high speed tracking errors
Posted: Tue May 07, 2013 12:35 pm
Hello everyone,
I am simulating a single leg jump landing using CMC. I am interested in extracting the quadriceps, hamstring and gastrocnemius muscle forces.
I have ran scaling, IK, ID, RRA but am struggling with CMC, specifically the Perr and reserve actuator for the ankle. This makes sense because GRF is large (~4x body weight) and the high speed at which the angle angle changes when it hits the ground (~60 degrees in <100ms).
I have locket MTP, and Subtalar joints and both kinematics and GRF are filtered at 15hz.
The general guidelines are: joint angles are "OK" if they are tracked within 5 degrees and "good" if within 2 degrees. On all my subjects the Perr term for the right ankle (the landing ankle)
shoots above the "ok" range sometimes as much as 9-10 degrees. A closer inspection of this error is that the large Perr is caused by the large rate of change in the ankle angle. A very slight shift in the tracking position leads to a large vertical difference between the CMC position and the Inverse Kinematics position. The steady state error is very close to zero. I have included figures to demonstrate my point. The first is the CMC Perr output and the second is the comparison between the IK Kinematics and the CMC Kinematics.
My question is if it is acceptable for the Perr to go above 5degrees in this situation? if so, how much is acceptable?
I am also seeing large reserve actuators in the ankle, usually 15-25% of peek joint moment. The interesting thing is that they are always in the opposite direction of the peek torque, and are activated at the initial impact. Below is a figure that includes the ankle reserve, torque and GRF I believe this may be due to the muscle dynamics, needing to provide such a drastic change in torque in a short amount of time. Does anybody have any clue to deal with this? I have tried the techniques presented in the documentation with minimal effect. Maybe I have not found the correct combination of parameters.
Both of these issues seem tied into one another, because when I am able to get one to budge, it changes the other. For instance the Perr could go to 15 degrees and reduce ankle reserve
I have also played with
1.Ankle tracking - interestingly, because there is not a lot of steady state error, increasing the weighting on the ankle has very minimal effect. I have had more luck decreasing the tracking so the ankle position is dictated more by the ankle reserve.
2. Ankle KP, KV - these parameters have large effects on the Perr term. In order to even get close to the recommended Perr values KP and KV need to be very large.
3. Decreasing the optimal reserve actuator torques- this helps minimally until I reach a threshold where the model launches off the ground.
4. Passive muscles forces, normalized fiber length and tracking weights.
Does anybody else have any experience with large forces and quick movements? It seems like this may be common issues.
Thank you for your time
I am simulating a single leg jump landing using CMC. I am interested in extracting the quadriceps, hamstring and gastrocnemius muscle forces.
I have ran scaling, IK, ID, RRA but am struggling with CMC, specifically the Perr and reserve actuator for the ankle. This makes sense because GRF is large (~4x body weight) and the high speed at which the angle angle changes when it hits the ground (~60 degrees in <100ms).
I have locket MTP, and Subtalar joints and both kinematics and GRF are filtered at 15hz.
The general guidelines are: joint angles are "OK" if they are tracked within 5 degrees and "good" if within 2 degrees. On all my subjects the Perr term for the right ankle (the landing ankle)
shoots above the "ok" range sometimes as much as 9-10 degrees. A closer inspection of this error is that the large Perr is caused by the large rate of change in the ankle angle. A very slight shift in the tracking position leads to a large vertical difference between the CMC position and the Inverse Kinematics position. The steady state error is very close to zero. I have included figures to demonstrate my point. The first is the CMC Perr output and the second is the comparison between the IK Kinematics and the CMC Kinematics.
My question is if it is acceptable for the Perr to go above 5degrees in this situation? if so, how much is acceptable?
I am also seeing large reserve actuators in the ankle, usually 15-25% of peek joint moment. The interesting thing is that they are always in the opposite direction of the peek torque, and are activated at the initial impact. Below is a figure that includes the ankle reserve, torque and GRF I believe this may be due to the muscle dynamics, needing to provide such a drastic change in torque in a short amount of time. Does anybody have any clue to deal with this? I have tried the techniques presented in the documentation with minimal effect. Maybe I have not found the correct combination of parameters.
Both of these issues seem tied into one another, because when I am able to get one to budge, it changes the other. For instance the Perr could go to 15 degrees and reduce ankle reserve
I have also played with
1.Ankle tracking - interestingly, because there is not a lot of steady state error, increasing the weighting on the ankle has very minimal effect. I have had more luck decreasing the tracking so the ankle position is dictated more by the ankle reserve.
2. Ankle KP, KV - these parameters have large effects on the Perr term. In order to even get close to the recommended Perr values KP and KV need to be very large.
3. Decreasing the optimal reserve actuator torques- this helps minimally until I reach a threshold where the model launches off the ground.
4. Passive muscles forces, normalized fiber length and tracking weights.
Does anybody else have any experience with large forces and quick movements? It seems like this may be common issues.
Thank you for your time