The major aspect of the device which we are designing is the wireless interface technology that will communicate between the EZRA II system and the hospital bed. The solution that must be implemented is required to operate with a minimum range of 25 meters and provide a reliable and secure point-to-point connection between the transmission device interfaced with the EZRA II system and the receiver device interfaced with the hospital bed. There are several solutions for achieving this wireless link effectively; the three most popular technologies considered being Zigbee(IEEE 802.15.4), WiFi (IEEE 802.11), and Bluetooth. Each of these three options carry with them various pros and cons in regards to signal reliability, ease of implementation, and potential for future additions and modifications to the EZRA II System which will be covered in the following chapter.The Invacare hospital bed interface is an RJ-50 cable with 10 connectors. 6 of these connectors correspond to the analog voltage signals causing the different sections of the bed to move. One of the pins provides a 12VDC reference voltage that is, under normal bed operation, routed to the desired pin via a hand-held pendant containing six buttons. To operate the bed the receiving unit must connect via this RJ-50 connection and be capable of providing a 12VDC signal to the required pins in order to operate the bed. The receiving device must also be capable of retaining the ability to have the bed controlled via this pennant, and therefore an interface must be designed that allows for manual control of the hospital bed by attendants and assistants. The EZRA II system has a DB-9 connector which currently provides these 12VDC signals to the bed via a serial cable. If we are required in the design to create a transmission unit, itwill interface with the EZRA II via this DB-9 port. This, however will likely not be required as the EZRA-II contains a Bluetooth communications module that could potentially be used for achieving the wireless link. The receiver design will be required to either draw power from the 12VDC provided by the Invacare bed, or by drawing from mains power via a wall socket. Regardless of what method is utilized to provide the unit with power, some voltage conversion will be required to reduce the supply voltage to the required value for operating the wireless and logic circuitry of the receiving unit. This level is likely to be either 3.3V or 5V depending upon the technologies chosen.

The Zigbee(IEEE 802.15.4) standard offers full wireless mesh networking capable of sup- porting networks of hundreds of devices while providing the ability to automatically detect and compensate for RF interference. A typical example of a Zigbee capable device would be the Digi XBEE module, which is a relatively low-cost and easy to implement device pro- viding a reliable wireless connection at ranges of up to 30 meters. The primary draw-back to this technology is that is has primarily only been adopted in industrial applications and therefore is not commonly pre-installed on many devices. To scale the EZRA II system with this device would involve designing a Zigbee transceiver unit for the EZRA II that would allow it to communicate with and Zigbee protocol devices. This inability to conveniently interact with several devices is the main drawback of a Zigbee based wireless solution. WiFi(IEEE 802.11) technology carries with it the benefit of being an extremely common protocol considering its extensive use with in-home wireless networking solutions. In this regard the protocol seems fairly attractive, however 802.11 protocol is primarily intended for use with in a WLAN style architecture, as 802.11 was intended as a wireless replacement for Ethernet, and very likely will not be particularly well suited to this application. Another disadvantage to using wi-fi technology is the potential for interference from devices included in the home network. Wi-Fi is the most weak technology with regards to the wireless bed interface application based off of these negatives. The Bluetooth protocol is a personal area network protocol designed initially as a wire- less serial cable replacement for device to device communication. Bluetooth technology is capable of achieving the desired transmission range whilst maintaining data integrity. The primary advantage of instantiating a Bluetooth based solution on this device is that this provides for future integration of the EZRA II system with a large variety of devices includ- ing mobile phones, video game consoles, and personal computers. The wide acceptance of Bluetooth as a wireless standard allows for a common interface amongst all of these devices making this wireless technology option The strongest of the three discussed technologies with regard to future system potential. In addition to these advantages, future versions of the EZRA system will be pre-installed with an existing Bluetooth transceiver. This will make the production of the bed control receiver device the primary goal of this project, and there will be no need to design a transmission device to connect to the EZRA II, as this can be handled simply with additional software to operate the included Bluetooth hardware. The next major issue with the design is in regards to processing the information from the EZRA II that is received via wireless transmission, and then translating this protocol in to discrete signals corresponding to the bed controls. Once the wireless signal is received from the bed, it will be translated via the transceiver hardware as a standard , most likely UART, signal to the chosen micro-controller. This micro-controller's purpose will be to take the packet received from the transceiver and use this data to send a discrete signal to the bed via an RJ-50 cable that controls the various positions of the bed. Therefore important features that the control circuitry must have available will be a minimum of 6 general purpose I/O pins or potentially 3 I/O pins sent to an external 3 to 8 decoder chip, a UART interface, And the micro controller must be easily programmable in a common language such as C/C++. Currently the primary choice for processing will be a micro-controller from the Atmel ATTiny line; specifically the ATTiny2313. This micro-controller provides 18 general purpose I/O pins and is the smallest micro-controller in the Atmel line to provide a UART inter- face. The processor also operates between 1.8-5.5VDC thus will easily interface with the Bluetooth communication hardware. Another advantage of this micro-controller is that the software and libraries for programming are free open-source software, thus reducing cost in development further. Team member Cyrus Metcalf is also experienced with working with this development tool-chain, and has all of the hardware necessary to work with this device, or other micro-controllers from Atmel in his home electronics workshop. The INVACARE bed is a 12V system which needs a 12V but needs a 3.3V control circuit. The different ways considered to power the wireless control system include: switch mode power supply and linear regulated power supply, AC/DC supply. The switch mode power supply are low weight, smaller size, higher efficiency, wide AC input voltage range and also reduced cost. There is presence of a switching regulator in order to be highly efficient in the conversion of electric power. The switch mode power supply effciently regulates an output voltage typically at a level different from the input voltage.The pass transistor of a switch mode power supply switches very quickly between full-on and full-off states , hence this gives the switch mode power supply an edge over the linear power supply. The output of the switch mode supply is regulated using duty-cycle control. The AC/DC power supply consist of a rectifier and a filter capacitor. The linear regulated power supply is used to set voltage to a precise value which eliminates fluctuations in the input voltage and load. The linear voltage regulator reduces the ripple in the output direct current. However, the switch mode power supply has certain benefits over the linear regulated power supply. The switch mode power supply has smaller transformer due to higher operating frequency, where as, the linear regulated power supply tend to have large transformers if implemented. Also heat sinks for high linear power regulators are setbacks seen in linear regulated power supply because they contribute to the larger size and weight of the linear regulated power supply. Voltages in switch mode power supplies vary insignificantly with load but in the case of linear regulated power supply there is a substantial voltage variation with load. Switch mode power supply happens to accommodate wider variations of input voltage before the output voltage changes.There is higher efficiency in the switch mode power supply as a result insignificant resistive losses between input and load unlike in the linear regulated power supply. The effect of synchronous rectifcation mode and higher input voltage makes the conversion process more efficient in the switch mode power supply.