In the spring of 2011, my undergraduate senior project tasked a group of 6 mechanical engineering students with designing and building an isokinetic exercise machine over the course of one semester. Using the technology of a local startup we built a fully functional prototype after iterating through benchmarks, initial concepts, manufacturing, and a final design. As the team leader I had the added responsibility of organizing team meetings, smoothing conflicts, and keeping everyone on the same page. This project helped spark a love of design and product development, and I'm still proud of what my team was able to accomplish. I've summarized the key points of the project in the following post.
From Solidworks plans...
To a working prototype!
Background
The typical weightlifting experience limits users to the weakest point in a workout's range of motion; the maximum weight this weak point can handle is the maximum resistance a workout allows. This neglects the stronger flexion points and thus reduces the effectiveness of the workout. Exerbotics LLC., a Tulsa startup, eliminates this facet by building machines in which a linear motor controls the speed of motion and the user exerts their maximum force relative to the current position. A touch-screen monitor then displays this force graphically providing a motivating goal to attain. This project is the design and construction of bicep/tricep machine that utilizes this novel approach by senior TU Mechanical Engineering students (and funded by Exerbotics).
The Exerbotics software charts user workouts in real time.
Objectives
The goal of the project was to produce a biceps and triceps workout machine based around the technology used by Exerbotics, LLC. Exerbotics uses a linear motor coupled with a force sensor to create exercise equipment that can chart user’s progress and give the user visual indications of their output.
The following quantitative goals were considered during design and manufacture:
- 160° Range of Motion
- Fit 95% of the population
- Linear Actuator Controlled
- Utilize Exerbotics’ Proprietary Software
- Concentric and Eccentric movements
- Withstand continuous 250lb workouts
Key Design Points
Demonstration of the triceps workout.
The final product is modeled after a preacher curl style workout. The range of motion required for this style of workout calls for an efficient way to convert the linear motion of the motor to rotary motion. A rack and pinion mechanism is used to accomplish this, and the other parts of the product are modeled after more conventional preacher curl style exercise machines.
Rack and pinion detail
A touch screen provides user friendly access to control the individual’s range of motion as well as starting and stopping the workout.
The base, built by the team, is derived from an existing Exerbotics leg press machine and the handle from a curling bar.
Because the user is never under a mass load, they are never in danger of being crushed. That means they can continue with their reps even after muscle fatigue prevents them from reaching their desired force output. This allows the user to reach their fitness or rehabilitation goals safely and efficiently.
This project served as a strong proof of concept for building the rack and pinion concept into Exerbotics machines that require rotary motion. Using rotary motors would be possible, but our research found them to be notably more expensive for a comparable force output. Exerbotics expressed their interest in continuing the development of this machine.
Testing brought up multiple suggestions for future production. Improvements to the rack stabilization system are necessary to improve the smoothness of the movement. The machine would fit more users comfortably with some minor modifications to dimensions. Further research into lubrication benefits is necessary, along with evaluating the trade offs of using a spur gear rather than helical.