Derek Dempsey, Anthony Capone
ABSTRACT
Our client is a 4 year old boy who has cerebral palsy. Our project’s goal is to create a tricycle that will let our client ride and exercise and strengthen our client’s core and leg muscles. The hybrid power tricycle (HPT) has a normal frame and and allows the rider to pedal under their own power. This was accomplished by converting a bicycle into a tricycle with a conversion kit. The motor mounted on the front wheel helps the rider with pedaling. Power is applied to the motor from a microcontroller that reads how fast the pedals are spinning. The microcontroller reads the speed from a sensor on the sprocket of the tricycle. The data from the sensor is calculated and the correct power is applied to the motor. For safety reasons, a wireless shut off switch is able to shut the motor off from a distance of about 300 feet.
BACKGROUND
Our Client, Pierce, is a 4 year old boy, who has cerebral palsy. He has poor strength in his core and leg muscles. Pierce loves to move around and ride his old tricycle, which he out grew, and did not have pedal assist.
Independence is an important attribute to everyone. It allows one to be and individual and operate under their own motivation. Assistive technology plays a key role in independence. Many projects are made to help individuals gain back their independence and individuality by allowing them to accomplish tasks on their own that would otherwise be undoable.
Projects of this nature have to address a current issue preventing a client from accomplishing something on their own. They need to be easy to operate and integrate rather seamlessly into day-to-day life. Giving someone independence to someone is a great accomplishment that means a lot to the person who is receiving it.
The Hybrid Power Tricycle does this exactly, in that it allows a child to ride a tricycle on his own.
PROBLEM STATEMENT
Pierce and his family need a tricycle that will let Pierce exercise and is easy to move around. The goal is to create a tricycle that lets Pierce exercise his core and leg muscles while being easy for him to pedal and able to be adjustable for him to grow while being safe to use. Also, the tricycle needs to be portable compared to other therapeutic bikes and resemble a normal tricycle.
DESIGN
Physical Structure
The Hybrid Power Tricycle (HPT) started with a children’s bicycle frame. A conversion kit was used to convert the back axle into a two wheeled axle. This bolted directly onto the frame. The conversion kit would transfer pedal power via chain to the back left wheel. The motor we chose came with a mounted plat that was welded onto the front fork of the tricycle. Motor power would be transferred via a chain to the front tire. 16-inch wheels were used for the HPT because it was the right size for our client. Two battery mounts made of metal were welded to the rear axle to hold two 12V sealed lead acid batteries. A rear handle was also added per request of the client. This was constructed from 1-inch PVC pipe and was clamped to the back of the frame. The handle is threaded and can be easily removed or attached to the tricycle. A back support bolted onto the base of the seat on the seat post. It has a chest strap built in and the pedals have Velcro straps. The frame and rear handlebar were painted green per the client’s request and the conversion kit is chrome.
Power Supply and Distribution
The HPT is powered by two 12V sealed lead acid batteries in series for the 24V motor. These batteries were chosen because they are designed to work with the motor we used and have a long life of 10Ah. The batteries connected to the 24V motor controller, which handled power distribution to the motor. The motor controller interfaced with the microcontroller with a 5V connection. A 5V signal gave full throttle and a 0V signal stopped the motor. The electronics on the HPT would draw power by taping off of one of the 12V batteries via a parallel connection. This would then feed into a 5V regulator. The front brake has a built in switch that ties into the motor controller so if the brake is pulled power is cut from the motor.
Main Electronics Box
The first part of the electronics was the 5V regulator, which would power all on board electronic components. This was made using a NJM78M05FA 5V regulator and two electrolytic capacitors. The Atmel 328 microcontroller is what controls the entire tricycle. The chip was interfaced with the Hall effect sensor to count pedaling rpm and also interfaced with an Xbee wireless module. The schematic can be seen in Figure 1. Also, the wireless shut-off switch, wired shut-off switch, hall effect sensor, motor controller, and power switch all interface inside this box. The project box is located over the rear axle of the tricycle.
Figure 1. Main Electronics Circuit
RPM Sensing
In order to apply the correct amount of power to the front motor, we needed to measure how fast the pedals were moving. A Honeywell SR17C-J6 hall effect sensor was used to accomplish this task. A mount was welded to the bike frame right next to the sprocket that the pedals were attached too. The sensor was attached to the mount and placed so the gear teeth of the sprocket would pass through it. See Figure 2 for the mounted of the sensor.
Figure 2. Hall Effect Sensor Attached to Tricycle
When a tooth passes through the sensor it breaks a magnetic field causing the signal pin to go from 0V to 5V. The microcontroller would sense this change of state and after 5 teeth passed through; it would do a speed calculation. This would loop as the bike was running so the after every 5 teeth went by a new speed calculation was generated. The hall effect sensor circuit can be seen in Figure 3.
Figure 3. Hall Effect Circuit
Motor Controller
The main power to the motor is handled by using a motor controller. The motor controller delivers the correct amount of power to the motor by interfacing with the microcontroller. Normally, the motor controller will use a throttle to control the speeds, but because of the hall effect sensor, it uses the lines that it would use for the throttle to connect to the microcontroller. The motor controller uses high power FETs to send the signal to the motor. It also has a line for the brakes to interface with it. When the front brake handle is pulled, a signal is sent to the motor controller to cut all power to the motor for safety reasons. The motor controller has a line that is a key switch too, which will effectively turn on and off the motor controller and power to the motor. A charging feature is build into it to recharge the batteries.
Wireless System
The HPT features a wireless shutoff system for our client’s parents to use. A remote was made out of a small project box and the actual wireless communication is accomplished using Xbees. A picture of the remote can be seen in Figure 4.
Figure 4. Wireless Shutoff Remote
The Xbee is a small, 1mW, wireless IC that is used to wirelessly send the state of the switch on the remote to the tricycle’s electronics box. It has a range of 100 feet indoors between walls or 300 feet outdoors with a direct line of site, which is far greater than the distance our client’s parents will be from the HPT. Two double batteries power the system. There is a power switch tied into the positive rail of the batteries and an LED to indicate power is on. A second switch is tied between the batteries, and LED, and an I/O pin on the Xbee that has a pull-down resistor on it. The system is setup so that the Xbee in the remote constantly sends a HIGH signal to the Xbee on the trike, which is read by the microcontroller. If the remote is not on, runs out of battery, or goes out of range, the HPT will not operate. The circuit for the remote can be seen in Figure 5.
Figure 5. Wireless Shutoff Schematic
RESULTS
In the end, our project has satisfied the needs of the original proposal. It allowed a young boy with Cerebral Palsy experience the joy of riding a tricycle with independence. The HPT was delivered to our client, Pierce on December 22, 2012. The HPT can be used for people with multiple disabilities and disorders.
DISCUSSION
In conclusion, our design meet all of the specifications from our client. It gives our client the ability to have a sense of independence, look like a normal tricycle, and relatively easy to move around. The ability for the parents to remotely and physically shut down the power to the motor adds a safety feature to the HPT.
One of first issues we ran into was choice of motor. Originally the design called for a hub motor that was directly built into the wheel. This was ideal because it was inconspicuous. The problem with a huba motor is that it is not geared so running it at low speeds would not be ideal. Electric motors are counter intuitive to what one would think in that at low speeds they use more power than at high speeds. This high drain state would lead to low battery life and performance. Instead we found a geared scooter motor that we could attach via chain which would fit power specifications and provide enough torque to move the tricycle.
ACKNOWLEDGEMENTS
We would like to thank our client, Pierce, and his family, the Conley family. Thank you to Dr. John Palma, Michael Darish, and ECE department Chairman and Head, Dr. Martin Margala from UMass Lowell. Thanks to our advisors, Donn Clark, Assistive Technology Program Director, Alan Rux, Technical Coordinator, and Senait Haileselassie. Also, thanks to UMass Lowell and the ECE department. Also, we would like to thank the Nation Science Foundation for funding the project.
Authors:
Derek Dempsey email: derek_dempsey@student.uml.edu
Anthony Capone email: anthony_capone@student.uml.edu