I have a concern about the Micro Gripper Kit with position feedback (https://www.pololu.com/product/3551).
I noticed that when the gripper is “closed” (when it takes an object for example), the servo becomes very hot in 2 or 3 minutes, because of the stall current.
Isn’t it dangerous ? Can it detroy the electronic or is it safe ?
The problem is that you cannot use the position feedback in order to add any limit: for example, if the object you grip is a 15 mm cube, as soon as you specify 15 mm for the “closing” command, the object is gripped, leading to a current and temperature increase. The position feedback gives 15 mm, so you cannot determine in the program whether the gripper is “closed” or not.
Of course, if you specify 2 mm as the closing command, the position feedback gives also 15 mm. So in this case you can determine that the gripper is closed.
So in fact the problem is when the gripper closed just a little more than the thickness of the object to grip. No mean to know that it is gripped and becomes very hot.
Thanks in advance for your answer,
It is normal for a servo to heat up a little when it is under load, such as when your gripper is closed around something, but it is safer if you can avoid sending target positions to your servo that it cannot reach by high margins. If you are gripping a 15mm wide object, you are much more likely to damage your gripper servo if you command it to a position where the gripper closes to a 2mm gap then if you command it to a position where the gripper closes to a 14.9mm gap.
If you already have methods for converting your target and feedback signals to specific gripper gap distances, then you can make a control routine that will allow you to determine if your gripper closes on something or not without constantly commanding it to a position where it is likely to damage itself. You just have to adjust your target after you have determined if you are closed on something. For example, you might start a gripping sequence by commanding your servo to close the gripper to make a 2mm gap. If after a little while your feedback signal indicates that your gripper closed to only a 15mm gap, then you can be confident that you are gripping something 15mm wide and adjust your target to something like 14.9mm, where the difference is great enough that your servo exerts enough force to hold the object but is not in serious danger of drawing too much power and damaging itself.
You could also look into adding some other sensors to your gripper to tell when it is holding something, like our force sensing resistors. Or, you could put a current sensor in-line with your servo’s power to monitor its power draw.
Thank you for your answer. I already put a current sensor and I noticed that, surprisingly, commanding a 2 mm gap or a 14 mm gap gives the same result: a 0.4 A current. But it is true that when the command is just below 15 mm, there is less current, but not much less: it is at about 0.3 A.
I think I will try to glue a temperature sensor at the back of the servo in order to deactivate the closing command (= open it) when it is too hot.