly inefficient in converting electrical power to mechanical work. This means you need a power supply many times larger than what a servo would require, and this adds further up-front expense.
Servos have guts. You can run them at multiples of their rated torque for short periods. This is exactly what you need to move a mechanism with stiction, which is to say any type of machine tool. Or to avoid stalling on a cut with a difficult momentary load. With steppers, moving and cutting is always stuck underneath the reciprocal of the peak force requirements, instead of the average.
Steppers are appropriate for slow, tiny, low-torque applications, like printer mechanisms and disk drive heads. They have no place in machine tools any more, other than as a hobbyist toy to get the cheapest possible motion no matter how dismal the speed.
Actually, closed loop in our context is exclusive to servos. It is not exclusive to DC servos. So far we have been using servo to mean DC servo and stepper to mean open loop stepper. A closed loop stepper setup (using encoder etc.) is also a servo. We seldom have to make the distinction since stepper based servos are very rare.
You just need an encoder and a closed loop stepper drive (encoder feedback). The most popular CNC software doesn't have a clue as to what is receiving the steps it ouputs since there is no feedback from the drive to the software. All it does is output a finite number of steps for a particular move. It will do that even if nothing is connected to the computer.
Don't discard anything if you're satisfied with it.
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.