John M. Tobias, Ph.D., P.E., CFEI, Electrical Engineering & Illumination Expert
The defendant suffered significant injury while riding on an urban street when the front wheel of an electric scooter braked unexpectedly, propelling them forward over the handlebars of the scooter. Investigating these mishaps, it was found that the scooter braking control is sensitive to external magnetic fields commonly emitted by utility power lines and similar electric utility equipment. The mishap location corresponded to the location of underground power utility equipment. Subjecting the scooter braking control to electromagnetic interference, specifically power frequency (60 Hz) magnetic fields, engaged the braking function. Further investigation revealed that the electric scooter was not properly tested for the effects of external electromagnetic fields on its safety-critical braking controls.
Electric Scooter Control and Braking:
The subject electric scooter had one small-diameter (8-inch) drive wheel at the front and two free-spinning tandem wheels at the rear, interconnected by a platform upon which the operator stands to ride the scooter. Control for the scooter was provided by a handlebar with integral controls for engaging the motor and braking on the right and left handlebars, respectively. The scooter incorporated a liquid-crystal electronic display/control panel, among other controls. Turning the handlebars provided directional control and steering, mechanically changing the angle of the front-wheel with respect to the long axis of the scooter, thereby changing the direction of travel, as with a bicycle.
In this particular case, an integral electric motor in the front drive wheel provided motive force for the scooter and employed what is termed ‘electronic braking’. Electronic braking utilizes the principle of ‘back electromotive force’ or back EMF, which is the converse of the motor function. Essentially, the motor becomes a generator when power is removed, which provides a retarding force. Creating an electrical short-circuit, or high resistance, across the motor creates a large back EMF (electromotive force) that arrests the spin of the integral motor/wheel assembly. In this arrangement there is no friction device, such as disc or drum brakes, for slowing the scooter.
The drive and braking controls are fully electronic. No mechanical links are present between the brake trigger and the drive wheel. Rather, the function of the motor is governed by a microprocessor, using inputs from the handlebar trigger switches for acceleration and braking. As one would expect, software or firmware controls the microprocessor and consequently the acceleration and braking functions. The particular scooter in this case was capable of speeds up to 20 miles per hour.
Electric Scooter Control Sensitivity to Magnetic Field Established by Testing:
Among other tests, including exposure to radio frequency radiation between 20 MHz to 3000 MHz, the scooter was subjected to power line frequency (60 Hz) magnetic fields. Although some of the scooter controls appeared affected by the radiofrequency radiation, the scooter brake control switch was found to consistently respond to external power line frequency magnetic flux density, or magnetic induction, between 0.5 milliTesla to 1.0 milliTesla. When the control was exposed to this degree of external magnetic field, the front wheel of the scooter ‘locked up’ abruptly and engaged the brake light that was affixed to the rear of the scooter.
An electromagnetic survey of the mishap location was conducted by reviewing nearby Federal Communication Commission license holder data, and through measurement. Magnetic fields comparable to what’s needed to trigger scooter braking function were found at the mishap location. Further investigation revealed that underground electric utility equipment (transformers and distribution circuits) was present at the mishap location. The degree of magnetic field near utility equipment and circuits correspond to the current load on the circuit, increasing during periods of high electric demand. So the scooter could traverse the same location without incident on some occasions during low electric demand, but malfunction and trigger the brake unexpectedly on other occasions, such as during periods of high electric demand.
Findings and Opinions:
The electric scooter was subject to abrupt and unexpected braking where magnetic fields were present at the mishap location, in this case a sidewalk in a major urban setting. Marketing materials for the scooter was targeted to urban users, highlighting its advantages and suitability for urban environments. As such, it was not safe to use in its advertised and intended application.
Product safety efforts were inadequate for the subject electric scooter. The US Consumer Product Safety Commission (CPSC) strongly endorses Underwriter’s Laboratories Standard for Safety 2272, entitled ‘Electrical Systems for Personal E-Mobility Devices.’ (UL 2272). That standard, in turn, invokes Underwriter’s Laboratories Standard for Safety 991, entitled ‘Standard for Tests for Safety-Related Controls Employing Solid-State Devices.’ (UL 991). Discovery and research efforts in the case revealed that little to none of the analysis and testing specified by these standards was conducted, although competitors marketing similar scooters did so, listing their products to the UL 2272 standard.
Moreover, exclusive use of front-wheel braking is conducive to loss of control and falling, contributing to the severity of the mishap. Aggressive braking of only the small diameter front wheel of the scooter creates a large mechanical moment about the axle of the front wheel, causing the rider to pitch forward over the handlebars. Consequently, if the brake for the scooter is applied suddenly, (and especially unexpectedly) there is a high probability of the rider falling.
Lastly, the degree of the magnetic field present at the mishap location exceeded the applicable State Public Service Commission guidelines.
Efforts in the case resulted in an equitable undisclosed settlement for the plaintiff.
John M. Tobias, Ph.D., P.E., CFEI, Electrical Engineering & Illumination Expert with DJS Associates, Inc., can be reached via email at experts@forensicDJS.com or via phone at 215-659-2010.
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