Abstract:
The invention comprises an actuator controlled harmonic drive transmission assembly for the speed and positional control of an output shaft of the harmonic drive transmission. The assembly includes a motor having a rotor shaft for providing rotational power to harmonic drive transmission and a control arrangement for permitting rotational positional and speed control between the rotor shaft and the output shaft of the harmonic drive transmission. The control arrangement may comprise an output speed, torque, vibration and/or rotational encoder mounted on the output shaft of the harmonic drive transmission. The control arrangement may comprise an output speed and rotational encoder mounted on the rotor shaft of the motor, each encoder feeding data to a control logic unit to control the motor driving the transmission.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to harmonic drive transmissions, and more particularly to sensor driven actuator arrangements to improve the positional accuracy of harmonic drive systems. 
     2. Prior Art 
     Harmonic drive transmissions, sometimes known as controlled-ratio deflection type transmissions, are used where rotary to rotary transmission is needed. In such a transmission, the gear tooth engagement is induced at a plurality of points by the deflection of a thin ring gear or the like. The tooth engagement at a plurality of points around the circumference is propagated along the periphery of a thin ring gear as the crest of the induced deflection ring is made to move around this periphery. As the deflection moves around the gear, each tooth moves radially in and out of engagement as it progresses from one tooth to the next, tracing during this motion, a curve which is generally of the character of a sinusoidal wave, giving rise to the term “strain-wave gearing”. 
     Examples of such early transmissions of this type are shown in U.S. Pat. No. 2,906,143, issued in 1959 to Musser, U.S. Pat. No. 2,931,249 issued to Musser, and U.S. Pat. No. 3,196,713 issued to Robinson, all of which are incorporated herein by reference. 
     Those transmission have found use in certain industries where rotary power is needed and increasingly so in the robotics industry. Such use, particularly in the robotics industry requires extreme accuracy. Heretofore, robotic transmissions and drive units have been empowered by direct drive motors which are able to repearably position the rotation of a shaft, within plus or minus 3 arc seconds. These motors typically are brushless and have a high output torque at a low velocity. Direct drive motors unfortunately are also very expensive. 
     It is an object of the present invention to provide a harmonic drive assembly, which may function as a direct drive motor replacement in the robotic industry. 
     It is yet a further object of the present invention, to provide a harmonic drive actuator arrangement, which permits highly accurate corrective positioning of the output shaft. 
     It is yet still a further object of the present invention to provide a harmonic drive apparatus with a sensor/encoder arrangement for speed and/or torque, position, vibration, temperature, performance degradation and/or tooth wear sensing utilizing self-diagnostic control of the apparatus. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention comprises a harmonic drive transmission coupled together with and being driven by an electric motor. This electric input motor is in a driving relationship with the input motor shaft connected to the input wave generator of the harmonic drive transmission. The electric input motor has a stator and a rotating rotor arranged within its housing. A sensor, here, an input shaft encoder, is disposed on the input end of the rotor of the electric motor. The input encoder, preferably of the optical type, or possibly the magnetic type, is arranged on the rotor of the electric motor, and is in electrical communication, via a proper circuit, with a control logic unit. 
     The flexspline in the harmonic drive transmission, is connected to an output shaft of the electric motor, the flexspline/harmonic drive transmission all driven by the electric motor. A sensor here, such as an output encoder, such as a magnetic or optical output encoder, may be arranged on the output shaft of the harmonic drive transmission. The output encoder is in electrical communication with the control logic unit, as is the input encoder. The control logic unit controls a power drive converter, which is in electrical communication with the electric motor. 
     A control signal from a proper operator control unit is inputted to the control logic unit. The control logic unit received signals from both the input encoder and the output encoder, making an analysis and comparison therebetween. The control logic unit governs the converter, to regulate the electric input motor. Alignment and matching of the signals between the output encoder on the output shaft of the harmonic drive transmission and the input encoder on the rotor of the electric motor permits the apparatus to have a power output from the harmonic drive transmission which is speed controllable and positionally governable with a repeatability to about 3 arc seconds of accuracy. The output encoder, which may be the magnetic or optical type, is preferably arranged as close to the output bearing on the harmonic drive transmission as possible, to minimize any possible error from axial loading upon the output shaft of the harmonic drive transmission. Thus, high precision rotational output control is achieved within the multisensor/encoder harmonic drive actuator assembly of the present invention. 
     A further embodiment of the present harmonic drive actuator assembly consists of a harmonic drive transmission similar to that of the aforementioned embodiment, having an input wave generator arranged within the housing and in wave generating communication with a flexspline attached to an output shaft of the harmonic drive transmission. An electric input motor has a shaft connected to the input shaft on the input wave generator. The electric motor has a stator and a rotor shaft at a first end thereof. A control logic and power drive unit is arranged about the rotor shaft on the electric input motor. An input encoder of the magnetic or optical type is attached about the rotor shaft adjacent the control logic and power unit on the electrical motor. 
     An output encoder, either the magnetic or optical type, is arranged about the output shaft of the harmonic drive transmission, extending from the flexspline. The output encoder is in electrical communication with the control logic and power drive unit on the electrical input motor. The input encoder arranged on the rotor shaft is in electrical communication with the control logic and power drive unit mounted adjacent the rotor shaft. An AC/DC converter is in communication with the control logic and power drive unit. An operator control signal is in electrical communication with the control logic and power drive unit on the electric motor rotor shaft, and operates the harmonic drive actuator assembly. Rotational input from the input motor drives the input wave generator to provide proportionate corresponding motion of the flexspline and the output shaft. Position and speed performance of the output shaft is monitored by the output encoder, which sends an output performance signal to the control logic and power drive unit. A corresponding speed and rotational position sensor on the rotor shaft of the electric motor sends a performance signal of the motor shaft of the electric motor, to the control logic and power drive unit. Correspondence between the input performance from the electric motor, and output performance from the harmonic drive transmission is maintained by the comparison between the sensor signals of the input encoder and the output encoder, to provide high precision position correction between the two assembly components. Torque control may be achieved with additional sensors such as torque (current) sensors in place at or in addition to the position sensors already identified hereinabove. 
     Thus, by having the appropriate sensors preferably arranged at one or opposite ends of a motor/harmonic drive transmission assembly, to optically, electrically, eletromechanically and/or magnetically determine speed, torque, and position performance between the input and the output of such assembly, permitting position control and accuracy of that assembly. 
     The invention thus includes an actuator controlled harmonic drive transmission for the speed and positioned control of an output shaft of the harmonic drive transmission, comprising: a motor having a rotor shaft for providing rotational power to harmonic drive transmission; and a control arrangement for permitting rotational positional and speed control between the rotor shaft and the output shaft of the harmonic drive transmission. The control arrangement may comprise an output speed and rotational encoder mounted on the output shaft of the harmonic drive transmission. The control arrangement may also comprise an input speed and rotational encoder mounted on the rotor shaft of the motor. The output encoder may be a magnetic or optical encoder or sensor, which is also known as a “resolver”. The input encoder may be an optical encoder. The control arrangement preferably includes a feed back loop which communicates with a control logic and power drive unit which governs the motor. 
     The invention also includes a method of controlling the output of a harmonic drive transmission, to permit positional and rotative accuracy of an output shaft of the harmonic drive transmission, comprising the steps of: arranging a rotor of a motor to rotatively empower an input wave generator of the harmonic drive transmission; coupling an output speed and positional encoder on the output shaft of the harmonic drive transmission; and attaching a circuit from the output encoder to the motor feed back control data to signal speed and positional information to control logic unit governing the motor for controlled accuracy thereof. The method may include the steps of: coupling an input speed and positioned encoder on the rotor of the motor empowering the harmonic drive transmission; and attaching a circuit from the input encoder on the rotor to feed back control data to signal speed and positioned information to the control logic unit for comparing input and output data to control positional speed characteristics of the input and output shafts of the harmonic drive transmission. The input encoder may be an optical or magnetic encoder to optically or magnetically sense the speed and position characteristics of the rotor on the motor. The output encoder may be a magnetic encoder to sense the speed position characteristics of the output shaft on the harmonic drive transmission. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and advantages of the present invention will become more apparent, when viewed in conjunction with the following drawings, in which: 
     FIG. 1 is a schematic representation of a harmonic drive assembly with an electric input motor and an encoder arrangement on each end of said assembly; 
     FIG. 2 is a schematic representation of harmonic drive assembly with an electric input motor and an encoder arrangement on each end of said assembly and a control logic unit on the input shaft; 
     FIG. 3 is a schematic representation of a harmonic drive assembly similar to that of FIG. 1, with a logic unit and encoder on one end adjacent the electric motor assembly; and 
     FIG. 4 is a schematic representation of a harmonic drive assembly similar to that of FIG. 3, with a control logic unit and encoder arrangement all on one end adjacent the electric motor assembly. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawing in detail, and particularly to FIG. 1, there is shown the present invention in which a harmonic drive transmission actuator assembly  10  having a harmonic drive transmission  12  coupled together with and being driven by an electric motor  14 . The electric motor  14  has an output shaft  16  which is in a driving relationship with an input wave generator  18  of the harmonic drive transmission  12 . The electric input motor  12  has a rotable rotor  20  and a stator  22  arranged within its housing  24 . An input shaft encoder  26 , is disposed on the distal end  27  of the input shaft  16  of rotor  20  of the electric motor  14 , as may be seen in FIG.  1 . The input encoder  26 , preferably of the optical type, in an alternate embodiment, of the magnetic type, is arranged on the end of the input shaft  27  of the rotor  20  of the electric motor  14 , and is in electrical communication, via a proper circuit  28 , with a control logic unit  30 . The control logic unit  30  is an electronic micro-controller and/or programmable logic device that controls the power drive unit  42  as needed to control position, speed and/or torque. The power drive unit  42  is a circuit to connect to and convert fixed DC voltage into a controlled voltage to permit the motor  12  to run at a variable speed and or torque in either direction. An AC to DC converter  43  is in communication with the power drive  42 , to rectify AC voltage from an AC source  45  into a constant DC voltage. 
     The harmonic drive transmission  12  has a flexspline  32  which is rotated via an advancing wave created by the wave generator  18 , the wave generator  18  being connected to the output shaft  16  of the electric motor  14 , as shown in FIG.  1 . The flexspline/harmonic drive transmission  32 / 12  is driven by the electric motor  14 . An output encoder  34 , preferably for example, a magnetic output encoder, is arranged about the output shaft  36  end of the harmonic drive transmission  12 . The output encoder  34  is in electrical communication via a proper circuit  38 , with the control logic unit  30 , as is the input encoder  26 . The control logic unit  30  controls the power drive  42  and the AC/DC converter  43 , which is in controlling electrical communication with the electric rotor  14  via a proper control circuit  44 . 
     A control signal from a proper operator control unit  46  is inputted to the control logic unit  30 . The control logic unit  30  receives signals from both the input encoder  26  and the output encoder  34 , making performance analysis and comparisons therebetween. The control logic unit  30  governs the power drive  42  and the AC/DC converter  43 , to regulate the electric input motor  14  as aforementioned. Alignment and matching of the performance signals between the output encoder  34  on the output shaft  36  of the harmonic drive transmission  12  and the input encoder  26  on the input shaft  27  of the rotor  20  of the electric motor  14  permits the harmonic drive actuator apparatus  10  to have a power output from the harmonic drive transmission  12  which is torque controllable, speed controllable and “positionally” governable to about 3 arc seconds of accuracy. The output encoder  34 , preferably magnetic, is preferably arranged as close to the output bearing  47  on the harmonic drive transmission  12  as possible, to minimize any possible error from axial loading upon the output shaft  36  of the harmonic drive transmission  12 . Thus, high precision rotational output control is achieved within the harmonic drive actuator assembly  10  of the present invention. 
     A further embodiment of the harmonic drive actuator assembly  50  is shown in FIG.  2 . The assembly  50  consists of a harmonic drive transmission  52 , generally similar to that of the aforementioned embodiment, having an input wave generator  54  arranged within the housing  56  and in wave generating communication with a flexspline  58  attached to an output shaft  60  of the harmonic drive transmission  52 . An electric input motor  62  has an output shaft  65  that is the input shaft on the wave generator  54 . The electric input motor  62  has a stator  66  and an input rotor shaft  68  at a first end thereof. A control logic unit  69  and power drive unit  70  are communicatively arranged about the rotor shaft  68  on the input end of the assembly  50  adjacent the electric input motor  62 . An input encoder  72 , preferably of the optical type, is communicatively disposed about the rotor shaft  68  adjacent the control logic unit  69  and the power unit  70  on the electric motor  62 . An output encoder  74 , preferably the magnetic type, is communicatively arranged about the output shaft  60  of the harmonic drive transmission  52 , extending from the flexspline  58 . The output encoder  74  is in electrical communication, via a proper output circuit  76  with the control logic unit  69  on the input shaft  68  of the electric input motor  62 . The input encoder  72  arranged on the rotor shaft  68  is in electrical communication, via a proper input circuit  78 , with the control logic unit  69  on the rotor shaft  68 . An AC/DC converter  80  is in communication with the power drive unit  70  via line  79 . An operator control signal unit  81  is in electrical communication, via a proper logic circuit  82  with the control logic unit  69  on the rotor shaft  68  of the electric motor  62 , and operates the harmonic drive actuator assembly  50 . An AC power source  84  provides electrical power to the AC to DC converter  80 . Rotational input from the electric input motor  62  drives the input wave generator  54  to provide proportionate corresponding motion to the flexspline  58  and the output shaft  60  of the harmonic drive transmission  52 . Position, speed, and torque performance of the output shaft  60  is monitored by the output encoder  74 , which sends an output performance signal via its circuit  76  to the control logic unit  69  and the power drive unit  70 . The corresponding speed, torque, and rotational position input sensor  72  on the rotor shaft  68  of the electric motor  62  sends a performance signal of the rotor shaft  68  of the electric motor  62 , to the control logic unit  69 . Correspondence between the input performance from the electric input motor  62 , and output performance from the harmonic drive transmission  52  is maintained by the comparison between the performances of the input between the two assembly components in the control logic unit  69 , such as is often required in “stop and start” movement, particularly in the robotic field. 
     Thus, by having an input encoder  72  and an output encoder  74  arranged at opposite ends of a motor/harmonic drive transmission actuator assembly  10  or  50 , to optically and/or magnetically determine speed, torque, and position performance between the input and the output of such assembly, such position control and accuracy of the assembly is maintained. 
     A further embodiment of a harmonic drive assembly  88  is shown in FIG. 3, having a hollow electric motor output shaft  90  arranged in the electric motor  92 , with an output shaft  94 ′ from the harmonic drive unit  96  through the hollow electric motor shaft  90  and out a first end of the assembly  88  (the left as seen in FIG.  3 ), and also an output shaft  94 , out from the flexspline  99 , (as seen on the right side of the assembly  88 ). In this embodiment, the input encoder  98  is arranged about the electric motor input shaft  90 , and the output encoder  100  is arranged about the output shaft  94 ′ which extends through the input shaft  90 . The input shaft  90  extends from rotative empowerment within the electric motor  92 , and is attached to the wave generator  97 , in the harmonic drive  96  to provide rotative power to that flexspline  99 , and hence rotative power to the output shaft ends  94  and  94 ′. The input encoder  98  disposed about the electric motor shaft  90  and the output encoder  100  disposed at the first end of the assembly  88  about the output shaft  94 ′, are connected to the control logic unit  30 , via proper circuits  102  and  104 , respectively. The control signal unit  46 , the control logic unit  30 , the power drive unit  42 , the AC power source  45  and the AC to DC converter  43  feed the harmonic drive assembly  88  similar to the setup shown in FIG.  1 . 
     Similarly, a still further embodiment of a harmonic drive assembly  110  is shown in FIG. 4, with a hollow electric motor output shaft  111  attached to the rotor  109 , the output shaft  111  having output shaft ends  112  and  112 ′. The first end  112 ′ of the output shaft  111  provides input to and turns the wave generator  113  in the harmonic drive unit  96 . The flexspline  115  in the harmonic drive unit  96  is connected to the output shaft  117  as shown in FIG.  4 . The output shaft  117  extends through the hollow input shaft  111 , as shown in FIG.  4 . In this embodiment, the input encoder  120  and the control logic unit  124  are both communicatively arranged about the first end  112  of the input shaft  111 . The output encoder  122  is arranged about the first end of the output shaft  90  of the harmonic drive transmission  96 . The output encoder  122  is connected to the control logic unit  124  by a circuit  125 . The input encoder  120  is in communication with the control logic unit  124  by a second circuit  127 . The control signal unit  81 ′, the AC power source  84 ′, the AC to DC converter  80 ′ feed the harmonic drive assembly  110  similar to the set up shown in FIG.  2 . In this embodiment, the output encoder  125  and the input encoder  120  are adjacent one another, both arranged about the central output shaft  90 , and the first end of the hollow shaft  111 , as shown in FIG.  4 . They permit control of parameters of input and output of a harmonic drive assembly  110  at a convenient common end of that harmonic drive assembly. Use of a hollow input shaft  111  and an output shaft  117  arranged through that hollow input shaft  111  is unique to the present harmonic drive invention.