Most motors I’ve seen spin at the leisurely rate of 6000 RPM or less (heck, many are limited to 3000 RPM).
My Emoteq BH02300’s can do 20,000 RPM.
I’ve drooled over the specs for motors from Emoteq, Pittman, and Maxon that can do 60,000->100,000 RPM.Â (Yes, I’d love to own one).
But a Swiss company, Celeroton, takes the cake: they have motors and controllers that can do 500,000 RPM!Â Those must be totally groovy.
Celeroton sells brushless motors, drives, and compressors that useÂ their motors and electronics.Â The drives have a minimum speed of 5000 RPM; some models can handle up to 1,000,000 RPM maximum speed.Â The drives (or inverters, as Celeroton calls them) are available in 400W to 3Kw models, and are sensorless (no hall effect, encoder, or other sensors required).
A friend is looking into using the Celeroton drives with a merely fast motor (~100,000 RPM).Â If I get any real world feedback from him, I’ll update this post.
November 4, 2014 No Comments
I’ve updated my trac page on the Electro-Craft E-3618 with a scanned datasheet with information on the E-3618, E-3622, E-3626, E-3629, and E-3633, and the BDC MINI-Series servo amplifier.Â Since I only own E-3618’s, I didn’t add a new page for the other motors.
By the way, the end of Electro-Craft catalog contains a nice little overview of servo motor theory and operation.
October 21, 2014 No Comments
Before digging into robotics, I am going to take a short detour to look at typical ways to move (motion technologies) and common ways to control (automation controllers).Â This sidetrack will help place robots into the wider automation world.
I am going to be pretty brief; for more information, start with wikipedia, internet searches, or books such as Industrial Automation: Hands On.
Pneumatic motion systems use compressed air to extend and retract cylinders. Pneumatic motion systems are easy to control (just turn the valve off or on), and are inexpensive to build. Providing a desired force is easy too: just use a regulator to control the pressure and use F=PA (force = pressure x area). On the downside, pneumatic cylinders are normally limited to two fixed positions (extend or retract), and compressed air can be expensive.
Stepper motors are brushless DC motors with rotors and stators designed so that a full rotation is divided into a number of equal steps. Since the stepper motor can move a step at a time or hold a position (assuming the motor has enough torque for the application), they can be run open loop (without feedback).
Since no feedback such as an encoder or resolver is required, stepper motors can be significantly cheaper than servo motors, they do not have servo dither (when the servo motor keep moving back and forth and never settles into position) and they are easy to control. You can buy a single IC stepper driver (from Allegro Microsystems, ST, TI, etc), connect it to power and the step motor, and get moving by giving the driver step and direction commands.
On the other hand, with standard drive technology, stepper motors quickly loose torque at higher speeds, waste energy (because the same amount of power is always used; it does not depend on the load), cannot dynamically respond to the load, and can lose position without warning.
Servo motors are brushed or brushless DC motors with feedback, typically encoder or resolver. They can be rotary or linear motors. They require a complex closed loop control algorithm (such as the classic PID method). Normally the control loop has to be tuned, and servo dither can be a problem.. Due to the added control and feedback, typically servo systems are more expensive than stepper systems.
Servo motors typically have a peak torque of 3-10x the continuous torque, their torque curve is much flatter than the stepper curve, and the maximum speeds are much higher. Peak torque is a great thing; often, a system just needs extra power for a short time to accelerate, overcome friction, or such.
Of course, there are other options such as hydraulics, proportional pneumatics, piezo motors, solenoids, and voice coils.
July 18, 2013 No Comments
I went to the three day TI Industrial Control Workshop in Santa Clara.Â Instead of repeating stuff (such as class outline) that you can read on the wiki link (above), I am going to give my impressions.
The bottom line: yes, the workshop is well worth attending if you like to (or just have to) control motors.Â 5/23/2013: I also want to add that I think this workshop is good for automation developers like myself.Â OK, I’m not sure it’s worth flying to another city to attend, but if there’s one close (next one is 17-19 September 2013 at Brookfield, WI) then it’s good to attend — you’ll learn a lot more about what goes on underneath the covers of your VFD or servo drive.Â I know I have a better understanding and appreciation of my drives.Â Also, you can just about do the course on your own by downloading the materials, but it’s not the same experience.
Disclaimer:Â I paid for this class myself (OK, at $79 it wasn’t a big deal – and the price includes snacks, lunch, and a F28069 controlStick); it was a very nice break from my typical workdays.
Update Feb 2014:I notice TI now has 4 videos from a more recent Control Theory Seminar, with the first episode here, and videos for the C2000 One Day Workshop, with the first module here (there’s also an older set of video modules).Â I couldn’t find videos for Day 2 (Motion Control Theory), although TI has a wide variety of other motor control videos.Â So download the materials, watch these videos, and you’re almost there!Â (But I still recommend attending in person if you can).
Considering TI must be subsidizing the workshop, it had amazingly little marketing content – less than a typical trade mag article.Â There was no mention of TI products at all on the first day (control theory) and very little on the second day (mostly pride in TI’s new instaSpin solutions).Â The third day was all about TI products (F28x DSP), but it was all about the product (architecture, peripherals, Code Composer Studio, etc), not marketing.
Overall, there were many good discussions, and lots of questions.Â I enjoyed learning about what other people are doing.
I think all three instructors did a good job; the biggest issue was time – each class easily could’ve been at least a week, so they had a real challenge trying to fit in as much material as possible, explaining it in an understandable manner, while still answering questions (and all three did a good job of answering the many questions).
Day 1 – Control Theory (Richard Poley)
On day 1 I felt like I was back in college; it was like a month (or more!) of college stuffed into one day — and the soft-spoken instructor, Richard Poley,Â reminded me of a college professor.
You do need to have a good math background to follow the theory.Â Fortunately I had a lot of math in college, and I did some reviewing via wikipedia before the workshop.Â I won’t claim I understood everything perfectly, but I felt I remembered enough to follow the basic concepts.
The theory got a little practical at the end of the day with sections on Digital Controller Design, Implementation Considerations, and a Suggested Design Checklist.
I’m pretty sure the vast majority of attendees don’t use control theory day to day.Â I know I don’t; for example, we rarely have problems tuning motion controller PID loops.Â So for me, the theory isn’t very useful for my day today tasks; in fact, trying to use it when it’s not necessary is a waste of time.
But it’s still good to know the theory for when the normal experienced-based approach doesn’t work.Â (The same applies to programming, say sorting: if you have a small set of data, all sorting methods will be reasonably quick.Â But if you have large data sets, knowing the theory of different sorting methods is critical).
I’m now interested in learning about state variable control theory, which is covered in the two day version of the Control Theory seminar, but it will be a while before I’ll be able to find time for project.
Day 2 – Motor Control Theory (Dave Wilson)
Dave Wilson is a motor geek and a primary contributor to TI’s Motor Control blog, which is a treasure trove of motor control information (even if you don’t use TI chips, since most of info isn’t TI-specific).
Dave Wilson emphasized AC induction motors and servo motors, because none of us were interested in stepper motors.Â He covered motor control theory and all the common algorithms (such as field oriented control).Â He discussed advantages and disadvantages of the different motor types and motor control algorithms.Â He did a good job of answering the many questions.Â And, yes, he is very excited by TI’s instaSPIN solutions (especially instaSPIN-FOC).
I really like Dave Wilson’s Power Point and VisSim animations that graphically showed what was going on to make the motors spin.
Day 3 – Intro to F28x (Ken Schachter)
This day was a rapid fire introduction to the F28x DSP series.Â The instructor, Ken Schachter, gave an overview of the peripherals, an overview of the available software such as controlSuite, and then we spent a lot of time doing labs that showed off some of the Code Composer Studio (CCS) goodness (like graphing memory).
I’d call the class an orientation – I wouldn’t even say I become comfortable, but I do feel like I got my feet wet with the tools, and have a better idea of how to start.Â CCS is pretty intimidating at first, and TI does provide a lot of libraries and examples.
May 21, 2013 No Comments
My final look is at NEMA17 motors.Â Today’s contestants are:
- In yellow, the IMS (now Schneider) MDI1PRL17C4x triple stack NEMA stepper motor with integrated driver and controller.Â I’ve used these cute little motors before; they are a great fit for the right application.Â As normal, the programming language sucks, but a CANOpen version is available.
- In red and blue, a Panasonic MUMS011A1 NEMA 17 servo motor.Â These motors have unfortunately been out of production for years; I loved their performance, encoder (10000 cpr), good looks, and price (about $250).
- In green, a Quicksilver Controls QCI-A17H3 stepper motor with encoder that’s driven like a servo.Â I’m including it to show how much improvement you can get from closed loop stepper control.
- The MDI1PRL17C4 shows typical stepper characteristics, with torque rapidly dropping off; it can’t even reach 2500 RPM.
- The MUMS011A1 shows typical servo performance, with a pretty flat torque curve, 3:1 peak to continuous torque, and compared to the steppers:Â higher speeds, less continuous torque at low speeds, more continuous torque at high speeds, and much higher maximum torque.
- The QCI-A17H3 doesn’t turn the stepper into a servo, but compared to an open loop stepper, it has substantially higher maximum speed (4000 RPM – the highest of any stepper I’ve looked at), and offers significantly higher torque at higher speeds — in exchange for a higher price, of course.
Some General Stepper & Servo Notes
- Steppers are much simpler to drive: you can easily build a low cost drive using an integrated chip (e.g. from Allegro Microsystems, ST, or TI) or buy a commercial driver for $100-$150 (from Automation Direct, Gecko Drive, etc).Â Many low cost PLC’s such as Panasonic’s FP0/FPG have built-in step/direction outputs.
- But open loop steppers are very annoying, because you have to figure out your needs before start: you have to specify fixed currents for holding and running torque.Â If your current is too low, the motor will miss steps.Â If your current and duty cycle are too high, the motor will get hot – and if a bit of extra torque is needed to overcome something unexpected, you’re out of luck.
- Closed loop steppers still aren’t servo motors.Â However, I’m looking forward to affordable sensorless closed loop control (available or coming from Trinamic, TI, and probably others), which will improve stepper performance and minimize stepper heat generation.
- I think servo motors’ torque curve fits well with many applications: a lot of time you just need maximum torque for a short time period.Â Servo motor system prices are also coming down (OK, not the basic motor, but there are affordable encoder, drive, and controller choices).
April 26, 2013 No Comments
In this post, I’m going to look at some fun NEMA23 single stack servo motors and a single stack stepper motor.Â The entrants are:
- In brown, the PacSci M21 with Sigmax technology.Â The M21 has a lot more torque than many (most?) NEMA23 single stack steppers.
- In blue and red, one of my favorite motors: the BH02300 servo motor.Â I’ve spun one of mine at 20,000 RPM.Â Like the Emoteq QB series, the BH series has great peak torque.
- In yellow and green, the MCG (now Ametek) I2351014NC servo motor.Â After all the high end motors, I decided it was time to feature a non-exotic and affordable (if you can get it) motor.
- The difference between the I2351014NC and the BH02300 is the peak torque (the BH02300 has much more) and maximum speed (20,000 RPM vs 12,000) and, of course, price; continuous torque is about the same.
- Both servos are can go much, much faster than the M21 stepper (although at 3000RPM, it’s pretty fast for a stepper motor).
- The M21’s continuous torque looks pretty good; it always has more continuous torque than either servo, and below about 2000 RPM has more continuous torque than the I2351014NC has peak torque.
- OK, it’s not really far to compare these motors — they obviously have their own areas (low end torque and low cost for the M21, high speed and moderate cost for the I2351014NC, and high speed, high peak torque, and high cost for the BH02300) but it’s fun to see big differences between motors that are all about the same size.
April 25, 2013 No Comments
NEMA34 Torque Curves
Today I am examining some interesting NEMA34 motors’ torque curves.Â The graph is a bit complex because I want to show a variety of motors on one graph; to make it a bit simpler, I am using dashed lines for servo continuous torque, and solid lines for servo motor peak torque.
I chose motors that are all roughly the same size.
The motors and their colors are:
- Burgandy Red – PacSci N32 PowerPac double stack NEMA34 stepper motor at 75V.Â I choose this motor because it’s a high end stepper, and I own a couple.
- Red – PacSci K32 PowerPac double stack NEMA34 stepper motor with Sigmax technology, also at 75V.
- Yellow – Emoteq QB03402 double stack NEMA34 servo motor.Â I own a similar motor (QB03403), plus the peak torque is very high.
- Green – Parker Compumotor BE342H double stack NEMA34 servo motor at 170V.Â Parker makes some really nice servo motors; the BE series has a lot of torque, and I want to look at the effects of voltage on torque curves.Â The BE342H and BE342K have different windings.
- Brown – Parker BE342H at 340V.
- Dark Blue – Parker BE342K at 170V
- Light Blue – Parker BE324K at 340V.
- The NEMA34 stepper curves are similar to the NEMA23 stepper curves; torque still drops off rapidly with increasing speed.Â One quirk: maximum torque is around 120 RPM, not 0 RPM.
- PacSci’s Sigmax technology does provide significantly higher torque at all speeds, but does not change the shape of the torque curve.
- Overall stepper vs servo comparison is similar: the steppers have much more continuous torque at low speeds, less continuous torque at moderate speeds, less peak torque at all speeds, can’t handle high speeds, and cost significantly less than servos.
- The Emoteq BH03402 has exceptional peak torque, but you’ll have to provide a lot of current (e.g. 50A for the 130V C windings).
- The Parker BE342 shows the impact of voltage and winding.Â When the servo motor does not get enough voltage, its torque can decrease like a stepper, but for a different reason: back EMF.
- The BE342K might seem better than the BE342H, since it has the “best” torque curve, but that comes at price: the same torque requires double the current of the BE342H.
- Stepper currents are much lower; the maximum current of any N32/K32 model is 10A.
- As always, it comes down to knowing your requirements: torque, speed, size, current, budget, etc.
April 24, 2013 No Comments
While I was working on my upcoming project, I got sidetracked by the issue of servo motors versus stepper motors.Â Since I’ve never seen direct stepper and servo torque curve comparisons, I created four comparison charts using a variety of interesting motors.
Disclaimer:Â I graphed all the torque curves myself, and their shapes should be pretty accurate; however, I didn’t have very good data for some motors (e.g. Emoteq peak torque).Â Motor pricing can very quite aÂ bit, depending on the exact model and quantity ordered, so any pricing is a rough guide.
If you click on the images, you will get a much bigger version.Â To avoid a megapost, I’ve split this topic into multiple posts.
Now that the preliminaries are out of the way, let’s start with the first contestants.
First up is a pair of motors: the Kollmorgen AKM21G single stack NEMA23 servo motor with 75V winding and the Pacific Scientific P22 double stack NEMA23 stepper motor.
I picked these motors because 1) they are both from the same company (Danaher), 2) I’ve used both, 3) they’re about the same size, 4) are designed for similar voltages, and 5) both are high end motors, so they should show off the best of their technologies.
The colors are as follows:
- Red – AKM21G peak torque
- Blue – AKM21GÂ continuous torque
- Green – P22 at 72V
- Yellow – P22 at 24V
- Notice how quickly the P22’s torque falls off at 24v – at slow speeds, torque is about 150 oz-in, but it’sÂ <50 oz-in by 1000 RPM (substantially less than the AKM21’s continuous torque).
- The P22 is considerably better at 72V – the torque curve is much flatter, and it has more continuous torque than the AKM21G up to about 2100 RPM.Â The higher drive voltages reduces the effects of the stepper motor’s inductance.
- Servo drives are the only option for higher speeds.Â The AKM21G is rated at 7800 RPM max, while the P22 torque curve only goes to 3000 RPM.
- The AKM21G has a much flatter torque curve, especially continuous torque.
- The servo’s peak torque is a big advantage: the peak torque available from the single stack AKM21G is ~50% greater than the maximum P22 stepper torque.Â The peak torque advantages gets much bigger as the speed increases.
- On the other hand, the servo is much more expensive: an AKM21G will run $400 or more, while the P22 is around $100.
March 30, 2013 No Comments
Although I use commercial motion control equipment, I enjoy learning about the fundamentals of servo motor control.Â I’m currently going through the posts in TI’s Motor Control blog; they have a high signal to marketing ratio, and include a lot of non-obvious tips, like the best time to measure current.
The posts include a number of simulations which are helpful in understanding the different control topologies.Â However, there is still no substitute for spinning actual motors.
Commercial controllers are great at getting you up and running quickly, but don’t let you play with different control techniques.Â For learning, motion control development kits are the way to go.
My dev kit is TI’s DRV8312-C2 kit with a F28035 DSP, DRV8312 brushless DC driver chip, and servo motor (unfortunately TI didn’t include a dual shaft model, but I have plenty of servo motors with encoders).Â TI’s ControlSuite software provides a variety of control methods.
May 3, 2012 No Comments