Dear friends,
We have purchased 24 RS-1270 servo motors for our humanoid project. We are facing certain difficulties in using them.
- The Motor is unable to hold its position: We set the servo to point at a particular angle. It does the job nicely. But, as soon as we put some load (torque) on the shaft, it moves to a new position because of the load and then holds the new position (approx 5-6 degrees away from the earlier position). This is creating a lot of problem in balancing the robot as the motor angle specified by the controller and the actual motor angle is quite different.
- Overheating: The motor is getting overheated at a load much lower than the rated (approx 60% of the rated). We have already burnt two motors due to this issue.
I have seen other humanoids using the same servos. I’m wondering, whether they did not face similar problems.
- Super SJ_01 (youtube.com/watch?v=B_wb_uGPEwc)
- TigerBot (youtube.com/watch?v=XyL2gxhsO8M)
Any help would be greatly appreciated.
Thank you.
Hi,
From what you are describing, it really looks like those servos are not powerful enough for your robot. At what voltage are you using the servos? When you say “…at a load much lower than the rated approx 60% of the rated” What is the load, and how have you calculated / measured it? Do you have any pictures of the robot?
The Super SJ_01 size seems to be decent for the motors. On the other hand, the TigerBot is a lot bigger and we think the roboard would not be powerful enough. They seem to be holding the robot in the video.
We found this in the design review of the TigerBot: “2 prototypes robot designs. One with all roboboard servos and the second with HQ servos in High torque areas (knees, hips, ankles)” So they probably encountered similar issues.
If you can, send us some picture of the robot, it should help us get a general idea.
Best regards,
Dear friend,
I do not have any photograph of the test setup. Therefore, I have attached a rough schematic of it.
Arm: Thin Aluminium
Load: water bottle
Moreover, the heating issue is there at much lower loads too. (Empty water bottle)
Hi,
The duty cycle is also an issue. You can probably be in the 60% zone ( 68% seems dangerous) but that does not mean the motor can hold it forever without burning. Look at the following images, it gives a good idea of the 3 zones. In order to maintain a weight forever, you would need to be in the safe zone which is around 1/4 of the Stall torque. If you need to go in the middle zone, you need to find a way to limit heat.
http://www.phidgets.com/wiki/images/thumb/d/df/Torque_safe_zones.png/500px-Torque_safe_zones.png http://www.phidgets.com/wiki/images/thumb/9/94/Torque_vs_rpm.png/500px-Torque_vs_rpm.png
As for the “not keeping its position”, make sure your power supply is capable of supplying enough current. Servos often create current spikes that may not be visible to voltmeters so your system may be able to supply the " mean current" but not be able to provide enough for the spikes that occur under heavy loads.
We hope this helps.
Thanks a lot for the valuable insight in the servo behaviour at different loading conditions. We will try to find a work around for the heating issue. Can you suggest something? We have no space to put heat sinks and our motors need to operate in considerably high torque region.
Holding the position:
We are providing the rated voltage (7.4V) using lipo batteries. (hobbyking.com/mobile/viewpro … oduct=9967)
Since lipo batteries are designed to provide high current, current spike should not be an issue. I checked the lipo manual and found the internal resistance to be ~30mohm. To check, I put an oscilloscope on the servo terminals and found out the voltage spikes to be ~180mV peak-peak. Is that an issue? Can something be done about it?
Hi,
We don’t know of any solution other than limiting the duty cycle, using a gearbox to improve torque or changing the servo for a stronger one.
These are 25C Lipo batteries, which is pretty good. The 180mV oscillations should not be an issue.
Best regards,
Dear friend,
If the current spikes or the 180mV ripple caused by that is not an issue then why would the motor not hold its position? Isn’t an errorof 5-6 degrees large? Assuming a deadband of 5us, we will get max 1.8 degrees of error.
Thanks
Hi,
Maybe that would explain the 5-6 degrees offset under load. At that point it is really hard to say. Maybe using some large capacitors could help but we cannot guarantee it. Have you tried contacting roboard ([email protected]) to get their feedback on this?
Sincerely,
Dear friend,
I did try putting some large capacitors but it didn’t change anything. I used electrolytic capacitor (~2000 uF). They have pretty high ESR. I guess the lipo battery has much lower internal resistance (~30 mohm) and we do not have capacitors with such low ESR. I will contact roboard for dealing with this issue.
Thank you for all the support.
