Actuator Improvements & Tuning, System Integration

posted Apr 3, 2015, 8:45 PM by Gene Merewether

Individual Progress

Actuator Robustness and Electronics

Since the last progress review, I worked to make the actuators more robust (Figure 1). I filed down the heat sink attachments so that they fit in between the threaded rods that attach the actuator to the joint at the top. I also ordered spacers from McMaster Carr to take the place of the old method of securing the threaded rods, namely pairs of nuts. Using the spacers reduces the fine-tuning step where the inner nut must be turned to be at the correct distance along the threaded rod, making assembly and disassembly more rapid. I have a greater appreciation for easy assembly and disassembly after having to take the actuators apart and rebuild them multiple times.

Figure 1: Knee and ankle actuators, showing limit switch relays, motor choke (ankle), and linear potentiometers

I also mounted the encoders on the head of the Maxon motors, using a laser-cut piece of acrylic. I used dowel pins to align the encoders. Next time, I would either order the motors from Maxon with the encoders already installed, or pick the adhesive-backed US Digital versions of these encoders because they provide an alignment tool that allows for easy assembly. Although they are more expensive ($120 instead of $40), the time savings would be significant. Our current encoders incorporate a line driver and differential outputs for high signal robustness to noise.

On that note, I used Loctite Threadlocker 242 to semi-permanently fasten the nuts that hold the ballscrews on. I kept having to retighten them due to the lubricant placed on the adjacent thrust bearings. After completing this laborious process several times (involving disassembly), I chose this glue because it can be loosened with hand-tools if necessary. I placed threadlocker on the threads and tightened the bearing hand-tight. The actuators still move smoothly and the ballscrew nut has not loosened at all since.

The large motor choke in the picture is necessary due to the low inductance of the Maxon RE40 motor on the ankle. The PWM signal output by the Elmo drive is not smoothed sufficiently, and confuses the driver, causing it to lose control at RPMs over about 2500. After consulting our contact in Professer Geyer's lab, I identified an appropriate inductor and wired it in series with the motor.

The limit switch modules consist of two relays per actuator connected to diodes. When the positive rotation limit switch is triggered, the relay switches the current path to include a diode, wired such that no current can flow in the positive direction. This means that the motor can only move away from the limit switch, causing it to release. The limit switch system is fail-safe, such that if wiring is cut or broken, the relays will return to their unpowered state, preventing current flow through the motor.

Entire Elmo Cabling Setup

I also finished the integration of the Elmo drives into a complete system that allows testing and tuning even while not mounted on the leg (Figure 2). I tried tuning the actuators while mounted on the leg, and the results were scary due to small amplitude, but high frequency vibrations. Additionally, tuning the motor parameters while the motor is not coupled to the ballscrew gives very different parameters than when the actuator is assembled.

Figure 2: Driver integration, showing knee and ankle actuators, USB-to-RS232 converters and cables, emergency stop, battery, and Elmo drives

Thankfully, the Elmo software has an autotuning function that works fairly well. At least for now, we will continue to use the autotune function.

Challenges

Elmo Whistle Digital Inputs

The digital inputs on both Elmo boards appear to be broken. We have been unable to determine the reason for this. Therefore, I created a hardware limit switch solution. We had planned to use the digital inputs to read the swing-stance transition using a threshold circuit on a force-sensitive resistor. Instead, we will use a DAQ device to read the potentiometers giving swing-stance transitions. Most likely, the optocouplers that operate the digital inputs are burnt out. Our contact in Professor Geyer's group has never used the digital inputs.

Analog Power Source

Ordinary low-dropout regulators do not provide a clean enough analog reference voltage to get high fidelity force measurement with the linear potentiometer. Instead, I am constructing a 5V positive and negative voltage reference circuit that will enable 14-bit resolution measurements with the Elmo, and 12-bit resolution measurements with the DAQ, giving us about 6 Newton resolution out of a force range of positive 3000N to negative 3000N.
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