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Aeroengineer
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Re: Motion solutions: motors- a mammoth post
Aeroengineer   7/3/2014 7:39:00 PM
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Yes, the BeagleBone black is high on the list of parts that we anticipate using.  We were able to speak to Jason Kridner at EELive and he said that he would be willing to help get us pointed in the right direction.

 

As to the encoders, I really think that it will depend on the overall stiffness of the system.  If you are mounting to the opposite end of a 3/8" leadscrew that is only 10 inches long, then at the maximum capacity of a plastic leadnut (which we would not be running at) the deflection due to torque would be .8°.  This would be a 350 lb load.  If we were to say that the gantry head weighed .5 lbs and we were to be running at a max accel of 10g's, then the deflection would be .01°.  For an encoder to be able to register this, it would need to be over 30,000 counts per revolution.

 

As for going with the Geko drives, that is probably out of the range of what we are able to handle.  Most likely we will have to leverage some other solutions that are already out there.

 

We did just get some information from Trinamic on a new dev kit that they have.  This is something that we may want to look at.  I know that the TI solution is a full closed loop control for BLDC motors, essentially creating a servo motor.  I have a feeling that we are going to have a lot to learn in this area. 

 

 

TonyTib
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Re: Motion solutions: motors- a mammoth post
TonyTib   7/3/2014 6:57:31 PM
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There's no reason to apologize for taking your time; family and job always come first.  So some more notes:

If you're using an encoder for commutation and/or speed control, it's probably best to have it mounted on the motor (or use dual loop feedback).  True, I've used Newport linear stages ($$$$) with an internal belt where the sole encoder was mounted after the belt -- but those used brush motors.

We also had issues with getting a circular encoder wheel mounted exactly in the center for a protoype stage (one reason it remained prototype).  That project had much tighter specs (IIRC, we were looking for ~1,000,000 counts per 360 deg), but little details like centering do matter.

If you're really going for affordability, you pretty much have to go steppers and custom drivers, since even affordable commercial drivers (such as Gecko Drives) will eat a lot of your budget.

ST does have some advanced stepper drive chips such as the dSpin models; I'm not sure how they compare to Trinamic.  Allegro Microsystems and TI are other "usual suspects" for integrated stepper drives, but as far as I know, neither one currently offers anything comparable to the more advanced Trinamic parts.  However, TI is also considering doing instaSPIN for steppers, but I have no idea when or if it will appear.  BTW, Trinamic was at ESC--I mean EE!!!LIVE!!!!--2014.

One possible approach would be to use sensorless control (such as instaSPIN) to control hobby motor speed and commutation, and use a cheap linear encoder for position (such as a US Digital EM1 + encoder strip; cheap but low res (for a linear, but probably good enough for this project) and looks hard to mount).

For overall control, an old PC is one approach.  I'd recommend taking a look at the BeagleBone Black, which is $55 for Rev C, runs Linux (including MachineKit, which is based on LinuxCNC), runs OpenCV, has good connectivity (Ethernet, USB, etc), good real time control (PRU-ICSS, which people are using for stepper control), 3 encoder inputs, a bunch of PWMs, an active community, some 3-axis (or more) stepper capes already created, and some attempts at using it for Pick and Place.

Aeroengineer
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Re: Desktop PnP--some inspiration
Aeroengineer   7/3/2014 6:25:12 PM
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Hey, I just came across this link for a mechanical solder paste dipsenser that might be applicable to somthing that we might want to do, and while it is not very detailed, it is an interesting concept.  I think that it could be done in a much smaller package, but it is some food for thought.

 

http://letsmakerobots.com/content/solder-paste-dispenser

Aeroengineer
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Re: Motion solutions: motors- a mammoth post
Aeroengineer   7/2/2014 8:54:02 PM
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I just got an email about the Trinamic parts.  I will have to look into their solution.  I fully expect that we will do a full trade and see what will work best within our constraints.

Aeroengineer
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Re: Motion solutions: motors- a mammoth post
Aeroengineer   7/2/2014 8:51:39 PM
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Tony,

Sorry for the late reply.  I have been traveling for the last month, and get to spend this week with my family.  I am catching up on my responses.

 

As to the issues that you highlight with the dual shaft motors, this assumes that you are going to mount the encoder on the shaft of the motor.  There are many other places that it can be mounted.  Some of these places are at the other end of a lead screw, on the opposite side of a pulley, or on another pulley that is not being driven by the motor.  Each of these locations have their advantages and disadvantages.

 

The issue that you bring up of speed, that is a potential issue, but most likely, we will not be running in those speeds as we will be limited by other things such as the PV ratio of a lead nut, critical shaft speed of a lead screw, belt stretch, etc.  It is something, though to take into account if we get there.

 

As to the magnets being placed in a hobby motor in incorrect locations, this could be an issue, and one that we will have to look into, but if we do a back of the envelope calc and assume that we have a 5° alignment error, and a pitch on a lead screw with a lead of .05", this would lead to a position error of ±.0007".  This would still be an acceptable placement error.  I will let Bob aka salbayeng comment on my math ;)

TonyTib
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Re: Motion solutions: motors- a mammoth post
TonyTib   7/1/2014 6:21:04 PM
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Of course, if you really need acceleration, speed, and precision, the way to go is linear motor, but they aren't cheap.

Another approach is to use steppers with advanced drives, which can extend the torque curve or detect stalling.  For example, Trinamic sells chips and drives which detect stalling, and stepper drives from Copley, Nanotec, and Quicksilver can all drive steppers using servo drive techniques (and require position feedback, and cost quite a bit, although Nanotec sells integrated NEMA24 stepper + driver + controller for <$250).

TonyTib
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Re: Motion solutions: motors- a mammoth post
TonyTib   7/1/2014 1:25:34 PM
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Just to give an idea of costs:

QB02300 / QS02300 motor is >$500 each, without encoder (I recently looked into them, because I was speccing out a nice NEMA23 servo with 80,000 count/rev encoder).

Typical modular or kit encoders run >$60, although CUI's AMT capactive encoders are ~$25 (and require a dual shaft motor).  BTW, most encoders have pretty low maximum speeds (e.g. 15000 RPM for AMT, 8000 for most Accucoders).

If you want cheap and good repeatability, you may want to think about software corrections (map the particular motor and encoder).  Hand-gluing a magnet on a carelessly built hobby motor looks like a recipe for cheap, but not precise and repeatable.


BTW, I have a lot of respect for our ME's, since they build fixtures that can measure a part with a repeatability of 2 microns -- and that's removing the part and putting it back on, again (if the part stays clamped, repeatability is more like 0.2 microns).

Aeroengineer
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Re: Motion solutions: motors- a mammoth post
Aeroengineer   6/30/2014 4:51:08 PM
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Would you mind if I put in a request to one of the powers that be that we be able to contact one another directly?

 

That or if you look me up in LinkedIn under Adam Carlson working at GE Aviation.

Aeroengineer
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Re: Motion solutions: motors- a mammoth post
Aeroengineer   6/30/2014 4:46:48 PM
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Sorry for the late reply.  I was able to travel back home, and I have been spending the weekend with the family.  This is really great information for the reason that it really opens up the design space of the motor selection.  I think that this combined with the hall effect, non-contact encoder from AMS that operates at high speed could make a great combination. 

 

I was able to do some basic researching over the weekend about using bldc in closed loop control for positioning.  It does seem like a promising thing to continue to look into as it could open up the speed profile for what we are trying to do. 

 

As you have mentioned before, though there are a few other reasons to continue to look at stepper motors.  Some of those include the large installed user base that already has experience with this type of technology.  At the same time, though, if you continue to do what other are doing, you will result in a machine that does just that, the same as everyone else is doing.  This could be a differentiating factor.

 

I have a few days off this week.  I am going to see if I can get a meeting with the guy at TI to talk through some of the things of their solution, and what it would take to implement in this type of design.  They do have a nice dev kit that I might see if they will allow us to use for a while as we really try and evaluate this option.  This would allow us to really dig a little more into this and see if it is an option.

salbayeng
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Re: Motion solutions: motors- a mammoth post
salbayeng   6/29/2014 6:28:39 AM
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oops just lost a bunch of typing.

OK three types of motors used (found in my shed) , experience suggests they are of right size to do the PnP job. All ~ 60mm size (NEMA23), The servomotors are typicaly of what pops up on ebay regularly.
  • Yaskawa : old fashioined , nice sinusoidal windings, needed an inefficient driver as you need 200v DC to make a 140V sinewave, very low distortion on back emf, zero cogging. about 300% torque overdrive available
  • Allied: newer style BLDC type, a bit coggy, moderate flux distortion, designed for trapezoidal drive, has a lot of reserve overdrive capacity with 1000% torque and 600% on speed.
  • no-name outrunner , very little data on the web, these numbers mostly from my measurements, very non-sinusoidal back-emf (3-5%) and a lot of cogging, a lot of asymettry as the motor is spun over a full revolution (the magnets are glued on haphazardly, and the airgap is non-uniform)

The motors have different nominal voltages (200,24,10) and different nominal currents (1,9,50) although all deliver nominally 3000rpm and 0.4Nm torque, so the electrical differences are just that some have more turns of thinner wire (if they were transformers they would have different turns ratios). These differences are reflected in the Kt and Kv values (which are analogous to turns ratios).

An important consideration for a robot is the dynamic stiffness, this determines how soft the drive is at high frequencies  (i.e. above the servo bandwidth). This also determines how well the position is held, when holding at a position (most servo controllers will drop to half current, and hold the last flux vector (i.e. this is open loop). So I've shown this as  for example Ks=4.69oz.in/deg This actually looks better than the real servo motors (mostly because the outrunner has twice as many magnets), but it should be considered in terms of the cogging torque of +/- 4oz.in , so the actual position error of the outrunner motor, due to some external disturbance torque of say 4oz.in could be around 1 degree plus or minus another degree (due to cogging) so position error is somewhere between zero and 2 degrees. 

While holding @ 50% Inom , all motors will dissipate 1/4 of "nominal" I2R losses, around 5W for the traditional servo motors, and 7.5W for the outrunner, most outrunners spin rapidly, and suck a lot of air through the windings, but in a hold situation the outrunner may overheat. (Possibly could hold at 33%, but cogging torques get proportionally more significant)

From a dynamic perspective, the motors are similar, with L/R time constant of 1.6,0.94,0.65ms so you could perform servo updates at 1kHz,1kHz,2kHz and do the PWM at 10kHz,10kHz,20kHz. Because of the irregular flux patterns of the outrunner , you will need to back off the servo gain to ensure stability, but you will probably end up with a similar GBW product as the "real" servomotors.

In summary , it looks like the outrunner could produce similar positioning outcomes to servomotors, at  less than half weight and 1/10 of cost.

So you just need to plug in say 78oz.in as torque and 6000rpm , with some leadscrew pitch number , and the putative mass of the head to see what trajectories are possible with the outrunner.  (Note, as is typical with most servo drives, the RPM is generally limited by external mechanical effects and bearing life, not by electrical constraints) 

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