Apr 21

Ankle torque testing

We finally have testing results in regard to our ankle system! While we were 100% sure that the system was capable of moving the mass of our user’s foot, we had not had any real lab data on the capabilities of our ankle system until now.

The physics department allowed us to use materials for testing. Here is the leg brace firmly held in place allowing us to test the torque output of the ankle system

The physics department allowed us to use materials for testing. Here is the leg brace firmly held in place allowing us to test the torque output of the ankle system

 

We evenly distributed weight through the shoe seen above and tested to make sure that the system still maintained the full range of motion required. We first added 0.5kg to the shoe and increased the weight in 0.5kg increments all the way to 5kg (5 times the mass of our specification) while still maintaining the full range of motion. Although our testing format was rather simple, it provided us with data proving the validity of the ankle system design.

Below is a video of the ankle testing in progress. The angle of rotation in the video is far beyond that which is required by specification and even beyond normal ankle motion, but it allowed for very thorough testing of the system.


While we were in the physics lab we decided to weigh our system to make sure that it was under the 4.5kg for the leg and 6.8kg for the box that was required by our specification in order to make it easy for our user move as needed. Below are the masses of the box and leg brace respectively.

Here we weighed the box containing all hardware components of our project aside from the leg brace and it came out to be only 2.7716 kg.

Here we weighed the box containing all hardware components of our project aside from the leg brace and it came out to be only 2.7716 kg.

Here we weighed the completed leg brace and to our surprise it came out to be only 2.7146kg.

Here we weighed the completed leg brace and to our surprise it came out to be only 2.7146kg.

 

Cumulatively our system only weighs 5.4862 kg which is fantastic! It is far below the user’s max which means it will be very easy to move around.

Mar 22

Armband level up

We have good news and bad news.

Part one of the bad news is that our knee motor broke. Our knee system uses a worm gear and regular gear; as we mentioned in the last entry, the regular gear is made of plastic and got stripped. In the meantime, we’ve ordered replacement plastic gears so that we can continue doing testing and integration of the knee system. We still haven’t found a metal gear that can perfectly replace our plastic. We are considering the following options:
– replacing the knee gearbox with a different gearing system/motor
– seeing if we can find a company that can custom make a metal gear for us
– seeing if we can make a gear ourselves on campus (via a metal shop)

Part two of bad news is that our ankle servo isn’t working. We are still investigating why it doesn’t work. This is especially troublesome because we have a presentation/demo later this week…

The good news is that we’ve made progress on the armband.

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Mar 11

Leg brace shown to the user

We’ve started integration for our project.

The fiberglass mold has been completed and we were able to incorporate that into the overall brace.

This is a photo of the user wearing the leg brace. The fiberglass works as a brace for their leg while the aluminum parts go down the leg and act as housing for the motors. They also acts as arms to drive movement of the leg as the motors turn.

This is a photo of the user wearing the leg brace. The fiberglass works as a brace for their leg while the aluminum parts go down the leg and act as housing for the motors. They also acts as arms to drive movement of the leg as the motors turn.

We made two fiberglass molds: one of the upper leg and one for the lower leg. As one can see from the photo, the molds are a little rough around the edges, but we plan to sand the edges and a little bit of the surface so that the molds are more aesthetically pleasing. The aluminum pieces running down the side of the brace are not taped onto the fiberglass (even though that’s what it looks like). The tape was used when cutting the fiberglass to approximate where straps would run.

We had one of our team members wear the brace for testing. We used the toggle handle to move the motor.

We also had the user wear and test the brace. Straps were attached so that the user could secure the fiberglass on their leg. Unfortunately, the leg brace did not work in this video (though it had been working prior to filming and they had been able to interact with it). This was due to our previous testing: the teeth of the gear for the knee motor stripped. (The gear was made of plastic, so we are looking for a metal gear as a replacement.)

The user had the following feedback for this iteration of the leg brace:
– They were satisfied with the toggle handle
– They liked the brace, especially how secure it was around their leg. However, the fiberglass pinched their leg, so they requested we trim the fiberglass for a better/more comfortable fit.
– They didn’t like the smell of the fiberglass.
– They preferred switches instead of buttons for the armband/user input.

We are now working on finding a replacement gear and to add the user’s requests to the leg brace.

Mar 01

Design details

We’ve completed a first iteration of the leg brace and delivered it to the user. Video and pictures should be coming soon (since the user will be interacting with the leg brace this weekend).

In the meantime, the following are some updates:

In a previous entry, we showed the box enclosure, which will hold the control system and other hardware and direct various wires. We’ve designed a shelf to hold the hardware components so that they won’t move around as the user carries the box (the box will have straps so that the user can carry it like a backpack).

A shelf to go into the box enclosure where all parts (the RPI, the vector board, etc.) are bolted down.

A shelf to go into the box enclosure where all parts (the RPI, the vector board, etc.) are bolted down.

We also made a carbon fiberglass leg mold. First we made a paper mache mold. Using that, we made a carbon fiberglass mold of the leg. We can mount the motors directly to the mold. Also, the fiberglass also provide structure. Instead of the user having to use several straps of velcro to attach the brace directly onto their leg, they can take the fiberglass on and off since the motors, leg brace, etc. will be attached to the fiberglass.

We made a fiberglass mold to mount the motors onto and to provide structure for the leg brace.

We made a fiberglass mold to mount the motors onto and to provide structure for the leg brace.

Feb 25

Toggle handle

We thought that we could use existing parts to make the kind of handle our user wanted (their description for the kind of handle they wanted was something like a fire extinguisher handle, i.e., could be worked in a squeezing fashion). However, we weren’t able to, so we decided to design and 3D print a handle instead.

I (Sarah) made the handle in SolidWorks and initially modeled it off a bike brake. A rim brake applies friction to a tire rim to slow/stop a bike. We decided to get rid of the tire and have the brake work as a contact switch instead.

Bike brake reference

A bike brake includes a lever, Boden cable, and the brake itself. Chase suggested combining the lever and brake (or contact switch in this case) so that we wouldn’t have to use Boden cable and to decrease the number of parts we have to make/print. Therefore, what we ended up with was something like this:

3D printed. A spring helps keep the contacts apart and helps with the squeeze motion.

3D printed. A spring helps keep the contacts apart and helps with the squeeze motion.

As you can see, the handle is made up of two separate pieces that are connected/held together with a screw and bolt. We have a spring to keep the contacts apart and to help with the squeeze motion (like wirecutters, which require force to bring the cutters together to cut the wire). A wire from one of the contacts was taped to the handle to secure the wire, but this is for now (i.e., the final product won’t look like this).

We will be making adjustments as the grips are a little short for the user’s handle. As the control team evaluated the handle, we realized that the grips should probably be thicker/sturdier (the cross section is 0.3″x0.3″). I’m going to redesign the grips so that I can slide and then glue PVC onto them.

The following is a video of the handle “in action”:

Feb 22

Many updates

Despite not updating since last December, we have several updates.

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Dec 09

Battery progress

The battery group is responsible for the powering the system. We decided to use a 12V 10Ah rechargeable battery. It should power all parts (maximum output) for two hours. The parts that need to the powered are: the control system (Raspberry Pi, 5V), LCD battery display (5V), ankle motor (6-7.4V), and knee motor (12 V). We need to make two voltage regulators to adjust the voltage from the main battery to the proper levels.

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Dec 03

Armband progress

The armband system is responsible for receiving user input and displaying it back to the user via an LCD screen. The system is composed of buttons, LEDs, and an LCD. The LEDs display the state of the system to the user and are implemented via Charlieplexing in order to reduce the number of GPIO pins used. The modes of the system are determined by buttons because that is the easiest method for the user. The LCD will display battery life information back to the user in the form of a 10-segment battery life bar and a percentage.

The knee and ankle motors are controlled via a toggle lever that the user holds. When the user holds the lever down, it moves the motor chosen, and when released, it stops.

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Dec 02

Knee progress

The knee subsystem is responsible for rotating the knee portion of the leg brace. It uses a gear motor that interfaces with a vertical worm drive gearbox in order to rotate a shaft that has an arm that is attached to the gearbox system and the brace to move the leg. The knee motor is controlled by the Raspberry Pi and can rotate the arm clockwise or counter-clockwise to get the necessary flexion and extension motions.

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Dec 02

Ankle progress

The ankle subteam is responsible for designing an ankle brace that will simulate movement for the user. The torque for ankle was calculated using T = m*g*l. The torque was calculated as T = 3.4 N*m. A 20% safety margin was used, increasing the required torque to 4.08 N*m. The selected servo has a maximum torque of 4.8 N*m at 7.4 V. The stall current is 3 A, and the idle current is 3 mA. The power consumption range was calculated to be from 0.022-22.2 W. The brace being used was donated to the group.

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