Hardware Development and Design

In the purpose of developing obstacle detection hardware circuit, there are several components involved. All these required items have their own functions in order to support the operation of the whole circuit in this obstacle detection system. Hardware will have a direct connection with the user since it is attached to the shoe. The design ofthis circuitry was done after careful considerations were made prior to developing the wearable device. The development of hardware for wearable obstacle detection system involves two parts which are the transmitter and receiver.The transmitter side consists of FSR sensors (as switching mechanism for distance sensors), IR and US sensors (as distance sensors), microcontroller unit (MCU) and wireless communication module for data (detected obstacle) transmission. The receiver comprises of second MCU, wireless

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communication module for data receiving and alarm units such as the buzzer, vibrator and audio messages.

5.2.1 Force Sensing Resistor

The use of FSR sensor in biomechanics research has been established for many applications or purposes. In this work, three pieces of Force Sensing Resistors (FSRs) type 402 manufactured by Interlink Electronics (Interlink Electronics, 2010) are used, which are in the circular shape as shown in Figure 5.1.

Figure 5.1:Illustration of force sensing resistor FSR 402 from Interlink Electronics (Interlink Electronics, 2010).

It is ideal for engineers, scientists’ or researchers who need to measure forces distribution under foot without disturbing the dynamics of their tests. This sensor can be used to measure both static can dynamic forces, and are thin enough to enable non- intrusive measurement. The resistive-based technology FSR sensors are a polymer thick film (PTF) device which exhibits a decreasing trend with an increase of the force applied to the active surface. The application of force to the active sensing area of the sensor results in a change in the resistance of the sensing element in an inverse proportion to the force applied which shared similar properties with theload cell or strain gauge (Interlink

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FSR1

FSR2

FSR3

Electronics, 2010). For simple force-to-voltage conversion, the FSR device is tied to a measuring resistor in a voltage divider configuration as displayed in Figure 5.2.

Figure 5.2:Configuration oftheFSR sensor circuit (Interlink Electronics, 2010).

The size, shape and thickness (very thin, less than 0.5 mm) of the sensor make it suitable to be mounted on the insole position of the shoe without disturbing the user during walking. The purpose of this sensor is to ensure that theanalogue distance sensor (US and IR) is only activated (sense the obstacle) when the entire sole of the foot (shoe) touches the ground. The foot is considered as fully touching the ground when all the values at theoutput of FSR sensors reach the setup threshold value (high pressure, low resistance). The high pressure value occurs at three top places under the foot which are heel, first metatarsal head (MTH) and toe as illustrated in Figure 5.3.

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(b) Infrared Sensors (a) Ultrasonic Sensor

5.2.2 Obstacle Detection Sensors

In this research, the size and weight of the sensors and their interfaces to a microcontroller are the important requirements since the sensors will be installed at the front part of the shoes of the user. The ultrasonic LV EZ1 from Maxbotics and infrared GP2Y0A02YK0F from Sharp family are chosen for this project because their specifications meet with the design requirement of the system. Both sensors are non-contact analogue distance sensor as shown in Figure5.4.

Figure 5.4:Analogue distance sensors used for obstacle detection includes (a) Ultrasonic LV- EZ1 from Maxbotics and (b) Infrared GP2Y0A02YK0F from Sharp.

The combination of these two medium range sensors enhances the reliable performance against the obstacle detection in a variety of shapes, sizes, materials and environment. The capabilities of the sensors towards obstacle detection have been elaborated in the previous chapter.

5.2.3 Microcontrollers

PIC microcontrollers from the Microchip Technology family are used in this system prototype due to its wide acceptance in industry, low cost, abundant information resources, easy to use, availability, versatility, ease of programming, small size and

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(a) PIC18F66K80 (b) PIC16F887

compatible for wireless application. Nevertheless, broad functionality allows this microcontroller to be physically embedded to the insole of the shoe to perform all the necessary control functions. The PIC microcontroller can also work with low cost development kits which are available in the market such as ESPIC40C. The proposed device utilized two 8 bits microcontrollers, which are PIC18F66K80 at the transmitter and PIC16F887 for the receiver as shown in Figure 5.5.

Figure 5.5:The 8 bits Microchip controllers used in the proposed system (see

Appendix B).

Both microcontrollers bring additional value to the developed system since it offers unique and exclusive peripherals (e.g., intelligent control capabilities, communication and networking, lowest cost and smallest form factors).

5.2.4 Wireless Transceivers Modules

The developed obstacle detection system utilizes thewireless communication technology for data transmission between the transmitter and receiver unit. The Xbee transceiver module provides wireless connectivity for data transfer without using any switch or connector. This type of wireless module is based on theZigbee network technology. The circuit operates at 3.3 V with a current of less than 40 mA during its operation and 3 mA

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