Help suggest low cogging outrunner for Univ robotics project

We have a haptic robotics project at school. It involves designing and making a force reflective robot wrist. The application is in tele-robotics, where a human moves one motorized arm, and those motions are replicated by an identical robot arm at a remote site. Here is an example picture and video of the robot sans wrist:

http://webdocs.cs.ualberta.ca/~vis/robotics/telemanip/openHouseDemo.wmv

It is important that the motors have low cogging torque. The motors are driven through a 1:20 ratio cable transmission, so any cogging of the motors feels greatly amplified to the human.

A simpler conceptual illustration of what we want to achieve is shown in this 1DOF teststand drawing: A human turns one end, and the other end replicates that motion. Any force that the other end feels (inertial or external) is replicated back to the human (this is all taken care of through control algorithms.)


We have difficulty finding good motors for this. For the wrist we bought and tested hobbykings two gimbal motors:
http://www.hobbyking.com/hobbyking/...ushless_Gimbal_Motor_Nex5_size_400_800g_.html
http://www.hobbyking.com/hobbyking/store/__45369__Turnigy_HD_5208_Brushless_Gimbal_Motor_BLDC_.html

However despite these are supposed to do precise control of a camera gimbal we found to our dismay that they have just as bad cogging as the average outrunner.

Now some outrunners will have low or no cogging torque, but this is not speced from hobbyking or any of the other web sites. (Not even for professional servo motors like this Maxon inrunner)
http://www.maxonmotor.com/maxon/view/product/motor/ecmotor/ecflat/ecflat45/339287?download=show

So we can't just keep buying motors at random and find they are not good.

Therefore, if you have some outrunners at home and you can test spin them slowly with your finger. Do you feel a cogging torque? That is, an uneven force/resistance where the motor will pop into certain positions, then require force to "cog" to the next?

Please let us know of any outrunners you find with low cogging.

We are primarily interested in 100-500g outrunners for now. Ideally low kV, but we can adjust controller (or worst case rewind).
In a later project we might want larger ones, so please report such also.

B.t.w. there are many gimbal motors here. Anyone tried them?
http://www.rctimer.com/gallery-248.html
Gimbal motors are attractive beacuse of low Kv, but any low cogging outrunner or high torque inrunner would do. We are looking for about 0.25-0.5nm peak torque for <10s (with no magnetic saturation) and maybe 1/10th that continuous.

Thanks,
Martin and haptics project group members.

Hi Martin,

One significant factor is the lowest common multiple of the slot/pole combination. This is what determines the number of detents per revolution. More steps and therefore less peak force for each step being better, of course. Looking at the list of motors on your link, they have a motor with 24t 22p, which has an LCM of 264. Much better than 12t 14p which has an LCM of 84.

Miles said:

Hi Martin,

One significant factor is the lowest common multiple of the slot/pole combination. This is what determines the number of detents per revolution. More steps and therefore less peak force for each step being better, of course. Looking at the list of motors on your link, they have a motor with 24t 22p, which has an LCM of 264. Much better than 12t 14p which has an LCM of 84.

Click to expand...

We bought a high pole count motor with LCM in mind, but in practice we found that the Quantum 4008 (24t 22p) cogs about as much as the Turnigy 5208 (12t 14p), despite the latter being much bigger. So stator and magnetic pole count is only one factor. Geometry of the magnetic paths and field strengths factor also.

So far we've found it difficult to guess from (the typically meager) motor specs if a motor will have lots or little mechanical cogging resistance.

That's where we were hoping that the readers of this post could try to spin their outrunners by hand and let us know which ones cog a lot and which ones have only little cogging.

Thanks,
Martin

Slotless?

Something like the Kontronic Tango?
http://www.kontronik.com/index.php?option=com_content&view=article&id=12&Itemid=12&lang=en

This gimbal motor is 51t 46p (LCM of 2346)

http://etecpower.com/html_products/GB106-65.html

Miles said:

Slotless?

Click to expand...

http://ams.aeroflex.com/motion/datasheets/motion-motors-zerocogging.pdf

Are there any small air-core motors available off-the-shelf?

Try ThinGap.

jag said:

Miles said:

Hi Martin,

One significant factor is the lowest common multiple of the slot/pole combination. This is what determines the number of detents per revolution. More steps and therefore less peak force for each step being better, of course. Looking at the list of motors on your link, they have a motor with 24t 22p, which has an LCM of 264. Much better than 12t 14p which has an LCM of 84.

Click to expand...

We bought a high pole count motor with LCM in mind, but in practice we found that the Quantum 4008 (24t 22p) cogs about as much as the Turnigy 5208 (12t 14p), despite the latter being much bigger. So stator and magnetic pole count is only one factor. Geometry of the magnetic paths and field strengths factor also.

So far we've found it difficult to guess from (the typically meager) motor specs if a motor will have lots or little mechanical cogging resistance.

That's where we were hoping that the readers of this post could try to spin their outrunners by hand and let us know which ones cog a lot and which ones have only little cogging.

Thanks,
Martin

Click to expand...

Try scorpion outrunners, lowest cog force of any motors I have measured. I have a test stand just for measuring cog force. They engineer the slot/pole width ratios for lowest torque ripple.

Beyond that you will need to go slotless, but the peak and continuous torque will suffer a lot.