Exemplary stability problems of motorized wheelchairs have been discussed in U.S. Pat. No. 6,202,773 and U.S. Pat. No. 6,615,937 with examples of typical front- and rear-wheel driven wheelchairs. As stated in these patents, on the one hand, rear-wheel driven wheelchairs are generally more stable than front-wheel driven wheelchairs, which allow for higher maximum speeds than the former. On the other hand, front-wheel driven wheelchairs generally have the advantage of better maneuverability and allow a user to get closer to an object, for example, in front of the wheelchair user. In general terms, rear-wheel driven wheelchairs are considered to be advantageous for outdoor use, where stability and high speed are preferred, while front-wheel driven wheelchairs are the preferred vehicle for indoor use.
In certain circumstances, wheelchairs can experience stability problems such as skidding, spinning out, or tipping due to loss of traction. Loss of traction can result from effecting a sharp curve while driving at high speed, for example. Furthermore, loss of traction may be observed when the surface is in some way slippery, for example, on a wet track, an icy surface, or sandy or dusty roads. A risk following loss of traction, particularly with front-wheel driven wheelchairs, is a spin-out or breakout which may cause the vehicle to suddenly and violently turn and sometimes spinout. This may leave the wheelchair facing in a different direction than when the spin-out started and may cause the wheelchair to tip over. A common way of dealing with this is to limit the speed of such wheelchairs.
A typical stability problem encountered with motorized wheelchairs is veering off course. For purposes of this application, “veering” refers to the wheelchair being off course from a desired course indicated by an input device for the wheelchair. In this manner of speaking, veering may be distinguished from loss of traction for purposes of determining whether the wheelchair is off course and subsequent measures to be implemented. Veer, for example, may occur when driving along a slope. Rear-wheel driven wheelchairs have a tendency of veering downwards instead of, for example, maintaining a straight path perpendicular to the slope. Front-wheel driven wheelchairs tend to veer upwards with respect to the slope. More generally, veering may occur when an external force acts on the wheelchair and thus changes the route the user has indicated via an input device. For example, wind blowing in a cross direction to a wheelchair's driving direction may also cause veering.
In U.S. Pat. No. 6,202,773 and U.S. Pat. No. 6,615,937, which are hereby incorporated by reference in their entirety, a rate-of-turn sensor (such as a gyroscope, an angular rate sensor (e.g., piezoelectric ceramic gyro, model no. CG-16D by TOKIN America), or a combination of accelerometers) is provided to sense or determine an actual turn rate of the wheelchair. The controller for the wheelchair may detect veering by assessing the difference between, for example, a turn rate command and the actual turn rate. Accordingly, inputs from the user's command input device and from the rate-of-turn sensor are compared in order to determine if the wheelchair's actual course is different from the desired course. In case of a difference, the wheelchair's speed may be limited and/or the turn rate may be limited or corrected. U.S. Pat. No. 6,615,937 illustrates a system that includes a second rate-of-turn sensor to detect failure of the first rate-of-turn sensor by comparing the outputs of each rate-of-turn sensor to one another. A failure of the first rate-of-turn sensor can be determined by a change in this comparison.
In U.S. Pat. No. 6,202,773 and U.S. Pat. No. 6,615,937 it is noted that an offset voltage associated with the rate-of-turn sensor may vary with temperature. These patents teach a controller that can determine the offset voltage to correct for temperature offset in the rate-of-turn sensor when the wheelchair is at rest. Temperature changes occurring while the wheelchair is being driven may cause an offset voltage error that may be corrected by the controller the next time the wheelchair is at rest.