The optimal controller delay for myoelectric prostheses

Abstract

A tradeoff exists when considering the delay created by multifunctional prosthesis controllers. Large controller delays maximize the amount of time available for EMG signal collection and analysis (and thus maximize classification accuracy); however, large delays also degrade prosthesis performance by decreasing the responsiveness of the prosthesis. To elucidate an "optimal controller delay" twenty able-bodied subjects performed the Box and Block Test using a device called PHABS (prosthetic hand for able bodied subjects). Tests were conducted with seven different levels of controller delay ranging from nearly 0-300 ms and with two different artificial hand speeds. Based on repeted measures ANOVA analysis and a linear mixed effects model, the optimal controller delay was found to range between approximately 100 ms for fast prehensors and 125 ms for slower prehensors. Furthermore, the linear mixed effects model shows that there is a linear degradation in performance with increasing delay.

Fig. 1.

(a) Photograph of PHABS detailing the components of the device. (b) Photograph of an individual wearing PHABS.

Fig. 2.

Linear speeds of all commercially available prostheses sold in the U.S. [23], [24], as well as the measured speeds for both the “fast” and “slow” prehensors used in these experiments. There are three prehensors that are marginally slower than our “slow” prehensor and the Motion Control ETD (when operated at 14 V) is the only device that has a higher speed than the “fast” prehensor. Based on this data it is believed that the spectrum of commercially available prehensor speeds was fairly well represented in these experiments.

Fig. 3.

Graphical illustration of myo-pulse control. Myo-pulse control provides proportional control of a motor by varying the pulse width and timing of a digital control signal. Whenever the EMG signal (bottom trace) is lies outside a predetermined threshold, the motor drive signal (top trace) is turned “on.” The ratio of motor “on” command time to motor ‘off’ time determine the velocity of the motor. Used with permission of the Northwestern University Prosthetics Research Laboratory (NUPRL).

Fig. 4.

Elements composing the time from the intention of movement to the completion of the movement by the prosthesis.

Fig. 5.

Photograph of a Box and Block apparatus. Subjects will pick up blocks from one side of the box, transport them over the barrier, and then release them on the other side of the box. The number of blocks transported over the barrier in 1 min is the score that the subjects receive.

Fig. 6.

Average Box and Blocks test scores (n = 20) when using the “fast” and “slow” prehensors. As the additional controller delay increases, the average scores on the Box and Block Test decreases. Additionally, as expected, subject performed better when using the “fast” prehensor.

Fig. 7.

Results of the repeated measures ANOVA analysis on the Box and Block data. Each cell represents the probability that there is no difference between the two conditions. Those combinations that produce a statistically significant result (p < 0.05) are highlighted in gray.