Motor Requirements and Selection

In order to choose the right motor for this device, estimates of the torque and speed requirements of gait in patients with osteoarthritis are needed. These estimates were found through a review of the literature on human gait.

Based on a study which conducted a systematic review of the literature, the device should provide about 5 Nm of torque to adequately assist the wearer. About 45 Nm of torque is required for walking on a flat surface. This data was obtained for a 90 kg individual with no muscle power whatsoever [75]. Individuals with osteoarthritis have a muscle ability deficit of about 10-15% on the lower end, and up to 50% on the high end [76]. Considering the aim was to work on preventative care with early stage patients, the group took the low end of patients, who have 85-90% muscle capability, and provide assistance for the

additional 10-15%. This yields the final maximum torque requirement of roughly 5 to 7 Nm. Since the capstan mechanism discussed in Section 5.1.3 has a gear reduction of about 12:1, the torque output of the motor only needs to be about 0.5 Nm. To run the motor around peak efficiency, a motor with stall torque around 1 Nm is ideal.

The speed requirement was reached in a similar fashion. The knee reaches a maximum angular velocity during fast walking (that is, walking in a straight line at 1.5 +/- 0.1 m/s) of around 400 deg/s [77]. This is equivalent to about 66 RPM, which would be the maximum rotational velocity required of the motor to comfortably assist with the gait.

Some additional constraints emerge through analysis of the usage of the motor as part of an exoskeleton device. Motors typically have high speed and low torque output relative to the design requirements, so gearing is likely to be necessary. However, back-drivability is also desirable because it means less resistance will be provided to the wearer while trying to move or while wearing the brace if power is lost. Gearing typically reduces the ability to easily back-drive a motor, so having minimal gearing or no gearing at all is important to the design. This is a delicate trade-off because too much gearing could make the device unusable, but not enough gearing would mean a large motor is required, or worse, the device is not effective at assisting the gait.

58 Figure 51: Motor and Capstan Drive Output Parameters

The original motor selected was a Globe brand brushed DC motor. To control the motor, an integrated circuit with an H-bridge was used. The Pololu MC33926 H-bridge was chosen because it was readily available and had a carrier board that easily interfaced with the rest of the circuitry. The H-bridge allowed for easy adjustment of motor speed using pulse-width modulation (PWM) from the microcontroller. The H-bridge carrier board also had other useful features, such as motor current sensing and protection against back-EMF generated by the motor. The carrier board is capable of feeding up to 5A of current at the desired 12V, so it was compatible with the motor needs. However, it was ultimately decided that with a maximum output torque of 0.2 Nm, this motor would not meet the needs of the design and something more powerful was needed.

Maxon motors are known for providing high performance (measured in torque, speed, and precision) while still being compact. The final motor selected to meet the needs outlined above was a Maxon EC- Max motor with a planetary gearbox, capable of providing about 1.2 Nm of torque at 12V. This motor is a brushless DC motor, making it more easily backdrivable, despite the large gear reduction in the gearbox. It also came with a built-in encoder for accurate position sensing, as well as Hall-effect sensors are used to allow a motor control circuit to accurately control speed. This circuit was acquired as a separate breakout board. The breakout board allowed for motor speed control using analog input voltage and direction control with a digital pin. A final diagram showing the motor requirements, selected motor, and output after gear reduction is shown in Figure 51.

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5.3.3 Sensor Selections

Sensing the rotation of the knee is crucial to device control, so a rotary position sensor was needed. Rotary potentiometers are a common choice for similar applications since they provide an absolute analog position reading. Potentiometers generally do not provide a full 360 degrees of rotation, but this is not a problem because the knee never rotates more than 180 degrees, at an absolute maximum. The

potentiometer chosen for this application is a simple 10 kilo-ohm potentiometer, which was readily available and easily mounted to the device.

Several other sensor types were considered, but not chosen to be included in the first prototypes. One such sensor is the Inertial Measurement Unit, or IMU. IMUs provide data about acceleration and rotational velocity. Position information can be obtained by using this data to compute the numerical derivatives. An IMU would provide extra information about the motion of the device on the wearer’s leg. However, IMUs are known for being hard to implement. They are subject to drift in the measurements, which can lead to large accumulated errors. Furthermore, correcting these errors requires a large amount of processing power to filter the data, which is not necessarily available on a small microcontroller. Ultimately, the position information gained from a potentiometer at the knee and an encoder at the motor is ample for the initial prototypes.

The Electromyography (EMG) sensor is another candidate sensor that was not selected for the initial design. EMGs are used for detecting the electrical signals in a patient’s muscles that are produced when the patient moves. This capability would be helpful for detecting the intent of the device’s wearer, such as when the wearer wishes to start walking from a standstill. An array of EMGs placed across several muscles could also detect more complex intended movements, such as standing up from a sitting position or climbing stairs. Unfortunately, EMG sensors are difficult to use because they are sensitive to oils in the skin and placement. It was also determined that complex movements were outside the scope of this project, and that the first iteration of the device should only focus on walking. Since the beginning and end of a sequence of steps can be detected in other ways using the potentiometer and motor configuration, the EMG was ruled out.