Stepper mo

[HandleUserName]

Hi,

I am trying to control a mass in the pitch and yaw axes with +/- 0.1 degree accuracy. I am new to all of this and have a couple of questions. So far my plan is to use absolute rotary encoders of 12+ bits resolution coupled with the mass to know it's position. This is for solar tracking applications where the sun moves ~1 degree every four minutes, so I will need to be moving the mass at minimum once every 24s. The sun's position is found using a solar positioning algorithm and should be accurate to .0003 degrees (may change but still most likely negligible). The motors will use slew drives to position the mass.

So my questions are:

1) Would a stepper motor like this be sufficient?

http://www.robotshop.com/ca/en/geared-bipolar-stepper-motor-3v-555-oz-in.html

Assuming the torque ratings are within my parameters (I require about 20Nm), the integrated gearbox for this motor has a backlash of <=1.5 degrees. Am I right in thinking this only comes into play when I change the direction of rotation? Or would the movement about my 2nd axes (one motor per axis) also cause inaccuracy from the backlash?

2) From what I have read BLDC motors are more desirable but harder to implement (and more expensive) for precision/accurate solar tracking. Would you need to have an encoder coupled with the BLDC as well as the mass (after gearing) to achieve my desired accuracy? And also would some sort of braking system be necessary? I am confused as to how exactly one controls position with a BLDC motor.

I would rather opt for the simpler solution provided it is sufficient for my application. If anyone has any suggestions or reading material I have left out that would be great also. Thanks!

 

[logbook]

converter
48 kg.cm = 4.7 N.m

Why are you tracking the sun? If it is to optimise power output from a solar panel, for example, this is massive overkill.

The backlash is wobble. If you are not driving the gearbox the wind load on the system can make it wobble. 1.5° seems a ridiculously large amount. Are you sure that the spec isn't in minutes of arc?

 

[HandleUserName]

It's for a parabolic dish reflector. It's part of a school project but the client involved wants +/- 0.1 degree tracking.

I just grabbed that backlash value from the data sheet of the motor:
Gearbox Properties
 Gearbox Type: Planetary
 Gear Ratio: 99.5:1
 Backlash Error: 1 1/2°
 Maximum Strength of Gears: 48 kg·cm
 Shaft Maximum Axial Load: 49.1 N
 Shaft Maximum Radial Load: 98.1 N

I have $400 for the motors/gearbox and drivers so the motor I selected there isn't at the top end of the budget, I just have 0 experience with selecting motors and controlling them with this level of precision and accuracy.

 

[HandleUserName]

Geez I just saw the first line of your post, you are right I misread it as 48kg.m. doh. I wondered how that tiny motor could pack that much torque at 1.6 Amps.

I had this motor in mind before I saw what I mistook as a better deal.

http://www.omc-stepperonline.com/ge...ue-nema-23-stepper-23hs302804spg47-p-282.html

It has the same 1.5 degree backlash though.

 

[MikeHalloran]

Here's a trick:
Use a gearbox with quite a bit less reduction. It will have about the same backlash. Use a timing belt as the last stage, with a huge driven pulley.

{ Another trick; that driven pulley only needs two teeth. The remainder can be smooth; you just have to get the diameter right so the belt rides on the correct radius. }

You can adjust the centers or add a spring loaded idler to keep the timing belt drive to pretty close to zero lash.



Mike Halloran
Pembroke Pines, FL, USA

 

[logbook]

The gearboxes you are looking at are rubbish for your application.

18 arc minute low-backlash gearboxes

The last thing you want to do is step once every 24 seconds. The motor needs to run continuously or it will wobble the load on every step. I would use a high ratio gearbox. The RGP60 at 320 reduction still gives 40Nm torque. (I haven't checked prices, so I hope it doesn't break your budget.) It is three stage so has 18 arc minutes backlash. Use a 400 step/revolution stepper motor and use a microstepping controller (at least 16x) to smooth out the steps. That way you can drive it continuously and thus minimise backlash and wobble.

Anti-backlash gears are readily available but for a one off you probably want an off-the-shelf solution.

 

[logbook]

I have another thought for the low cost end of the market.

Use the highest reduction ratio gear box you can find, regardless of backlash, and put a mechanical brake on the driven shaft. The backlash in the gearbox will be irrelevant as the brake will hold the load until forced to move. This will damp out any wobbles. The output shaft is essentially rotating at zero speed so there is no power involved with the brake. Some spring loaded rigid plastic pushed onto the output shaft should do the job.

 

[btrueblood]

Another problem: your 12-bit encoders give a resolution of 0.087 degrees or thereabouts. You will end up "dithering" +/- 1 encoder step at minimum, unless you filter and average or do other fancy software tricks, but figure at least 2 bits of error worst case. Either improve the angular resolution or live with the larger error than your estimated 0.1 degree.