Education in Motion / Clinical Corner / December 2016 / Steer Correction for Power Mobility

Steer Correction for Power Mobility

Have you ever driven your vehicle and felt like you had to correct the steering to continue to go in the direction you intended? Most likely, the answer is yes. This may have occurred if you drove over a bumpy dirt road or other rough terrain. It may have occurred if two wheels were on one surface, such as a road, and the other two wheels were on another surface, such as a soft shoulder, or when there was a transition between two surfaces. Drivers of power wheelchairs also may experience circumstances when correction of the steering is required. This month, Clinical Corner will focus on technical solutions to address steer correction in power mobility.

In a vehicle, we notice when our wheels need alignment when the car consistently veers or pulls to one side, and we have to steer to correct this. In a power wheelchair, this may occur if there is an imbalance in the motor values due to active wear and tear. (Recall from the Clinical Corner article De-mystifying Power Wheelchairs: Batteries and Motors that electric wheelchairs are operated by 2 motors.) If a qualified service technician determines that the cause of the veering of the wheelchair is a result of an imbalance that can be corrected through programming, the technician can use the steer correct programming parameter to make changes that will enhance the tracking of the wheelchair and eliminate the veering. (See Power Programming Basics for an overview on programming.)

Typically, however, the need for steer correction is felt in everyday power wheelchair driving, even in new wheelchairs, such as when climbing curbs, thresholds or small obstacles, making flooring or grade transitions, and when driving on uneven terrain. Directional stability, or tracking, depends on the extent to which the wheelchair "wants" to turn, even when the individual wants to go in a straight line. (For more on directional stability of wheelchairs and drive wheel position, refer to Power Mobility: Comparing Mid-Wheel, Rear-Wheel and Front-Wheel Drive.) Depending upon the physics involved, the wheelchair may drift to the left or right. Steer correction technology can be added to motors to provide assistance to ensure the wheelchair tracks in a straight line, reducing the need for the individual driver to make steering corrections.

One type of steer correction technology is encoders. Encoders are designed to reduce the drive corrections needed to maintain the direction and speed of a wheelchair due to the variability of terrain. This means that encoders help to maintain the intended speed going up or down hill, across a side slope, or going in a straight line, while maintaining the desired direction. Encoders work by sending feedback multiple times per second on the revolutions per minute (RPM) of each motor to the encoder module. If one of the motors is not turning as quickly as the other motor, despite receiving equal current (indicating that the driver intends to drive in a straight line), the encoder will send more current to the slower motor to allow the wheelchair to run in a straight line. Below is a graphic illustration that shows the benefit of steer correction on a mid-wheel drive wheelchair.

While we can see that an individual using a standard joystick can benefit from using steer correction technology to enhance ease of driving, steer correction technology is an important consideration for individuals who use specialty input devices. When an individual is unable to use a standard joystick to operate a power wheelchair, either proportional or non-proportional specialty drive controls may be used. (See Specialty Controls for Power Wheelchairs for more information on specialty input devices.) As non-proportional drive controls are programmed for a set speed and direction, it may be frustrating and fatiguing for an individual if steer corrections are constantly required throughout the course of the day. Encoders work well for individuals using specialty controls to help the wheelchair maintain the desired speed and direction, without needing additional corrections.

Steer correction technology should be considered for individuals who will be driving power wheelchairs. While not every individual who drives a power wheelchair will require this technology, it is a tremendous benefit to those who are using specialty input devices and/or those who are very active users in their power wheelchairs, traversing over different terrains and flooring.

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