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Abstract—An automated wheelchair with a guide sensor, guided by a magnetic ferrite marker that is resistant to the presence of dirt, is described. The wheelchair permits the severely disabled, as well as the older population, to move about freely, both indoors and outdoors. This is due to the simple operation involved (pushing a button), and a magnetic ferrite marker lane which is minimally influenced by dirt or other nonmagnetic materials. For increased comfort, a nonlinear signal-processing circuit and pulse-steering drive method have been developed to provide a smooth running operation. In addition, a function that provides for stopping at desired destinations has been added for convenience, and a collision prevention function using infrared sensors has been added for safety.
Key words: automated wheelchair, biomechanics, ferrite marker, magnetic sensor.
INTRODUCTION
In response to the demands of wheelchair users for equal access, hand-propelled wheelchairs, electrically-controlled wheelchairs, and automated-guided wheelchairs (AGW) have been developed. However, because upper body strength is required, a hand-propelled wheelchair does not permit an older or severely disabled person an extensive range of travel. An electrically-controlled wheelchair can be controlled by a manually-operated joystick—but because this type of wheelchair sometimes zigzags, and a slight
Address all correspondence and requests for reprints to: Mr. Hiroo Wakaumi, Resources and Environment Protection Research Laboratories, NEC Corpora¬tion, 4-1-1, Miyazaki, Miyamae-ku, Kawasaki 213, Japan.
movement of a joystick can cause a quick turn, driving an electrically-controlled wheelchair requires the opera¬tor to be skilled both in turning, and in direction-change operations—especially on narrow or curved roads. There¬fore, it is difficult for many severely disabled and/or elderly persons to operate them skillfully.
Conventional AGWs, which are guided by reflective tape markers laid on the road or floor, are influenced by dirt on the tape. That is, when reflective tape markers are coated with dirt or mud, the photo-detection sensor (with photo diodes) installed on the wheelchair cannot dis¬criminate these markers from the surrounding road or floor surface; therefore, the user cannot control the steering wheel, making this type of wheelchair difficult to use.
In addition to the photo-detection guidance technique for the AGWs, other automatic vehicle guidance techniques, such as visual machine guidance and buried-wire guidance systems might be used (1). As a visual machine guidance technique, optical guiding along a painted track, using a video camera, has been proposed (2). Since much process¬ing time is necessary for this guidance system to recog¬nize the position of the track in front of the AGW, it is impossible to move very fast; this technique is also not useful in a dirty, rain-covered, or snowy environment, be¬cause the camera cannot recognize the track covered with dirt or snow.
When using a buried-wire guidance system, if the wire is cut for some reason, it cannot operate due to lack of a magnetic field generation. Changes in the guidance lane location are awkward because the wire must be buried. Therefore, in addition to a high implementation cost, this guidance system lacks reliability.
DOI:10.1682/JRRD.1992.01.0 02 7he magnetic ferrite marker technique is useful because the vehicle can move relatively quickly (due to simple recognition of the marker position by magnetic sensors) and is not influenced by a local marker being cut off (3).
This paper describes an automated wheelchair guided by a magnetic ferrite marker which is relatively free from adverse influence by dirt or mud and because of its sim¬ple operation, permits easy use by severely disabled and older people (4). This wheelchair would be especially use¬ful for mentally alert people with severe motor impairment (e.g., quadriplegics and cerebral palsy patients), since it allows them to move wherever they want to go along a laid¬out route, merely by a simple push-button operation. A nonlinear circuit and a pulse-steering drive method (de¬veloped to achieve a smooth running operation), an infrared sensor system for stop operation, and safety functions are described. Investigation results were derived from the run¬ning characteristics of this wheelchair.
GUIDANCE TECHNIQUE PRINCIPLE
Figure 1 shows an AGW with a magnetic sensor guided by magnetic ferrite markers which are laid in/on the sidewalk or floor (3,5). Any electric motor-driven wheelchair can be partially modified by installing the magnetic sensor, signal-processing circuit, and operation button, allowing a change in its mode of operation. The magnetic sensor is installed under the footrests at the front of the wheelchair to control the steering wheels. This sensor, which is 7 cm from the road surface or floor, picks up guidance signals from magnetic ferrite markers. The magnetic ferrite marker lane, using soft ferrite material bound in place with resin, is 10 cm wide, 5 mm thick, and can be extended as far as necessary.
Figure 2a shows the magnetic sensor and ferrite marker configuration. Figure 2b shows the magnetic sensor signal-processing circuit. The magnetic sensor consists of an exciting coil, L, at the center of the sensor unit and two detecting coils, II and 12, placed on its right and left sides. Exciting coil L generates a magnetic field. The ferrite marker is magnetized by this field and sets up a new resonant magnetic field; the result being that the original magnetic field is deviated. Detecting coils, LI and L2, pick
up the magnetic field deviation. The detected output sig¬nals obtained by two detecting coils, VLI and VL2, reduce linearly, as shown in Figure 3, as the sensor position devi¬ates from the center of the marker lane. These signals reach a peak level and then reduce to a small value as the sensor position deviates further. These characteristic signals cross at the center of the marker lane. These output signals are subtracted from each other to obtain an S-shaped charac¬teristic, suitable for use as a steering control signal. As a result, since the sensor output signal (i.e., the difference between these detection-output signals), is proportional to the wheelchair deviation from the center of the ferrite mar¬ker lane in a line or curved form, the difference signal per¬mits controlling the movement direction of the wheelchair. For example, consider the case where the sensor position deviates slightly from the center of the marker. When the sensor output signal voltage is increasing to a higher posi¬tive level, a controller for governing the steering motor rotation direction permits it to rotate the forward wheels in such a direction as to bring the sensor position back to the center of the marker lane. When the sensor output voltage is decreasing to a lower negative level, the con¬troller permits the forward wheels to rotate in the oppo¬site direction. Thus, the wheelchair can be controlled in a route approximately along the center of the marker lane by using the magnetic sensor.
Smooth running operation for comfort
In order to provide a comfortable ride, a partially steering-free operation by a nonlinear circuit and pulse-steering drive method was developed.
In general, when the wheelchair's steering is controlled according to a sensor output signal indicating sensor posi¬tions (steering angle changes linearly with the sensor out¬put signal), the wheelchair movement tends to oscillate slightly. This oscillation phenomenon occurs when a sys¬tem, including the sensor, signal-processing circuit, con¬troller, drive motor, drive wheels, and magnetic marker, is set up for a certain oscillation condition. The magni¬tude of the steering angle changes almost linearly with the sensor output signal. Since a steering control signal (sen¬sor output signal) is applied continuously to the wheel¬chair, it is almost immediately brought back toward the center of the route whenever it deviates slightly from the center of the marker. In this case, the wheelchair could move from side to side at a certain variation rate (a rela¬tively higher perturbation frequency than a few cps). When this return operation frequency matches and reinforces a system oscillation frequency, the result is the system oscil¬lation frequency being different from the ordinary inter¬mittent and irregular steering control state. Once the system starts spurious oscillating, it is difficult for the system to shed this movement variation.
To prevent this wheelchair from wobbling during operation, a steering-free operation within a small devia¬tion range (i.e., a small area along the center of the ferrite marker lane when the magnetic sensor deviates slightly) is realized by a nonlinear signal-processing circuit (Fig¬ure 4). This circuit consists of an amplifier and several diodes, used to shift about 1 to 2 V of the sensor output voltage point, when the steering control voltage rises. When switch A is on, about ± IV steering-free operation voltage range is obtained. When both switches A and B are off, about + 1.5V steering-free operation voltage range is obtained. Even when the magnetic sensor deviates within these steering-free operation ranges, accurate steering con¬trol is not immediately achieved. In this case, system steer¬ing control oscillation is retarded, because the return
NEW TECHNIQUES FOR HIGH LEVEL PERFORMANCE
Safety, comfort, and convenience are very important for wheelchair users: therefore, several considerations are required to realize a high-level performance wheel¬chair system.
operation from side to side becomes slower than the oscillation, enhancing safe driving.
The wheelchair also has a tendency to zigzag (not oscillate) due to any quick movements of the steering wheel when partially steering-free operation is achieved by the nonlinear circuit. That is, after the steering angle rises sharply when steering control starts in a conventional continuous steering drive method, subsequent steering movements continue to be controlled within the steering operation ranges. Therefore, the steering wheel movement sometimes overruns to the other side and follows a zigzag course. If the wheel movement inertia moment is too large at a subsequent change in its movement direction by the steering control signal, the wheelchair cannot change this moment direction immediately. This zigzag running is a slow movement with a large arc locus and differs from the above mentioned oscillating operation.
To minimize this zigzag running to the smallest extent, a new pulse-steering drive method is used within a small steering wheel deviation range. With the pulse-steering drive method (Figure 5), intermittent drive pulses are added to the sensor output signal and passed through the nonlinear circuit. As a result, steering is gradually changed from a lower sensor output voltage than that required for the steering-free limit (does not cause steering to change greatly when steering control starts). The intermittent drive pulse variation consists of a positive pole pulse, when the sensor output signal is positive, and a negative pole pulse, when the sensor output signal is negative. Its variation fre¬quency is 10 times per second (with a 10 percent to 25 percent duty range). With this drive method, a gradual steering control is achieved. When the wheelchair devi¬ates a little from the center of the marker lane, the control steering angle is set relatively small. As it deviates fur¬ther from the marker lane center, the steering control angle becomes relatively greater. Thus, a delicate steering wheel angle control is possible and is not expected to cause over¬run. As a result, the wheelchair will move smoothly without zigzag running.
Automatic stop operation for convenience
People who use wheelchairs may be severely visually-disabled or lack the full use of their hands. When the user needs to move to several destinations in sequence, the auto¬matic stop operation is required at specified positions. (This route can be set up before wheelchair movement starts.) To permit the wheelchair to stop automatically at desired destinations, an infrared position detection sensor system has been constructed which detects the reflection tape previously placed at the destination (laid on the floor or sidewalk). The reflecting tape material has a different reflection coefficient from that of the surrounding floor or pavement surface. In experiments, road sign reflecting material now on the market was used as reflecting tape. When the position detection sensor on the wheelchair detects this reflecting tape, it automatically stops the wheel¬chair movement.
When there are several destinations, the sensor counts the number of tape markers as the wheelchair passes them so that the user can move easily to a desired destination.
Emergency stop operation for safety
Maintaining safety is important for every user. To pre¬vent a possible collision with people, chairs, animals, etc., two infrared obstacle detection sensors were constructed on the front of the wheelchair. When an obstacle appears in front of the wheelchair, it detects the obstacle, stops, and remains stationary until the obstacle is moved out of the path of the wheelchair.
Figure 6 shows a control circuit block diagram. A selection switch used to select either automatic mode or manual mode is provided for achieving flexible movement. (For example, the manual mode allows users to operate the wheelchair even in locations where the marker lane has not been laid out.) In the automatic mode, an automatic-manual selection switch circuit permits current to flow in an electromagnetic relay and selects the automatic mode. In this case, the magnetic sensor output signal is used for steering control through the nonlinear circuit and the pulse generator. Also, the velocity, predetermined in a velocity regulation part, is used for drive motor speed control (front wheel movement speed control).
However, when an obstacle or position detection sensor detects either an obstacle in the path or reflecting tape laid at the destination, the operation mode is automatically changed to manual mode and the wheelchair stops. The joy¬stick is held in the center of its control area and steering- and drive-motor control signals, supplied from a handle opera¬tion, are not present. In the manual mode, steering and speed are completely controlled by operating the joystick.