# Joint Analysis¶

## Different Stiffness Values¶

Following are graphs for a simple stiffness test. The request for a joint was changed in one frame by a lot, which was repeated for different stiffness values. One graph each shows the joint positions and the other the change in position.

Positions Changes per Frame

The position change for the hip, knee and ankle pitches for the time windows from frame 90 to 110 are shown below:

## Joint Play¶

As the robots get older and have been used more, the joints will gain more and more play. As far as we know, the position sensors can detect most of this play. But this does not help to let the joints execute the request perfectly. Following two examples are shown of joint play while walking for joints of the supporting foot. The dotted lines show the most expected position for the joint. We assume, that a joint can always hold its position, and if it has enough strength to move, it will follow the request. This means, that a measured position can differ by a huge amount relativ to the requested position, yet has actually no joint play. But if it moves too far or in the opposite direction, then it is moving in its joint play.

Here the knee is in the beginning part of the swing foot until about frame 12. Afterwards the foot becomes the support foot and the joint gets stuck. Interestingly at about frame 33 the request changes in direction of the measured position, which causes the joint to move by about the same amount, but shifted. The robot itself is in an state of being tilted a lot backwards. Here the knee is unable to move positive, because this would tilt the robot forward and therefore would need to work against the weight of the robot. But moving negativ tilts the robot even more backwards which the joint can easily do. The shift itself is assumed to be a result of the controllers of the motors. This effect can be reproduced on a new robot too, but less extreme because of the missing joint play.

Here the ankle pitch becomes part of the support foot at the beginning. The robot is in a far forward tilted state. The request goes more negativ and the joint executes the movement change, but once again shifted by the already existing difference between the request and measured position. This effect can also be reproduced on a new robot, but also less extreme.

Note that this robot used for the grafics has about 5 degrees of play in the shown knee and about 4 degrees of play in the shown ankle. The differences of the positions and requests are mostly caused by this play, but also to some part because of the overall state of the robot, e.g. being tilted far back or forward. A robot with such high play is just a lot more likely to get into such state.

Last update: July 18, 2023