Model a knee motor as a CoordinateActuator

[Míriam Febrer-Nafría]

Hi all,

I’m trying to model the knee motor of an active KAFO, which is intended for SCI patients. In the model, it is a CoordinateActuator that acts on the knee_angle coordinate.

The workflow I want to use is to define an excitation pattern for the motor based on the controls for the knee torque that I get from a RRA using the experimental data. Then, run an Actuation Analysis, and compare the motor torque with the knee torque obtained from ID. Since an SCI patient cannot produce torque at knee level, I suppose that all the torque needed for the motion will be produced by the motor. But also a small passive torque due to stiffness can be at the patient’s knee, so perhaps the motor will need to produce more torque to counteract the effect of this stiffness that the patients present.

Some questions that I have:
1. It is OK to assume that the difference between knee motor actuation and the knee torque obtained from ID will be the knee torque produced by the subject?
2. It is OK to try to define the motor excitation pattern using the experimental motion and the knee torque obtained from this?

I am also thinking, as future work, to do some simple tests with patients, measuring the resistance torque that they present when another person tries to flex their knees, to model the stiffness that they present, and add it as a spring element (Bushing Force) in the subject’s knee joint. What do you think about this? Has someone worked in a similar project?

Thanks in advance,
Míriam Febrer

[Thomas Uchida]

I'm not sure your proposed workflow will give you the results you seek. It sounds like you want to track experimental motion data while applying a motor torque to the knee with no activity in the surrounding muscles. If so, you will simply be using the reserve actuator to achieve whatever net knee joint torque is required to track the motion (required net joint torque = motor + passive muscles + reserve). Perhaps you could track the motion data with no motor and use the reserve actuator torque to inform what the KAFO should be applying?

I am also thinking, as future work, to do some simple tests with patients, measuring the resistance torque that they present when another person tries to flex their knees, to model the stiffness that they present, and add it as a spring element (Bushing Force) in the subject’s knee joint.

That sounds like a good idea. Existing models already include passive muscle forces, but you could perform simple experiments as you describe to calibrate the model. You could add bushings, or you could simply adjust the passive force–length curve in the muscle model.

[Míriam Febrer-Nafría]

Thanks, Tom,

I will try to follow your suggestions.

Míriam