Servos & Steppers.

Ok so I have an application where I might need a servo or stepper (both of which I know exactly ZIP about). The application is low rpm to drive a timing belt at 1ft per sec. with about 30ftlbs of force. The other problem is space. The motor cant be more then 3-1/2 inches long from face to back. I looked at the AD stuff but not powerfull enough. The application is to open & close a sliding door for a demo unit where the installiation overhead is not high enough to accomidate our standard swing-arm door operators. I was hoping that somebody could point me to a good primer and some reccomended manufactures of steppers & drives.

Thanks,

30 foot lbs is alot of force for a servo, it translates to 40,474nm ( thats 40.674 Newton meters, the metric system uses a comma instead of a period 0r 350 in/lbs.).

Yaskawas Sigma series will probably work for you:
http://www.yaskawa.com/site/products.nsf/productGroup/ServoSystemsAndMotionControllers.html

They also offer some e-training modules associated with their products: http://www.yaskawa.com/site/Training.nsf/training/MotionELearningModule.html

The motor cant be more then 3-1/2 inches long from face to back.

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This may hinder the process a little, the torque requirements may require a little more length.

I'm not sure how they are on price but i'm pretty fond of the Bosch Rexroth Ecodrives we have in our equipment for servo applications. The ones we use are too small for 30ft/lbs since they've mainly been used to replace small air cylinders for more concise control. The typical design seems to be longer with a smaller circumfrance than a similar standard motor. Obviously, talking to a rep. would be much better than listening to me though.

The Drivetop software is a little confusing at first but gives you a ton of control and offers a simple interface to the PLC. A signal from a discrete output triggers the drive to run one of it's preset programs (Set through drivetop) that tells it how many degrees to run, and at what RPM. The drive's program can also trigger another program. This is handy when you want to trigger a series of events with a single output on the PLC.

Might be total overkill for your application. They are pretty maintenance free. We have one that spins 120 degrees somewhere around 30,000-40,000 times every 24 hours and hasn't been touched in 4 and a half years since it was installed except to blow the dust off it once in a while and it's taken a good lickin' over the years. They also making a cloning module for easy replacement of the drive.

I use only Pac-Sci servo motors, but I don't know if any are suitable for your application. I always let my distributor size them for me...

Have you considered other options? Perhaps a right angle drive will rotate the motor to where it fits better (plus you get gear reduction!). You said "sliding door" so another thought is a linear actuator. I've used IDC actuators on a number of projects with great success.



-Eric

Mike:

Yaskawa Sigma II "P" type 400 W motor (SGMP series, a "stubby one") is about 3 in long and delivers almost 1.3 Nm of continuous torque. It can run continuously at 3000 rpm although may go as high as 5000 rpm for short periods of time.

If there is a way in your app to provide 1:40 gearing (say, 10:1 gearhead and 4:1 pulleys), you would get the torque you need at about 75 rpm. Another nice feature of a servodrive is that there are discrete inputs to limit the maximum torque when needed - and I guess, as far as elevator doors are concerned, it is a desirable thing to have in case someone's arm is stuck.

Call Yaskawa, ask for advice, get in touch with their local rep. I am sure they would be able to help, if you want to go that way.

Rexroth and Pac-Sci are excellent choices too - I just am not familiar with them.

With a highish torque and low speed the usual solution includes a gearbox. With your space requirements I would look to a right angle gear box so you have some room for a motor (assuming you have space in the other dimesion).

Even at that, 3.5" is a pretty small space but you may be able to find something. A NEMA size 23 rt. angle is about 3.3" from mounting face to the back end but you probably won't get the 360 in-lbs out of it that you need (for very long). The next size up (NEMA 34) will have the torque but is going to be around 4.25". Perhaps someone makes a special Right Angle gearbox with reduced profile in that frame size. The dimensions above came from www.danahermotion.com in the Standard Components/Gearheads/NEMA true section.

Also... being a door application, you might need to provide something that can be backdriven when there is no power for emergency opening purposes. This can be really tough to do with higher gear ratios. When I was working at a transit door operator company, we always had to have a way to open the doors manually under loss of power (This was always a deadly serious part of the design). If you can't backdrive the gear train, you can also provide a clutch that couples the load until a manual/emergency release is pulled.

There is one motor that I can think of that will fit your 3.5" requirement directly but you don't say the RPM required so I'm not sure on that. It is called a Direct Drive Rotary brushless servo motor. You have to do a lot more design to fit it into the application but it has the torque and length specs you requre. Note that it is 10.2" in diameter (You have to get the torque somehow... it still takes a certain amount of copper and iron )
Here is a link to info:
http://products.danahermotion.com/danaher/modelDetail.asp?User=DDR2&PkgID=423407

Disclaimer... I work for a Danaher Motion distributor.

Eric:

Just wanted to let you know that the links you provided are static and only still there for reference purposes (The pac-sci one states this but the IDC one doesn't seem to :-( ). Since Pacific-Scientific and IDC were bought by Danaher, all new and up to date information is at http://www.danahermotion.com Also, Danaher is using the Pac-Sci brand name only for steppers from now on. The Kollmorgen brand name will be for servos. I believe their new/prefered servo line (the AKM brushless AC servos) are a hybred design taking the best design features from the pac-sci and kollmorgen motors. I'm pretty sure they are made in Rockford, IL.

I always let my distributor size them for me...

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So does everyone else...

Thanks for the info, Norm. I know about the 'Danaher went on a shopping spree' thing, but I didn't realize those sites were static.

You'd think they would make them redirect to Danaher pages...


-Eric

Time out people!

30 pounds of force moving something at 1 foot per second doesn't necessarily equate to 30 foot pounds of torque from the servo or stepper motor.

Mike has not given enough information to properly size the motor. All he's given is how fast the object to be moved needs to travel. In order to size the motor, you need to know the diameter of the pulley that timing belt is wrapped around. You also need to know if there is going to be a gearbox or some method speed reduction between the motor and the timing belt pulley.

"30 foot lbs is alot of force for a servo, it translates to 40,474nm ( thats 40.674 Newton meters, the metric system uses a comma instead of a period 0r 350 in/lbs.)"

Not in the least bit relevent , but potentially very confusing .

40,474 nm or NM or pink elephants is not the same as 40.674 nm NM PE
the comma delimiter as far as I can tell from SI is never to denote a decimal fraction , only as a "thousands" indicator - Maybe somebody knows something to the contrary , but I would say anyone who used 40,474 instead of 40.674 would be a thousand times "out"

Wow! Thanks for the links and info guys. Eric, at 1st I liked the liner actuator idea, but then I realized there ball screws. This has to be "backdrivable", as you would say. My thought was to place the motor on the support header with the shaft sticking through, and placing a 3 or 4 inch timing pully on it. On the other end of the opening there would be an idler pully and the endless belt, between the motor & idler, would be clamped to the top door roller/hangers. I've looked a gearmotors but cant find a stock unit to meet the application specs.

Again thanks for the links an suggestions, lots to read up on....

Steve Bailey said:

Time out people!

30 pounds of force moving something at 1 foot per second doesn't necessarily equate to 30 foot pounds of torque from the servo or stepper motor.

Mike has not given enough information to properly size the motor. All he's given is how fast the object to be moved needs to travel. In order to size the motor, you need to know the diameter of the pulley that timing belt is wrapped around. You also need to know if there is going to be a gearbox or some method speed reduction between the motor and the timing belt pulley.

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Steve,

The size of the pully will depend on the motor speed & torqe output chosen to fit the space behind the header track, (3-1/2" max depth). I'll be moving about 150 lbs at 1ft per second with a max force of 30 lbs measured on the leading edge of the door. The door is going to move a distance of 21". Additionally if there is a power failure one has to be able to push the door open. Other then that I'm working with a blank peice of paper.....

Mike,

If you're willing, this could be a good lesson in the process of sizing a motor to an application. Can you post a picture or a drawing of the application? With that, we can step through the physics and calculate the required forces and inertias. You will also need to specify how fast you will need to accelerate the load up 1 ft/sec. That number will be as important in determining the final size of the motor as the load.

If you can post the details, I'll be willing to help explain the calculations. I can't speak for them, but I expect that Norm and Peter may also be willing to pitch in.

tutichip said:

the comma delimiter as far as I can tell from SI is never to denote a decimal fraction , only as a "thousands" indicator - Maybe somebody knows something to the contrary

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I used to think that until I received a price estimate from a Swedish company. I told them they were way too expensive, then they informed me that the Swedes use a comma as the decimal place.

Mike,

Sorry mate, I haven't got a clue about servos. But I will follow this thread to learn a bit more .

I will play.

The commas and period use are a european thing. Not SI.

I help with the motor sizing but actually I rarely size servo motors. I usually am sizing servo hydrualic systems.

30 foot lbs is alot of force for a servo.

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ft-pounds is torque or energy, not force. It is important to keep the units straight.

Mike, we use these for our demo equipment.
http://www.zero-max.com/products/rohlix/rohlixmain.asp

The bearings hold the drive shaft by friction. We used these because an accident with a lead screw could tear the equipment apart. These bearing will slide once and obstruction is hit. A demo unit should be safe!