29th Annual RESNA Conference Proceedings

Wireless Monitoring of Power Wheelchairs and Wheelchair Users

Eli Wasserman, BS

Human Engineering Research Laboratories, University of Pittsburgh and VA Pittsburgh Healthcare System, Pittsburgh, PA 15206


Wireless or remote monitoring can be used to track wheel chair performance, patterns of use, user vital statistics, or detect an emergency. In the event of an emergency, a 911 call can be automatically made. Design considerations for sensors, data logging, data processing and wireless interface are presented. A low cost solution for in home monitoring is juxtaposed with a fully mobile solution for wireless medical telemetry.


Emergency alert system, wireless monitoring, remote monitoring, wireless medical telemetry


In 2000, the FCC allocated the following frequency bands to be used exclusively for wireless medical telemetry (WMT): 608-614 MHz, 1395-1400 MHz, and 1429-1432 MHz (1). The FCC has made it illegal to use WMT bands at home or in moving vehicles effectively limiting device use to the doctor's office or hospital. This leaves the industrial, scientific, and medical (ISM) frequencies for the production of unlicensed devices that seek FCC authorization. Four ISM bandwidths are most frequently seen in consumer electronics: 26957-27283 kHz used primarily for RC toys, 433.05-434.79 MHz used for security systems, garage door openers etc, 902-928 MHz used for cordless phones, and 2.4-2.5 GHz where the majority of data transfer standards are developed such as IEEE 802.11, HomeRF, and Bluetooth. While data transfer is possible at any frequency, the 2.4 GHz band has solicited more product development as a result of spread spectrum technology. The larger bandwidth of the 2.4 GHz range allows redundant data to be sent at more than one frequency, reducing errors from interference. Furthermore, frequency hopping algorithms that change the transmission frequency increase signal to noise ratio and provide security from standard RF scanners.

At least 50 different models of emergency alert systems (EAS) are available commercially (3). Most follow a simple formula: a remote control worn around the neck or wrist communicates with a base station that dials for help when a button is pushed. Some have special features such as a full duplex pendant, reminder alarms, or subscription services that connect the user to a relay station with access to a his or her medical history. Only two devices stand out the iLife fall detection sensor by AlertOne Services Inc and the Magnavox MobilePal. The iLife sensor is novel because it is the only EAS that can activate itself and therefore is a form of remote monitoring. The MobilePal is the only EAS that utilizes cellular technology to be fully mobile. Prices for an EAS range from $40-$400, some with additional subscription fees.

Photo shows the Micropaq monitor being worn around a person’s neck, a QRS PC card sensor, the egg shaped iLife sensor worn on a gentleman’s waistband, and MobilePal emergency phone that looks like a cordless phone only slightly larger.Photo 1: Commercial Devices: (from left to right) Micropaq, QRS PC card, iLife sensor, and MobilePal (Click image for larger view)

More advanced sensors have data logging and real-time processing capabilities such as Welch Allyn's Micropaq and QRS Diagnostics PC card sensors. Micropaq records and transmits heart rate, ECG and pulse oximetry with the IEEE 802.11 wireless standard. It is also equipped with an alarm system that may be triggered by any of the parameters it monitors. Intended for hospital use, individual monitors cost upwards of $4,000 and $60,000 for a base station. The QRS Diagnostics product line can measure blood pressure, ECG, Spirometry, Oximetry and other vital signs. By building their sensors on PC cards, QRS has given users the ability to interface with laptops and PDAs. Depending upon software and hardware options, cards range from $1,500 to $6,000.


Wireless Interfacing

Schematic shows the pin designations and proper wiring for HCS series encoder and decoder circuits using TWS434 and RWS434 RF transmitter and receiver chips.  Data bits are set at the encoder with dip switches connected to ground.  Data decoded is sent to a microprocessor where appropriate output signals are generated to activate the auto-dialer, alarm or other device.Schematic 1: 4 bit RF Encoder/Transmitter and Reciever/Decoder (4) (Click image for larger view)

The desired end function of the remote monitoring system can dictate the operating frequency of the device. An alert system can be fully implemented in the 27 MHz or 433 MHz range using the inexpensive technologies that have flooded the EAS market. Sensor design for an alert system comes down to building a switch that will replace the button on a traditional RF keychain style remote. Each sensor may be connected to its own RF encoder and transmitter enabling unique detection by a base station. The base station is comprised of a RF receiver and decoder interfaced with 8051 and PIC micro controllers for decision making. The user interacts with the base station by use of a key pad and LCD to store phone numbers in the 8051 chip and the station dials out by means of a Dual Tone Multiple Frequency (DTMF) MT8880C chip. See Schematics 1 and 2.

Schematic shows the pin designations and proper wiring for an automated phone dialer as described in the body of the paper.Schematic 2: Dialer (5) (Click image for larger view)

Once it is detected that the line has been answered, a message may be played or a speaker phone enabled (circuits not shown).

If real-time data communication is desired then the 2.4 GHz frequency range should be considered in conjunction with 802.11 or Bluetooth standards. Following QRS Diagnostic's approach, sensors could be connected to a PC card and interfaced with a laptop or PDA with wireless capabilities. However, that would make for an unfavorable 1 to1 sensor to computer ratio. Using the Bluetooth wireless standard, wireless sensors can be made with data transfer rates up to 3 Mb/s while allowing up to 8 wireless devices to communicate with each other. Devices such as Palm's Treo650 Smartphone, which is difficult to distinguish between a PDA, cell phone and pocket PC, come standard with 802.11, Bluetooth, and cellular technologies. The Treo650 could serve as an all in one solution. Sensors built with Bluetooth RF chips could communicate data to the phone where software could then be used to process and route data via WLAN or cellular data transfer. If an emergency situation is detected then a 911 call could be made. Furthermore the on board GPS of all cellular phones made after 2002 could be used to locate the user in trouble.

Sensor Design

In the home based design, the sensor must raise the "VE" input on the remote's encoder to +3V. This makes most sensor designs really simple. A fall sensor could be realized with a tilt or mercury switch. Seizures could be watched for with accelerometers. If a user leaves the chair without activating a bypass switch (i.e. unintentionally or without required supervision) a pressure sensor could sound the alarm. A more involved sensor would be a heart rate monitor that goes off when the user's pulse drops below 50 or goes above 120. Voltage from the skin would have to be amplified and filtered before being used as the clock signal for a counter that is measured and reset at a fixed interval. The output of the counter could then be put through Boolean gates or a microprocessor to trigger "VE." Solving the same problem using Bluetooth could use the same amp and filter but then interface with an analog to digital converter that would cue information to transfer wirelessly. An ECG could be constructed on the computer assuming the sampling rate is at least twice the highest frequency component.

Counters and A/D converters are also key in implementing sensors for power wheelchair function. Measuring the current that the DC motor draws can detect when debris is inhibiting wheel movement or when motor brushes need to be replaced. A Bluetooth odometer could track use patterns and have a software interface let friends and family know when a loved one is becoming less active.


The home monitoring and alert system presented has its strengths in low cost and robust embedded design. However it would be rendered useless in the presence of high RF noise at its frequency of operation. Data logging abilities could be added incorporating a microprocessor and memory chip into the sensor design. Computer interfacing to the logger could be accomplished through a docking feature or direct USB 1.0 connection. This would likely result in making a sensor significantly larger than its Bluetooth counterpart and is certainly less convenient for the user.

Bluetooth offers resistance to frequency jamming and noise, but interfacing with a cellular system requires service that might not be available between mountains, in the middle of the dessert or behind metal lined walls. A wireless Bluetooth system will cost more to develop and produce but ultimately provides more services and could prove to be more economical by working with technologies a user intended to purchase. Using a Bluetooth array of 5 sensors, a person could still use their wireless device with two more wireless I/O devices, perhaps a form of AT.

Another consideration is technological lifespan. The first generation of cellular internet connectivity for Palm devices such as model i705 has come and gone. The greater functionality, standardization and upgradeability of today's products should extend their lives, but open them up to attacks like the current bout of cell phone viruses.


  1. Federal Communications Commission. (Sept 2005). FCC Online Table of Frequency Allocations 47 CFR 2.106. Available at: http://www.fcc.gov/oet/spectrum/table/fcctable.pdf
  2. Federal Communications Commission. (2001). BAS Technical Rules 47 CFR 74. Available at: http://www.fcc.gov/Bureaus/Engineering_Technology/Notices/2001/fcc01092.doc
  3. Product Searches performed on: http://www.abledata.com, http://www.wirelessrerc.gatech.edu/
  4. Isotronics. RF Application Note. http://www.ishtronics.com/rfapnote.doc
  5. eBlocks: Embedded Systems Building Blocks. http://www.cs.ucr.edu/~snematbakhsh/dialer/
  6. Bluetooth Standards. (2006). http://www.ieee802.org/15/pub/TG1.html




Eli Wasserman, BS
Human Engineering Research Laboratories
VA Pittsburgh Healthcare System
7180 Highland Drive, Building 42nd Floor
East Pittsburgh, PA 15206
Office Phone (412) 365-4850
EMAIL: wassermantechnologies@gmail.com

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