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11/2/06
10:07 AM
Page 35
Chapter 8
Electromagnetic Interference (EMI)
Our environment is saturated with radio (electromagnetic) waves that originate from
transmitters of television, radio and communication signals. The waves are invisible
and their strength at any location is unknown but in general, the strength of a given
e.m. wave increases as you move toward the transmitting source. All electrical con-
ductors act as antennas to the e.m. signals. To varying degrees, all powered wheel-
chairs and scooters are susceptible to electromagnetic interference. Abnormal oper-
ation in the form of unintentional movement and / or erratic control of the vehicle
could result if the interference is such as to override the control system of the vehicle.
The U.S. F.D.A. has suggested that the following statement be incorporated to the
owner's manual for all powered wheelchairs.
Electromagnetic Interference ( EMI ) From Radio Wave Sources.
Powered wheelchairs and motorized scooters (in this text, both will be
referred to as powered wheelchairs) may be susceptible to electromagnetic
interference (EMI) which is interfering electromagnetic energy (EM) emitted
from sources such as radio stations, TV stations, amateur radio (HAM) trans-
mitters, two-way radios and cellular phones. The interference (from radio
wave sources) can cause the powered wheelchair to release its brakes, move
by itself or move in unintended directions. It can also permanently damage
the powered wheelchair's control system. The intensity of the interfering EM
energy can be measured in volts per meter (V/m). Each powered wheelchair
can resist EMI up to a certain intensity. This is called its "immunity level".
The higher the immunity level, the greater the protection. At this time, cur-
rent technology is capable of achieving at least a 20 V/m immunity level,
which would provide useful protection from the more common sources of
radiated EMI. This MPV 4 powered wheelchair model, as shipped, without
any further modification has a tested immunity level of at least 20 V/m.
There are a number of sources of relatively intense electromagnetic fields in
the everyday environment. Some of the sources are obvious and easy to
avoid. Others are not so obvious and exposure is unavoidable. However, we
believe that by following the warnings listed below, your risk to EMI will be
minimized.
D82008204 REV B 1/14/04
Chapter 8
35
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