Wheelchair and method for controlling the assist power of a power assist wheelchair

Abstract: A wheelchair comprises a human power drive means, an assist power drive means and an assist power control means. This assist power control means controls an assist power commensurate with the magnitude of the human power detected by a human power detection means. Said assist power control means is adapted to calculate the assist power applicable to one drive wheel in accordance with the magnitude of the human power applicable to both drive wheels

Description: The present invention relates to a method for controlling the assist power of a power assisted wheelchair according to the preamble portion of claim 1. Furthermore, the present invention relates to a wheelchair according to the preamble portion of claim 8. As an intermediate existence between the manual wheelchair and the motor-operated wheelchair, a power-assisted wheelchair has been proposed. The power-assisted wheelchair is constituted to detect human power intermittently applied to the left and right drive wheels, and to apply assist power commensurate with the detected human power to the left and right drive wheels to alleviate the physical effort of the rider handicapped in walking. The rider can operate it with the same feeling as that with the manual wheelchair and is also relieved from mental pain. The wheelchair comprising a human power drive means for driving said wheelchair by human power, an assist power drive means for driving said wheelchair in support of said human power, and a control means for controlling an assist power as well as a method for driving such a wheelchair are known from EP 687 454. According to EP 687 454, the wheelchair is driven solely by an assist power for a period of time after the supply of human power has stopped. Furthermore, according to EP 687 454, the assist power for each of the drive wheels is independently calculated in accordance with the human power applied to the hand rim of the respective drive wheel by the rider and the grip for a helper on the side of the respective drive wheel. Since the power-assisted wheelchair is constituted that an assist power in proportion to the human power applied to a wheel is added to the wheel, turning motion (yaw motion) is more likely to occur with increased propulsive power, and straight running property could be adversely affected. Furthermore, since an arrangement is employed in which the assist power is added only when the human power is applied, a problem may occur on an uphill for instance that the wheelchair suddenly loses speed and stops as soon as the human power application is stopped. Another problem is that, since the assist power is added to both forward and reverse directions, after the human power application is discontinued the motor and the drive system adversely serve as loads and offset the effect of the assist power. Therefore, the applicant has developed a power-assisted wheelchair arranged that the assistpower remains even after the human power application has ceased, and has submitted an application. The power-assisted wheelchair developed as described above has no problem as long as the power characteristic is completely identical for both left and right assist power systems, and the human power is completely identical for both left and right wheels. However, when different magnitudes of human power are inputted to left and right drive wheels as in the turning motion on a level ground, the ordinary wheelchair without an assist power makes a turning motion due to difference in the drive power during the input. When the input is over, the wheelchair moves almost straight forward in the direction in which the wheelchair is directed when the input is over. With the power-assisted wheelchair developed as described above, on the other hand, since assist powers on respective drive wheels remain independently in different magnitudes each other, undesirable turning motion remains even after the human power input, is stopped and the rider has an inconsistent feeling. Furthermore, even if a completely identical human power is inputted, variations in manufacture of the staicture from the input sensor to the drive system cause difference in magnitude and duration of the remaining power, which also causes a turning motion. It is an objective of the present invention to provide an improved method for controlling the assist power of a power assisted wheelchair as indicated above which allows a supply of an optimum assist power to an assist power drive means with a high reliability. This objective is solved by a method for controlling the assist power of a power assisted wheelchair having the features of claim 1. In order to provide a more stabilized run or drive, respectively, it is advantageous when said assist power in addition is calculated in accordance with the resultant power applied to both drive wheels. A better adaptation to the needs of the user is obtainable when amplification ratios and combination ratios preset according to physical conditions of a user are stored for controlling the respective assist power. Further preferred embodiments of the method as indicated above are laid down in the respective dependent claims. With regard to an apparatus aspect, it is an objective of the present invention to provide an improved wheelchair as indicated above, which is adapted to allow a supply of an optimum assist power to an assist power drive means with a high reliability. This objective is solved by a wheelchair having the features of claim 8. According to a preferred embodiment of the present wheelchair the assist power control means is adapted to calculate said assist power in addition in accordance with the resultant power applicable to both drive wheels. In case the assist power should remain even after the human power input has been stopped, it is advantageous when the assist power control means is adapted to maintain the assist power component calculatable in accordance with the resultant power of the applicable human power even after the supply of said human power has been terminated. A more comfortable wheelchair is obtainable when both of said drive wheels are provided with assist power drive means and assist power control means. According to an advantageous embodiment of the present invention, said two assist power control means are interconnected with each other for information exchange. According to another preferred embodiment of the present wheelchair, said assist power control means comprising a sensor drive I/F for inputting human power applied detected by the human power detection means, a CPU for calculating target values, a motor I/F, a motor drive for feedback-controlling the assist power drive means, and a communication I/F for interconnecting left and right CPU's. Other preferred embodiments of the present wheelchair are laid down in the respective dependent claims. In the following, the present invention is explained in greater detail with respect to several embodiments thereof in conjunction with the accompanying drawings, wherein: FIG. 1 is a side view of an embodiment of the power-assisted wheelchair; invention; FIG. 2 is a plan view of the above wheelchair; FIG. 3 is a rear view of the above wheelchair; FIG. 4 is an axial view of the hub portion, with its cover removed, of a wheel of the above wheelchair; FIG. 5 is a cross-sectional view taken along the line A-A in FIG. 4; FIG. 6 is a rear view of the hub portion of a wheel of the above wheelchair; FIG. 7 is a cross-sectional view taken along the line B-B in FIG. 6; FIG. 8 is a cross-sectional view taken along the line C-C in FIG. 7; FIG. 9 is a block diagram showing a constitution of a controller for the above wheelchair; FIG. 10 shows a relationship between human power input signal and target torque with the assist ratio as a parameter; FIG. 11 is a characteristic graph of human power and output torque; FIG. 12 shows a system constitution for controlling the assist power for the above wheelchair; FIG. 13 is a flow chart for describing the control action for the assist power for the above wheelchair; FIG. 14 is a flow chart for describing the control action for the assist power for the above wheelchair; FIG. 15 is a flow chart for describing the control action for the assist power for the above wheelchair. Embodiments of the invention will be hereinafter described in reference to the appended drawings. FIGs.1 to 15 are drawings for describing an embodiment of the power-assisted wheelchair. FIG. 1 is a side view of the wheelchair. FIG. 2 is a plan view of the wheelchair. FIG. 3 is a rear view of the wheelchair. FIG. 4 is an axial view of the hub portion of a wheel with the wheel cover removed of the wheelchair. FIG. 5 shows a cross section taken along the line A-Ain FIG. 4. FIG. 6 is a back view of the wheel hub portion of the wheelchair. FIG. 7 shows a cross section taken along the line B-B in FIG. 6. FIG. 8 shows a cross section taken along the line C-C in FIG. 7, partially broken away. FIG. 9 is a block diagram showing a constitution of a controller for the wheelchair. FIG. 10 is a graph of relationship between the input signal and target torque with the assist ratio as a parameter. FIG. 11 is a graph of input signal characteristic. FIG. 12 is a diagram showing control actions of an assist power for the wheelchair. FIGs. 13 to 15 are flow charts for describing the assist power control actions for the wheelchair. The power-assisted wheel chair 1 of the embodiment is made by attaching a power assist system to an exsting wheelchair of folding, manual type. The wheelchair 1 is constituted by attaching removable wheels 2 as drive wheels on the left and right sides of a vehicle body. The front and rear portions of a frame 3 made of pipe materials are supported with paired left and right casters 4 and wheels 2 for free movement of the vehicle. A canvas seat 5 (See FIGs. 2 and 3) for a rider to seat on is stretched in the center of the frame 3. As shown in FIG. 3, the frame 3 has paired front and rear cross members 3a crossing each other in X shape with their intersection pivoted with a shaft 6. Paired left and right handle arms 3b are erected in the rear parts of the frame 3. The upper parts of the handle arms 3b are bent rearward and provided with grips 7 for a nursing person. Paired left and right arms 3c extending horizontally forward from the middle height points of the handle arms 3b of the frame 3 are bent by about right angles at their front ends vertically downward and their lower ends are provided with casters 4 for free rotation. At a portion of the arm 3c located on the right as seen from a rider on the seat 5 and bent at about right angles (upper part of the vertical portion), is attached a main switch 8. Front parts of paired left and right arms 3d disposed below the arms 3c extend obliquely down forward and their extended (front) ends are provided with paired left and right steps 9. As shown in FIG. 5, each of the paired left and right wheels 2 is supported through ball bearings 12 on a wheel shaft 11 supported on a boss 10 welded to the frame 3 and its outer side is provided with a ring-shaped hand rim 13 to be turned by hand by the rider. A disk 14 is supported for rotation through a bush 60 on the boss portion 2a-1 formed on the hub 2a of the wheel 2. The hand rim 13 is attached to the disk 14 through three spokes 15 with a bolt 16. Therefore, the hand rim 13 can rotate independently of the wheel 2. By the way, in this embodiment, as shown in FIG. 5, a sealing 17 made of an elastic material is interposed between the hub 2a of the wheel 2 and the disk 14 covered with a cover 19 secured with a bolt 18. The sealing 17, with sealing function, serves also as a damper for restricting vibration in the circumferential direction of the disk 14. Here, the hand rim 13 is elastically connected to the wheel 2 at three circumferential points with the structure shown in FIG. 4. In other words, as shown in FIG. 4, a spring 21 is disposed in each of spaces of a shape widening radially outward and formed between paired stoppers 20 formed in the hub 2a of the wheel 2. Each of the springs 21 is prevented from coming off by means of a holding member 22 secured to the hub 2a. Both ends of the spring 21 are received with spring receivers 23. In a neutral state in which no human power is applied to the hand rim 13, as shown in FIG. 4, the spring receivers 23 are in contact with the paired stoppers 20. By the way, a groove 20a is formed through the center of each of the stoppers 20. On the other hand as shown in FIG. 4, three brackets 24, with their positions adjustable, are attached to three circumferential positions on the disk 14. Both ends of each of the brackets 24 are provided with paired pins 26 extending inward. The paired pins 26 are in contact with the end surfaces of the spring receivers 23 in a neutral state of no human power being applied to the hand rim 13 as shown in FIG. 4. Both end portions of each bracket 24 are provided with elongate holes 24a extending in the radial direction. A bolt 25 is inserted in each of the elongate holes 24a. By loosening the bolts 25, the bracket 24 may be displaced in the radial direction for adjusting its position. Since the end surfaces (for the pin 26 to be in contact with) of the spring receiver 23 are tilted as shown, by displacing the bracket 24 in the radial direction in the neutral state as described above, the positions of the pins 26 may be adjusted relative to the spring receiver 23 so that the paired pins 26 are respectively brought into contact with the spring receivers 23. As shown in FIGs. 4 and 5, a potentiometer 27 with its position adjustable for zero point calibration for detecting magnitude and direction of human power applied to the hand rim 13 is secured to the disk 14 of the hand rim 13. One end of a lever 28 is secured to one end of an input shaft 27a of the potentiometer 27. The other end of the lever 28 is connected through a rubber cap 30 to a pin 29 projecting from the hub 2a of the wheel 2. The rubber cap 30 is for preventing the lever 28 from becoming loose. Here, the spring 21, the potentiometer 27, and others constitute human power detection means for detecting the human power applied to the hand rim 13 by the rider. The human power detection means is housed in a closed space surrounded with the hub 2a of the wheel 2, the disk 14, and the cover 19. On the other hand as shown in FIGs. 5 and 6, a disk-shaped fixed plate 31 is secured to a wheel shaft 11 on the inner side, with respect to the vehicle width direction, of the hub 2a of each of the paired left and right wheels 2. A cylindrical holding member 32 covering a boss portion 2a-2 of the hub 2a of the wheel 2, and a holding ring 33 are secured with a bolt 34 to the inside surface, on the hub 2a side, of the fixed plate 31. A controller 35 is also disposed on the inside surface, on the hub 2a side, of the fixed plate 31. As shown in FIGs. 6 and 7, a drive motor (assist power source) 36 and a wheel side coupler 37 are attached to the outer, vehicle body-facing side of the fixed plate 31. As shown in FIGs. 5 and 6, a plural number of vertical heat radiation grooves 31a are formed on the outside surface of at least part of the fixed plate 31 where the controller 35 is disposed. Here, an inside space defined with the hub 2a of each of the wheels 2 and the fixed plate 31 is divided into chambers S1 and S2 with a ring-shaped partition wall 38 secured to the fixed plate 31 and the holding ring 33. As shown in FIG. 7, an opening 38a is formed in part of the partition wall 38. A ring-shaped inner transformer 39a is secured to the boss portion 2a-2 of the hub 2a on the rotating side. An outer transformer 39b is interposed between the holding member 32 and the holding ring 33 on the fuxed side. The inner transformer 39a and the outer transformer 39b are coaxially disposed with a small gap in between to constitute a rotary transformer 39 constituting signal transmission means between the controller 35 and the potentiometer 27. The controller 35 is disposed in the chamber S1. Here, the assist power produced with the drive motor 36 is transmitted to the wheels 2. The power transmission mechanism will be described in reference to FIGs. 7 and 8. The power transmission means comprises components including pulleys 40, 41, a belt 42, and a plural number of gears G1 to G4. The pulley 40 of a smaller diameter is secured to the end of an output shaft 36a of the drive motor 36. The pulley 41 of a larger diameter is secured to one end of an intermediate shaft 43. The endless belt 42 is routed around the pulleys 40, 41. As shown in FIG. 7, the intermediate shaft 43 and a drive shaft 44 parallel to the former are rotatably supported through bearings 46, 47 with the fixed plate 31 and a cover 45, respectively. The intermediate shaft 43 is integrally formed with the gear G1 engaging with the gear G2 secured to one end of the drive shaft 44. The other end of the drive shaft 44 penetrates an opening 38a formed in the partition wall 38 (Refer to FIG. 5) and extends into the chamber S2. The gear G3 of a smaller diameter integrally formed with the extended end of the drive shaft 44 engages with a ring gear G4 of a larger diameter secured to the inside circumference of the hub 2a. By the way, since the pulleys 40, 41, the belt 42, and the controller 35 should be free from lubrication oil, they are housed in the chamber S1 separated with the partition wall 38 while the gear s G3, G4 are housed in the chamber S2. As described above, an assist power system is constituted with; the human power detection means consisting of the spring 21 and the potentiometer 27, the signal transmission means consisting of the rotary transformer 39, the control means consisting of the controller 35, and the power transmission means consisting of the drive motor 36, the pulleys 40, 41, the belt 42, and the Gears G1 to G4. The assist power system is disposed as compact as possible with respect to radial and axial directions around the wheel shaft 11 of the hub 2a of each of the wheels 2. The two wheels 2 of the identical structure each consisting of the assist power system disposed at the hub 2a are removably attached to left and right side of the vehicle body. Here, the removable attachment structure of the wheel 2 will be described in reference to FIG. 5. The wheel shaft 11 supporting the wheel 2 for rotation is formed hollow with a rod 48 of a small diameter passing through. To the inside end portion of the rod 48 is secured an engage-stop member 49 engaging with the inside end surface of the wheel shaft 11. To the outside end portion of the rod 48 is secured a pressing member 50. The engage-stop member 49 and the pressing member 50 having greater diameters than that of the rod 48 are slidably inserted in the wheel shaft 11. The rod 48, the engage-stop member 49, and the pressing member 50 are constantly urged outward (to the right in FIG. 5) with a spring 51. A snap ring 61 in FIG. 5 serves as a stopper. The inside end portion (where the engage-stop member 49 is fitted) of the wheel shaft 11 is formed with a plural number of round holes 11a in which balls 52 are retained. In the central portion of the cover 19 is fitted a flexible rubber cap 53 the inside of which faces the pressing member 50. On the other hand, a cylindrical sleeve 54 is inserted in the boss portion 10 welded to the frame 3. The sleeve 54 is secured to the boss portion 10 with a nut 55 which is in screw engagement with the outside circumference of the sleeve 54. Here, each of the wheels 2 is attached to the vehicle body by inserting the inside end portion of the wheel shaft 11 from outside into the sleeve 54. In the state of the wheel 2 attached, as shown in FIG. 5, the balls 52 are pushed radially outward to project from the outer circumferential surface of the wheel shaft 11 and made to engage with the inside end surface of the sleeve 54. As a result, the wheel shaft 11 is prevented from coming off and the wheel 2 is securely attached to the vehicle body. Next, in order to remove the attached wheel 2 from the vehicle body, the rubber cap 53 should be pressed by finger to displace the pressing member 50, the rod 48, and the engage-stop member 49 as a whole toward the inside of the vehicle against the urging force of the spring 51. Then, the engage-stop member 49 retracts from the position of the balls 52, and the small diameter rod 48 is located in the position of the balls 52. As a result, the balls 52 move radially inward of the wheel shaft 11 to be recessed from the outer circumferential surface of the wheel shaft 11. If the wheel 2 as a whole is pulled outward in that state, the wheel shaft 11 may be taken out of the vehicle body. Therefore, the wheel 2 may be easily removed from the vehicle body by a single hand operation. In order to reattach the wheel 2 to the vehicle body, the wheel shaft 11 is inserted into the sleeve 54 while the pressing member 50, the rod 48, and the engage-stop member 49 toward the inside of the vehicle body by pressing the rubber cap 53 by a finger, and then the finger is removed from the rubber cap 53. Then, the balls 52 are pushed in the radial direction out of the outside circumferential surface of the wheel shaft 11 and engage-stopped with the inside end surface of the sleeve 54. Thus, the wheel shaft 11 is prevented from coming off. In this way, the wheel 2 is easily attached to the vehicle body by a single hand operation. As shown in FIGs. 5 and 6, a rotation stop member 56 opening in a U shape toward the outside of the vehicle body (namely in the removal direction of the wheel 2) to the outside circumferential edge of the fixed plate 31 of each of the wheels 2. An engage-stop member 57 is secured to the frame 3. When the wheel 2 is attached to the vehicle body as described before, the rotation stop member 56 fits into the engage-stop member 57 to prevent the fixed side including the fixed plate 31 from rotating. By the way, the power-assisted wheelchair 1 of this embodiment as shown in FIGs. 1 to 3 is provided with a removable battery 58 attached on the right wheel 2 side. A wiring harness 59 is disposed on the vehicle body (frame) 3 side. Here, since the left and right wheels 2 is of the identical structure as described above, when they are attached to the vehicle body, they are disposed in symmetric positions with respect to the longitudinal center of the vehicle. With such an arrangement of the left and right wheels 2 of the identical structure, as shown in FIG. 3, the inward projecting drive motors 36 are disposed in different height from each other so that they do not interfere with each other when the wheel chair 1 is folded. As a result, the wheelchair 1 is folded easily in a compact size. Once the left and right wheels 2 are attached to the vehicle body by the procedure described before, and the coupler 37 is connected to a coupler 59A of the wiring harness 59 disposed on the vehicle body side, electric power is supplied from the battery 58 disposed on the right wheel 2 through the wiring harness 59 to the drive motor 36 and the controller 35 disposed on the left wheel 2. Next, the constitution of the controller 35 will be described in reference to FIG. 9. FIG. 9 shows a block diagram showing the constitution of the controller 35 which comprises; a sensor drive I/F 70 for inputting the human power applied to the hand rim 13 and detected with the potentiometer 27 through the rotary transformer 39, a CPU 71 for calculating a target value of the assist power based on the input human power, a motor output I/F 72 for interconnecting the CPU 71 and the drive motor 36, a motor driver 73 for feedback-controlling the value of current applied to the motor 36 so that the output torque of the motor 36 becomes the target torque calculated as described above, and a communication I/F 74 for interconnecting left and right CPUs 71. Furthermore, the left and right communication I/Fs 74 are interconnected through serial cables (serial communication means) 75. The magnitudes of the left and right human powers inputted as described above are transmitted through the communication I/Fs 74 to the left and right controllers 35 each other. In this way, when the human powers FL, FR are intermittently applied to the hand rim 13, the human powers are detected with the potentiometer 27, and the detected signals Vin are inputted to the controllers 35 Each of the CPUs 71 of the left and right controllers 35 calculates a target torque τ according to an assist ratio required for the input signal Vin outputted from the potentiometer 27 and outputs a control signal commensurate with the target torque τ through a motor output I/F 72 to a motor driver 73. FIG. 10 shows the relationship (characteristics of the motor output I/F 72, and the motor driver 73) between the input signal Vin and the target torque τ with the assist ratio as the parameter. As apparent from the figure, while the value of the input signal Vin is between Vi1 and Vi2, the target torque τ is zero, which forms an electrically insensitive zone. Here, control of assist torques τL', τR' supplied to the left and right wheels 2 will be described in reference to FIG. 12 which shows a system constitution of control actions for the assist power for the wheelchair 1 of this embodiment. In the controller 35 are stored amplification ratios KL, KR, KM and combination ratios α, β preset according to physical conditions of the rider. In this embodiment, assuming that the rider's left arm is stronger than the right arm, the values are preset as KL = α = 0.4, KR = β = 0.6, and KM = 1.0 First, products of input signals from the human power detection means 27 constituted with the potentiometer 27, namely the human powers FL, FR and the amplification ratios KL, Kr are calculated, and assist powers Assist L, Assist R are calculated for assisting, for example, turning force during a turning when the human powers are being inputted. A product of the human power FL and the combination ratio α, and a product of the human power FR and the combination ratio β are calculated. A product of the sum of the two products and the amplification ratio KM is calculated. Using the calculated results, assist is supplied while the human power is being supplied. At the same time, an assist power Assist M is calculated with a remaining torque section 76 for carrying out straight coasting after the human power supply is stopped. In other words, the assist power Assist M is for assisting straight running power which is caused to be outputted with the CPU 71 even after the human power supply is stopped, and it is arranged that its magnitude decreases gradually with time. A sum of the assist power Assist L or Assist R and Assist M is set as command values τL* or τR* to the motor driver 73, and the value of the current supplied to the motor 36 is feedback-controlled so that the assist torque τL' becomes the torque command value τL*. The sum of the assist torque τL' and the human power torque to the left wheel becomes the left wheel propelling torque τL. Also, the value of the current supplied to the motor 36 is feedback-controlled so that the assist torque τR' becomes the torque command value τR*. The sum of the assist torque τR' and the human power torque to the right wheel becomes the right wheel propelling torque τR. As described above, in this embodiment, the assist powers TL, TR or the assist torques τL', τR' applied to the left and right wheels 2 are determined as sums of values obtained by combining together the left and right human powers FL, FR with the combination ratios α, β and values obtained by amplifying the left and right human powers FL, FR by the amplification values KL, KR. That is to say, the assist powers TL, TR are obtained as functions of the combined force of the left and right human powers FL, FR and the left and right human powers FL, FR. Next, control actions with the CPUs 71 of the left and right controllers 35 will be described in reference to FIGs. 13 to 15. FIGs. 13 to 15 are flow charts for describing the control actions of the assist power for the wheelchair 1. In the control with the controllers 35 as shown in FIG. 13, various memories and timers of the controllers 35 are reset as a preliminary process (step S1). Next, as an interrupt standby and communication process, calculation of the target torques of the assist powers supplied to the left and right wheels 2 and communication between the controllers 35 are carried out (step S2). The interrupt standby and communication process (step S2) is repeated. In the interrupt standby process in the step S2, shown in FIG. 14 an AD port input process of converting the analog human power input signal into a digital signal (step S3), an assist torque calculating process of calculating the target torque of the assist power supplied to the wheels 2 (step S4), a torque outputting process of outputting the calculated torque to the motor driver 73 (step S5), and error correction processes of correcting various errors detected with the previous processes (step S6) are carried out in sequence and repeated. In the assist torque calculating process (S4) shown in FIG. 15, first a determination is made whether the human power F1n inputted to one wheel is within the range between a lower limit value Flow and an upper limit value Fhigh (step S7). When the human power F1n is outside the specified range, the input value is determined as an error and an error correction process is carried out (step S8). The symbol n denotes the number of the control processes. Next, a polarity process of determining the direction of applying the human power F1n is carried out (step S9). That is to say, a value obtained by subtracting Fnull shown in FIG. 11 from the F1n is newly set to Fin. If the newly set value is greater than zero, the direction is determined as forward, while the direction is determined as reverse when the value is smaller than zero. By the way, FIG. 11 shows the characteristic of the input signal from the potentiometer 27 when the wheelchair 1 is running. In the figure, Fnull or Vnull shows the value of the input signal when the wheelchair is at rest. A register process (step S10) for the data exchanged between the left and right CPUs 71 in the communication process (step S2) is carried out. That is to say, the value F1n is set to a signal sending register Tx for storing data to be sent out, while a human power F2n inputted to the other wheel is set to a signal receiving register Rx for storing received data. A product of the inputted human power F1n and the combination ratio α, and a product of the inputted human power F2n and the combination ratio β are respectively calculated, and the sum FMn of the products is calculated as an assist torque component, namely the assist power Assist M mentioned above, acting on the center of gravity of the wheelchair 1 (step S11). Next, when the magnitude of the FMn (assist M) shown in absolute value is determined to be not less than a specified threshold value h and to be not in the insensitive zone (step S12), process of integrating input values is carried out (step S13). That is to say, a product of the calculated FMn and a specified constant a, and a product of the previous value and a specified value b are calculated respectively. The sum of the products is calculated and the result is set as an integrated calculation value Yn of the input signals FMn. When the magnitude of te FMn is determined to be less than the specified threshold value h and to be in the insensitive zone, a value obtained by attenuating the previous value Yn-1 with a specified constant c smaller than one is set as a new calculation value Yn (step S14). Next, whether the absolute value of the Yn is greater than a predetermined limit value Ymax (step S15). If greater, the Yn is set to the Ymax (step S16). When the Yn is determined to be not greater than the limit value Ymax in the step S15, a product of the Yn above and a specified constant d, and a product of the F1n and a specified constant e is calculated as the torque command value Assist τ* (step S17). By the way, the constants d and e denote specified coefficients; d corresponding to the amplification ratio KM, and e corresponding to KL and KR. With the torque command value Assist τ* calculated as described above and with the determination of forward or reverse run, rotating direction and torque of the drive motor 36 are set, and the motor 36 is controlled with the CPU 71. The power-assisted wheelchair 1 of this embodiment is arranged that the magnitudes of the assist powers applied respectively to the left and right wheels 2 are set according to the resultant force of the human power applied to the left and right wheels 2 and respective human powers. As a result, the propulsive force of the wheelchair 1 is, when one drive wheel is noted, a resultant force of a propulsive torque caused by a human power inputted to the wheel, a direct assist torque on the wheel, and an assist torque caused by a virtual momentum reserved in the center of gravity as a result of input to the left and right wheels. Since it is arranged as described above that the assist power to be applied to one drive wheel is calculated from the human power applied to both drive wheels, the turning motion (yaw motion) which is otherwise likely to occur due to increased propulsive force is reduced. It is also arranged that, when the user's left and right arms have different strengths, the amplification ratios KL, KR of the direct assist powers to the left and right wheels are set to different values, and the combination ratios α, β for determining the resultant force deemed as acting on the center of gravity are also set to different values. As a result, difference in the strengths of the rider's left and right arms is absorbed, the rider's effort is alleviated, and the running characteristic is improved. In this case, even when the human power is applied to one wheel 2 only, the assist power may be applied to both wheels depending on the setting of the amplification ratios and the combination ratios. This also alleviates the rider's effort. It is also arranged that identical left and right assist powers are applied after the human power input is stopped and that the magnitude of the assist power is decreased with time. As a result, after the human power supply is stopped, it may be deemed as if a virtual momentum were applied to the center of gravity. Therefore, the wheelchair is prevented from continuing a turning motion when the rider removes hands from the hand rims 13 during the turning. This gives the same operation feeling as that on the manual wheelchair. Furthermore, on a grade, the wheelchair is prevented from stopping suddenly upon stopping the human power input. This also alleviates the rider's effort. Furthermore, since the potentiometer 27, the motor 36, and the controller 35 are disposed separately on the left and right wheels 2, ease of assembly work of the wheels 2 is improved, production cost is reduced, degree of freedom of the assist ratio is increased, supply of assist power commensurate with the rider's condition is made possible, and the rider's effort is further alleviated. Furthermore, since the two CPUs 71 are interconnected through the serial cables and a serial communication for sending different data at time intervals is employed, number of signal lines is reduced, the cables may be interconnected with connectors so that the wheels 2 may be easily handled independently of each other. Furthermore, since the target torque with the electric motor 36 as a source of assist power is calculated, resistance against changes in speed is reduced and therefore turning by human power is made easy. As an alternative of calculating the target torque of the motor 36 as described above, it may also be arranged to calculate a target rotation speed of the assist power. In that case, since the vehicle speed is maintained irrespective of changes in load, an uphill run may be made with the same number of operations as that on a level road and stabilized straight run may be made. As still another alternative, it may also be arranged to calculate a target application voltage of the motor 36. As described above, with the power-assisted wheelchair according to the embodiment described above each of the assist powers TL, TR applied respectively to the left and right drive wheels is set as a function of both of the human powers FL, FR applied respectively to the left and right drive wheels. That is to say, the assist power applied to one drive wheel is set according to the human power applied to both of the left and right drive wheels. As a result, such effects are provided that virtual action point to which the drive force is assumed to be applied is brought near the center of gravity of the wheelchair, turning motion of the vehicle which is more likely to occur with increased drive force is reduced, and the rider's effort is reduced. With another embodiment, each of the assist powers TL, TR applied respectively to the left and right drive wheels is set as a function of the resultant force FM of the human powers FL, FR applied respectively to the left and right drive wheels and FL or FR. That is to say, the assist power applied to either wheel is set according to the assist powers applied respectively to left and right drive wheels. As a result, effect of more stabilized run is provided. In other words, since the resultant force FM may be considered as the straight run component for the entire vehicle, if the assist power components of the resultant force FM are outputted to the left and right drive wheels, the rider feels as if the center of gravity is pushed, the same effect is felt as if the rider's own weight were lightened, and a stabilized run is made possible. A further embodiment is arranged that the assist power remains even after the human power input is stopped. In addition, it is possible that the magnitude of the remaining power decreases gradually. As a result, it may be assumed that a virtual momentum is reserved in the center of gravity and an effect is provided that the rider feels as if the rider's own weight were lightened. Since a still further embodiment is arranged that the target torque of the electric motor is calculated as the assist power, there is no resistance against changes in the running speed and therefore an effect is provided that turning by human power is made easily. Further, it may be possible to calculate the target speed of and the target voltage applied to the electric motor as the assist power source, an effect is provided that stabilized straight run is possible even if there are changes in load. Further , since the human power detection means, the assist power source, and the control means may be provided for each of the left and right drive wheels, effects are provided that, ease of assembly work is improved, the assist ratios for the left and right drive wheels may be optionally set according to the difference in strengths between left and right arms of the rider, and the rider's effort is further alleviated. Another embodiment is arranged in which the control means are interconnected serially, the number of signal cables is reduced, and the left and right wheels may be handled separately. This also provides an effect of improving ease of assembly work.

Claims: Method for controlling the assist power (TL,TR) of a power assisted wheelchair (1) comprising human power drive means (2,13) and assist power drive means (36), wherein said method comprising the steps of: applying human power (FL,FR) to said wheelchair (1) by means of said human power drive means (2,13), detection of said supplied human power (FL,FR) by means of human power detection means (27), calculating of the assist power (TL,TR) commensurate with the magnitude of the human power (FL,FR) by means of a power assist control means (35), characterized in that the assist power (TL,TR) for one of the drive wheels (2) applied by the assist power drive means (36) is calculated in accordance with the magnitude of the human power (FL,FR) applied to both of the left and right drive wheels (2). Method according to claim 1, characterized in that the assist power (TL,TR) applied to either wheel (2) in addition is calculated in accordance with the resultant power (αFL + βFR) of the human power (FL,FR) applied to both of the left and right drive wheels (2). Method according to claim 2, characterized in that said assist power component calculated in accordance with the resultant power (αFL + βFR) of the applied human power (FL,FR) is maintained even after the supply of said human power (FL,FR) has been terminated. Method according to claim 3, characterized in that the magnitude of the remaining assist power component is attenuated with the lapse of time. Method according to at least on of the preceding claims 1 to 4, characterized in that said assist power is a torque value andlor a rotating value of said assist power drive means (36). Method according to claim 1 or 2, characterized in that the amplification ratios (KL,KR,KM) and combination ratios (α, p) preset according to physical conditions of the user are stored for controlling the respective assist power (TL,TR). Method according to at least one of the preceding claims 1 to 6, characterized in that the assist power (TL,TR) applied to the left and right wheels (2), respectively, are determined as sums of values obtained by combining the left and right human power (FL,FR) with the combination ratios (α, p) and values obtained by amplifying the left and right human power (FL,FR) by the amplification ratios (KL,KR,KM) Wheelchair (1) comprising a human power drive means (2,13), an assist power drive means (36) and an assist power control means (35) for controlling an assist power (TL,TR) commensurate with the magnitude of the human power (FL,FR) detected by a human power detection means (27), characterized in that said assist power control means (35) is adapted to calculate the assist power (TL,TR) applicable to one drive wheel (2) in accordance with the magnitude of the human power (FL,FR) applicable to both drive wheels (2). Wheelchair (1) according to claim 8, characterized in that the assist power control means (35) is adapted to calculate said assist power (TL,TR) in addition in accordance with the resultant power (αFL + βFR) applicable to both drive wheels (2). Wheelchair (1) according to claim 9, characterized in that the assist power control means (35) is adapted to maintain the assist power component calculatable in accordance with the resultant power (αFL + βFR) of the applicable human power (FL,FR) even after the supply of said human power (FL,FR) has been terminated. Wheelchair (1) according to claim 10, characterized in that said assist power control means (35) in addition is adapted to attenuate the magnitude of the remaining assist power component with the lapse of time. Wheelchair (1) according to at least one of the preceding claims 8 to 11, characterized in that the assist power is a torque value and/or a rotating value of said assist power drive means (35). Wheelchair (1) according to claim 12, characterized in that said assist power drive means is an electric motor (35). Wheelchair (1) according to at least one of the preceding claims 8 to 13, characterized in that each drive wheel (2) comprising a human drive means (13), an assist power drive means (36) and an assist power control means (35) which are connected with each other for exchanging information. Wheelchair (1) according to at least one of the preceding claims 12 to 14, characterized in that said assist power control means (35) is adapted to control said torque valve and/or said rotating speed such that the assist power (TL,TR) become target values. Wheelchair (1) according to claim 14 or 15, characterized in that said assist power control means (35) are interconnected through serial communication means (59). Wheelchair (1) according to at least one of the preceding claims 8 to 16, characterized in that said human power detection means comprising a potentiometer (27). Wheelchair (1) according to at least one of the preceding claims 8 to 17, characterized by a rotary transformer (39) for transmitting signal between the assist power control means (35) the human power detection means (27). Wheelchair (1) according to at least one of preceding claims 8 to 18, characterized by a power transmission means comprising pulleys (40,41), a belt (42) and a plural number of gears (G1-G4). Wheelchair (1) according to at least one of preceding claims 8 to 19, characterized in that said drive wheels (2) are detachable. Wheelchair (1) according to at least one of preceding claims 8 to 20, characterized in that said assist power control means (35) comprising a sensor drive I/F(70) for inputting human power applied detected by the human power detection means (27), a CPU(71) for calculating target values, a motor I/F(72), a motor drive (73) for feedback-controlling the assist power drive means (36), and a communication I/F(74) for interconnecting left and right CPU's (71).

Inventors: OGATA HIROAKI, UCHIYAMA ATSUSHI, OGATA, HIROAKI, UCHIYAMA, ATSUSHI
Applicants: YAMAHA MOTOR CO LTD, YAMAHA HATSUDOKI KABUSHIKI KAISHA
Application year: 1997
Main classification: A61G5
IPC codes: A61G5, B62M23