NSF-Funded Student Design Projects:

ETL Mobile Robot

Designers: MultiSemester see Student Teams Table
Client Coordinator:Patricia Nizio, Detroit Institute for Children
Supervisors: Dr. Robert Erlandson, Mr. David Sant
Department of Electrical and Computer Engineering
Wayne State University
Detroit, MI 48202

Multi-Semester Project Student Teams

Introduction

Preschool children who are disabled and will require a powered wheelchair as they mature must learn the cause-effect relationships between a joy-stick, or other controller, and wheelchair movements. Many of these same children must also be evaluated for switch placement on their wheelchairs because they have consistent volitional movement from limited regions of their body. The ETL Mobile Robot is a therapeutic robot designed to facilitate preschoolers in acquiring cause-effect, pre-wheelchair mobility skills while supporting therapists in the switch assessment process.

Summary of Impact

Past experience with a variety of small mobile robots at the Detroit Institute for Children has demonstrated the effectiveness of these devices to engage children and thereby facilitate their learning cause-effect relationships and “rules-of-the-road” [Erlandson, 1991 #35][Erlandson, 1995 #37]. Furthermore, the children are so thoroughly engaged that therapists can conduct switch placements and evaluations with compliant and task focused children [Erlandson, 1991 #35][Erlandson, 1995 #37].

The previously used robots were donated mobile robots that were retrofitted with remote radio frequency controllers. The mobile robots worked well for a while and then broke down. Since they were donated, no readily available replacement parts existed and over time the units were retired. Current radio controlled toy cars and trucks are designed to move either too fast or too slow. They tend to be small and not easily controlled by the target population of children with disabilities. Hence, the need to build the ETL Mobile Robot.

Technical Description

The first step was to design and build the mobile robot chasse. The chasse is aluminum and the unit uses two drive wheels, with two additional wheels for stabilization. The drive wheels are powered by permanent magnet DC gear motors with a 150:1 gear ratio motors. See Figure 1.

Figure 1. ETL Mobile Robot Base Unit

If both drive motors run at equal speeds, the robot moves straight ahead. The robot turns by driving the wheels at different rates. If the drive motors are concurrently run in opposite directions, the robot stands in one place and turns. Each drive wheel has a custom designed and built encoder. Figure 2 shows a functional block diagram of the mobile robot’s main systems.

Figure 2 Functional block diagram of the mobile robot's main systems

The RF Joy-Stick Controller is a separate unit. Its functional diagram is shown in Figure 3.

Onboard the mobile robot, all the subsystems communicate over the Controller Area Network (CAN) bus. This network topology was chosen primarily for its ease of use and modularity. Additional devices can be added to the bus with minimal software changes.

Figure 3. RF Joy-Stick Controller functional diagram

The RF Joy-Stick Controller uses a joy-stick commonly used in powered wheel chairs. See Figure 4. The joystick is connected to a PIC microcontroller and an RF transceiver to make it completely wireless. The microcontroller reads in the analog data from the joystick using its built-in analog-to-digital converters and packetizes the data in digital form for use by the RF transceiver. The RF transceiver has built-in error correction algorithms to ensure that the data arrives intact to the robot.

Figure 4. RF Joy-Stick Controller and joy-stick

At the base unit the RF transceiver decodes the paketized data and places the data on the CAN bus. These CAN messages are read by all of the modules on the bus and are used as inputs to the Navigational Sensor System (NSS) and motor controller.

The NSS is designed so that as a child controls the robot’s movement, it will not run into walls, another person, furniture, or objects. If there is no place to go, the robot stops. Using information from two front and one rear ultrasonic sensors, the NSS calculates the distance to the nearest object and places the appropriate navigation messages on the CAN bus for use by the motor controller. The NSS performs collision avoidance by diverting the robot’s motion away from detected objects or obstacles. A speaker is used to produce a tone whenever the robot has detected an object in its way.