Dixon Correa, Anish Dabhi, Avik Kathuria, Rogelio Ortiz
Children with disabilities have varying degrees of limb mobility. While devices exist to engage students with disabilities in sporting activities, very few are actually able to adapt to varying physical capabilities of these students. This paper describes a device that is designed to hit a ball over a net with a racket simulating a serve in racket sports. The device accommodates varying degrees of motion capability in that it allows actuation either via a button or by swinging a dummy racket. To make the device easy for instructors to use, the device embodies a ball-feeding mechanism that is designed to hold balls in a chute and drop them onto the racket whenever the machine is actuated. Safety of students being a top priority, several safety considerations incorporated in the device are discussed. Additionally, the device accounts for other ergonomic factors like accommodating students of different heights and suitability for right/left handed students.
Introduction and Background
Our client was Melissa Devine, a physical education instructor at Rosedale School in Austin although the device was to be designed to cater to the needs of the students of Rosedale School. Melissa (or Coach Mel as she is fondly called at Rosedale) was instrumental in conveying to us the needs of her students, many of whom lacked the cognitive abilities to communicate with us. One of the students, Carlos had really good range of motion. However, Coach Mel told us that he can sometimes lose control over his movements. Angie had good mobility too but she was in better control of her movements. Savannah’s hand movements were heavily restricted and she barely had enough strength to push on the big button. Tony could only move his left arm in one direction. Clearly, we were dealing with students with vastly varying needs which made the project even more challenging.
Rosedale School needs a portable, electrically powered device designed to assist individuals with disabilities in hitting a ball over a net using a racket. The device should accommodate students with varying degrees of disability while also taking into account that some students are bigger/taller than others and may favor their right hand over their left or vice versa. The device needs to be absolutely safe.
Design Process, Approach, and Solutions
In order to come up a solution to our client’s problem we brainstormed for ideas. After building some initial proof-of-concept models for these concepts, we were able to eliminate those that were infeasible or inconsistent. The concepts that we selected ended up being the embodied design. We envisioned a motor driven racket that hits a ball that is fed to it by a feeder placed above it. The racket motion would be timed with the ball feeder so that it consistently hits the ball. The actuation was envisioned to take place by a big button that students at Rosedale currently use for various activities. Alternatively, we decided to provide a dummy racket to act as an actuator for students with better motor abilities. These are seen in Figure 1.
To allow for students with different heights and wheelchair requirements to use the machine, we decided to mount these subsystems on a slider that can be locked into place at a desired height with the help of a handle as seen in Figure 2.
Ball Feeding Mechanism
In order to release one ball at a time when the device is actuated, we needed a mechanism to let one ball pass while preventing the others from passing. The result is an ingenious double cam mechanism mounted on two separate shafts and linked by a set of gears. The upper shaft is motor driven while the bottom shaft is driven by the gear pair in the opposite direction. The cams are designed in such a way that one half rotation of either shaft results in a single ball being dropped through the unloading chute. To achieve an exact half rotation every time, a stepper motor was used to drive the ball feeding mechanism. The ball feeding mechanism is seen in Figure 3. It is designed to be loaded with multiple balls at a time and to accommodate balls of different sizes.
Racket Swinging Mechanism
The racket swinging mechanism seen in Figure 4 is essentially a four-bar mechanism with a wire connecting the input and output links. The wire works as a rigid link in tension and as a flexible link in compression. This allows the racket arm which travels at high speed to vibrate on impact without exert a back driving force on the DC motor that drives the mechanism. The mechanism has limit switches at the racket’s start and end positions that communicate with the microcontroller and drive the racket in the desired direction. The racket swing is timed with the ball feeding mechanism using the microcontroller so that it hits the ball consistently every time.
Dummy Racket Actuator
The dummy racket is used to actuate a switch that initiates the device. As a safety measure, students with motor capabilities can swing this dummy racket instead of holding on to the racket that does the hitting. The dummy racket is held in a clamp to allow for easy replacement. The racket attachment can also be unscrewed and fixed at the other end for use by right-handed students.
The microcontroller used was an Arduino Uno R3. The microcontroller was responsible for implementing all the logical decisions needed for the device to perform as intended. The DC motor for the racket swinging mechanism was driven by a Sabertooth v1.03 motor driver. The unipolar stepper motor for the ball feeding mechanism was driven by a Darlington array (ULN2803A) integrated circuit. All motors operate on a 24V DC power supply.
Results and Discussion
The device works like a dream! A demonstration of the device was arranged at Rosedale School with members of the staff and a few students in attendance. A student used the big button to actuate the device while another used the dummy racket. In both cases, the device worked perfectly. The staff wanted to try balls of different material and size which also posed no problem for the device.
There were a few inconsistencies observed in the timing of the racket. These can be attributed to:
- Different balls have different sizes and shapes and take different amounts of time to reach the racket from the unloading chute.
- Inconsistencies in the chutes because of friction.
- Misalignment of the cams.
Timing errors can be rectified by manually repositioning the initial cam position. The same can be done to ‘tune’ the machine to balls of different sizes and material.
Safety being an important consideration for the device, an acrylic shield was introduced to cover the moving components in the racket swinging mechanism. A switch was provided to disarm the motors in case of an emergency. Additionally, an intentional delay of five seconds was incorporated between swings so that repeated willful actuation of the device could be avoided.
Some of the important customer needs met by the device include the following:
- Adjustability for different student heights.
- Suitability for both left and right handed students.
- Portability in the form of a wheeled base.
- Safety of the students.
- Suitability for students with varying motor capabilities and wheelchair requirements.
- Permit the use of the standard big button used at Rosedale School.
- Permit the interchangeability of rackets.
- Allow the use of balls of different sizes and material.
We would like to thank Dr. Richard Crawford for his constant help and input during the course of this project, our clients, Melissa Devine and the students and staff at Rosedale School, who were always available for advice and feedback which drove the project forward, and lastly, our friends at the Longhorn Maker Studio who were gracious to let us use their 3D printers beyond regulation hours which turned out to be crucial in the completion of this project on time.
Dixon Malcolm Correa