Patent Description:
Servomotors are automatic control systems that include a DC motor, a speed reduction gear set, a sensor, and a control circuit. The existing speed reduction gear sets for servomotors are mainly spur gear reducers and some of them are planetary gear reducers. Because of performance needs, the reduction ratio of servomotors is usually <NUM>/<NUM>, and can even be <NUM>/<NUM>. It usually needs three to five stages of speed reduction for the spur gear reducers and the planetary gear reducers to obtain the above-mentioned reduction ratio. It usually needs four stages of speed reduction for obtaining the reduction ratio. Considering the actual machining and assembling capability, given that the total efficiency of a single stage gear set is about <NUM>-<NUM>%, then the total efficiency of four stage gear set is about <NUM>-<NUM>%. When there is improper design/assembling, the efficiency will be lower, resulting in that the gear sets to have a large size and low efficiency. For a smooth transmission, the gears of the spur gear reducers and the planetary gear reducers are arranged to have gaps, resulting in that reducers to theoretically have backlash. The unit for measuring the backlash is arcmin (arc minute). Generally, reducers having a backlash smaller than <NUM> archmins are called high precision type, and reducers having a backlash greater than <NUM> archmins are called low precision type. The backlash of ordinary planetary gear reducers can be <NUM>-<NUM> archmins, and the backlash of ordinary spur gear reducers is greater than <NUM> archmins.

Another type of reducers, the harmonic gear reducers, are a new type of reducers that have been developed based on the principle of planetary gear transmission. The harmonic gear transmissions are a type of planetary gear transmission that transmits power and motion through the mechanical wave generated by flexible components. The harmonic drives have been widely used as important components of robot joints for the advantages of simple structure, small size, low noise, high transmission ratio, high transmission precision, and high efficiency. Harmonic drives have been fully studied by main industrialized countries all over the world since the first harmonic drive was invented by the American C. Musser in <NUM>. Various types of harmonic drives with different sizes have been developed, among which harmonic fine-pitch gear reducers are most widely used. Since the limitations in structure and technology exist, when the existing harmonic drives are used, the position of the output side is usually controlled by the detection value of the encoder at the back of the motors. Because the motor output error and the harmonic transmission error are not considered, the error generated during the transmission process can not be eliminated, causing the backlash error of the output side to be uncontrollable. The backlash of the harmonic drives is usually less than <NUM> arcmin, and the backlash of some high precision harmonic drives is less than <NUM>/<NUM> arcmin. After the harmonic drive is used, the gears may be worn, causing the backlash to increase.

Angular transmission accuracy or angular transmission error, refers to the difference between the theoretical output rotation angle and the actual output rotation angle. The above-mentioned backlash, motor output and harmonic transmission error will affect the motor output angle and lower the angular transmission accuracy of motor.

<CIT> relates to a hollow driving module comprising a hollow driving module including a hollow motor including a stator, a rotor rotating with respect to the stator and having a perforated center portion, and a rotation shaft disposed at and coupled to the perforated center portion of the rotor; a decelerator connected with the rotation shaft positioned at an output side of the hollow motor to decelerate rotation of the rotation shaft; a torque transmission unit being connected to an output side of the decelerator to be driven by a decelerated rotation angle; a first encoder positioned at an input side opposite to a side connected with the decelerator of the hollow motor; a second encoder positioned at a side opposite to a side connected with the hollow motor of the first encoder; and a link connection means connecting the second encoder with the torque transmission unit.

<CIT> describes 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.

A machine-translated abstract of <CIT> reads: 'The utility model belongs to the technical field of a servo motor and specifically relates to a lock-free electromechanical servo mechanism capable of realizing no-power locking. The utility model, on one hand, is to realize locking of an engine in a lock-free condition, and on the other hand, is to realize the function above without increasing weight of the servo mechanism so as to help to increase range. The lock-free electromechanical servo mechanism comprises a servo motor , a decelerator and a displacement sensor, wherein the servo motor and the displacement sensor are arranged on the decelerator. The servo motor of the servo mechanism is provided with a locking torque, and then, the locking torque is amplified by utilizing the decelerator of the servo mechanism, so that no-power locking of the servo mechanism is realized, the servo mechanism can still realize a locking function under the condition of not using a locking device, the unlocking and locking operation is cancelled, and the reliability of the whole system is improved.

<CIT> discloses an encoder rotor of an absolute magnetic encoder mounted on a servomotor shaft having a first drum of a bipolar magnet, and a second drum with a Q-bit multipolar magnetic pole track and a reference track. A signal processor generates absolute signals on the basis of detected signals, which differ in phase by <NUM>° and in which a single rotation represents a single period, from X-phase and Y-phase magnetic sensors disposed facing the first drum, and on the basis of A- and B-phase signals, which differ in phase by <NUM>°, and a reference signal obtained from A-, B-, and Z-phase magnetic sensors disposed facing the second drum.

According to aspects of the present invention there is provided a servomotor, as defined in the accompanying claims.

Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

Many aspects of the present embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiments. Moreover, in the drawings, all the views are schematic, and like reference numerals designate corresponding parts throughout the several views.

The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings, in which like reference numerals indicate similar elements. It should be noted that references to "an" or "one" embodiment in this disclosure are not necessarily to the same embodiment, and such references can mean "at least one" embodiment.

Referring to <FIG> and <FIG>, in the present embodiment, a servomotor includes a control circuit <NUM>, an electric motor <NUM>, a harmonic drive <NUM>, and a sensor. The control circuit <NUM> is electrically connected to the electric motor <NUM>, and used to control the angular displacement, rotational speed, and torque of the electric motor <NUM>.

The harmonic drive <NUM> includes an outer casing <NUM>, a wave generator <NUM>, a flex spline <NUM> and a circular spline <NUM> that are arranged within the outer casing <NUM>. The wave generator <NUM> is a cam having varying radial length, and includes a rolling bearing mounted on its periphery. The electric motor <NUM> is arranged at one end of the outer casing <NUM>. The wave generator <NUM> is driven by the electric motor <NUM>. The flex spline <NUM> is sleeved on an exterior of the wave generator <NUM>. Controllable deformation of the flex spline <NUM> can be generated by the wave generator <NUM>. The circular spline <NUM> is fixed within the outer casing <NUM>. An outer gear is arranged on an exterior of the flex spline <NUM>. The circular spline <NUM> is a ring shaped internal gear, and the flex spline <NUM> is located within the circular spline <NUM> and engages with the circular spline <NUM>. When the wave generator <NUM> rotates, the shape of the flex spline <NUM> keeps changing, thereby promoting the engaging state between the flex spline <NUM> and the circular spline <NUM> and realizing the reverse rotation of the flex spline <NUM> relative to the circular spline <NUM> along the wave generator <NUM>.

Transmission ratio is controlled by the difference between the teeth of the flex spline <NUM> and the circular spline <NUM>, and the gear teeth of the flex spline <NUM> of the wave generator <NUM> move radially and evenly during the transmission process. Therefore, the relative sliding speed of gear teeth is low even if the input speed is high. The wear of gear teeth is thus mild and the efficiency is high and can be up to <NUM>-<NUM>%. Compared with conventional reducers, when realizing the same output torque, the size of the harmonic drive can be reduced by <NUM>/<NUM> and the weight can be reduced by <NUM>/<NUM>, allowing the servomotor to be small-sized and light-weighted.

The flex spline <NUM> is connected with an output member <NUM> that is used to output power, and a post <NUM> is arranged along a rotation axis of the output member <NUM>. The sensor is arranged within the outer casing <NUM> and used to detect angular displacement of the post <NUM>.

In the embodiment, the electric motor <NUM> includes an output shaft <NUM>, and one end of the output shaft <NUM> extends to an inside of the outer casing <NUM> and includes a first gear <NUM> arranged thereto. The wave generator <NUM> includes a second gear <NUM> arranged thereto. The first gear <NUM> engages with the second gear <NUM> for a first stage transmission. The electric motor <NUM> outputs torque via the output shaft <NUM>. The torque is then transmitted to the wave generator <NUM> via the first gear <NUM> and the second gear <NUM>, realizing the torque input to the harmonic drive <NUM>, and the speed-reduction transmission output through the harmonic drive <NUM>.

The first gear <NUM> and the second gear <NUM> are spur gears, and the number of the teeth of the first gear <NUM> is less than the number of the teeth of the second gear <NUM>. The first gear <NUM> and the second gear <NUM> form a spur gear speed-reduction transmission. It is noteworthy that the spur gear speed-reduction transmission is just one preferred embodiment. Other gear transmission technologies used as alternative technical solutions, such as bevel gear transmission and leadscrew transmission, should also be covered by the protection scope of the present invention.

The harmonic drive <NUM> further includes a flexspline pressing member <NUM> and a bearing inner ring <NUM> that are used to connect with the flex spline <NUM> and transmit the torque output of the flex spline <NUM>. The flexspline pressing member <NUM> is a T-shaped truncated cone structure and includes a position limiting portion <NUM> and a protruding portion. The position limiting portion <NUM> is a ring structure and the protruding portion <NUM> is cylindrical. The flex spline <NUM> defines a first through hole <NUM> in its bottom, and the first through hole <NUM> is located at the central axis of the flex spline <NUM>. The bearing inner ring <NUM> defines a second through hole <NUM>. The protruding portion <NUM> of the flexspline pressing member <NUM> passes through the first through hole <NUM> and the second through hole <NUM> in sequence. The flex spline <NUM> is arranged between the position limiting portion <NUM> of flexspline pressing member <NUM> and the bearing inner ring <NUM>, fixing the flexspline pressing member <NUM> and the bearing inner ring <NUM> to the flex spline <NUM>. When rotating, the flex spline <NUM> can drive the flexspline pressing member <NUM> and bearing inner ring <NUM> to simultaneously rotate about its central axis.

The outer casing <NUM> defines an opening in an end away from the electric motor <NUM>. The output member <NUM> is located at the opening of the outer casing <NUM>. The output member <NUM>, the bearing inner ring <NUM>, the flex spline <NUM> and the flexspline pressing member <NUM> are fixed together. Specifically, a number of threaded holds are defined in the corresponding positions of the output member <NUM>, the bearing inner ring <NUM>, the flex spline <NUM> and the flexspline pressing member <NUM>. The threaded holes are respectively arranged in a circular array in the surfaces of the output member <NUM>, the bearing inner ring <NUM>, the flex spline <NUM> and the flexspline pressing member <NUM>. The output member <NUM>, the bearing inner ring <NUM>, the flex spline <NUM> and the flexspline pressing member <NUM> are fixed by a number of screws that pass through the threaded holes therein. In the present embodiment, the output member <NUM> is a drive disc. In other embodiments, the output member <NUM> may be structures, such as a drive shaft.

A first bearing <NUM> is arranged between the outer casing <NUM> and the output member <NUM>. The peripheral of the bearing inner ring <NUM> is partly embedded in an inner ring of the first bearing <NUM> to guarantee the rotation accuracy of the flex spline <NUM> and the output member <NUM>.

In the present embodiment, the second gear <NUM> is a hollow T shaped gear and includes a gear portion <NUM> and an extending shaft <NUM>. The gear portion <NUM> includes on its exterior outer teeth that engage with the first gear <NUM>. The gear portion <NUM> includes therein a second bearing <NUM> supported between the gear portion <NUM> and the outer casing <NUM>. The gear portion <NUM> and the wave generator <NUM> are fixed together through screws, and the wave generator <NUM> is sleeved on the extending shaft <NUM>. A third bearing <NUM> is arranged between a periphery of an end of the extending shaft <NUM> that is away from the gear portion <NUM> and an inside surface of the position limiting portion <NUM> of flexspline pressing member <NUM>. The rotation accuracy of the second gear <NUM> and the wave generator <NUM> is guaranteed by the second bearing <NUM>.

The flexspline pressing member <NUM> defines a third through hole <NUM> in a center thereof. The post <NUM> pass through the second gear <NUM>, the third through hole <NUM> and the output member <NUM> in sequence. One end of the post <NUM> is arranged in a center of the output member <NUM>. A fourth bearing <NUM> is arranged on a periphery of the post <NUM> and supported between the post <NUM> and the outer casing <NUM>. The output member <NUM> defines a fourth through hole in its center, and the post <NUM> includes a threaded portion at its lower portion. The threaded portion pass through the fourth through hole and a nut is provided for engaging with the threaded portion.

In the present embodiment, the sensor is an encoder <NUM> that includes an encoder circuit board <NUM> and a magnet <NUM>. The magnet <NUM> is arranged at one end of the post <NUM> away from the output member <NUM>. The encoder circuit board <NUM> is fixed to the outer casing <NUM> and faces the magnet <NUM>. The encoder circuit <NUM> can detect accurately the actual motion data of the output member <NUM> by detecting the change of magnetic field of the magnet <NUM> at the center of the post <NUM>. Encoder circuit boards can detect data of <NUM> bits, which means that the minimum detection angle is: <NUM>/<NUM> = <NUM> degrees = <NUM> arcmin. The backlash and the angular transmission accuracy can thus be <NUM> arcmin.

In the present embodiment, the outer casing <NUM> includes a first casing <NUM> and a second casing <NUM>. A chamber for mounting the harmonic drive <NUM> is formed between the first casing <NUM> and the second casing <NUM>. A first step groove <NUM> and a second step groove <NUM> are defined in positions where the first casing <NUM> and the second casing <NUM> join. The circular spline <NUM> is located in the first step groove <NUM> and a second step groove <NUM>. The first step groove <NUM> and the second step groove <NUM> clamp the circular spline <NUM> therein to hold the circular spline <NUM> in position. The first casing <NUM> further includes a tube-shaped supporting portion <NUM> that is located in the chamber. The outer surface of tube-shaped supporting portion <NUM> is connected with the inner ring of the second bearing <NUM>, and the inner surface of the tube-shaped supporting portion <NUM> is connected with the outer ring of the fourth bearing <NUM>.

The first casing <NUM> defines a space in a side opposite to the chamber. The electric motor <NUM> and the circuit <NUM> are located in the space. A lid is arranged at the end of the space that is away from the chamber for closing.

In the servomotor of the present embodiment, the speed-reduction output to the electric motor <NUM> is conducted via the harmonic drive <NUM>. The output member <NUM> of the harmonic drive <NUM> is provided with a post <NUM>. The angular displacement of the output member <NUM> can be accurately detected by the sensor that is capable of detecting the rotational state of the post <NUM>, thereby obtaining the deviation value between the angular displacement of the output member <NUM> and the angular displacement theoretically derived according to the initial motion planning. The deviation value is fed back to the control circuit board, and one or more motion planning is added. The rotational deviation caused by the motor output error and the reducer transmission error is then offset, thereby increasing angular transmission accuracy. When the existing harmonic drives are used, sensors are arranged at the end of the electric motor. In the case that the performance of the electric motor is constant, it needs to utilize harmonic drives with greater accuracy for increasing the output accuracy of the reducer. The present invention utilizes ordinary harmonic drives and controls the actual output accuracy of the harmonic drive by using the angular displacement detection value of the sensor. The cost of ordinary harmonic drives is about <NUM>/<NUM> of the cost of the precise harmonic drives, which is much lower than the solution using the precise harmonic drives.

As shown in <FIG>, in the present embodiment, the servomotor includes most of the components of embodiment <NUM>. The differences between them are described as follows.

In the present embodiment, a transmission shaft <NUM> is arranged along the central axis of the wave generator <NUM>. The transmission shaft <NUM> includes a shoulder <NUM> that is connected to the wave generator <NUM> by screws. The second gear <NUM> is sleeved on the transmission <NUM> by interference fit. The second gear <NUM> includes outer teeth that engage with the first gear <NUM>. The transmission shaft <NUM> is hollow and the wave generator <NUM> is sleeved on the exterior of the transmission shaft <NUM>. A fifth bearing <NUM> is arranged between one end of the transmission shaft <NUM> and the outer casing <NUM>. A sixth bearing <NUM> is arranged between an opposite end of the transmission shaft (<NUM>) and the flexspline pressing member <NUM>. The rotational accuracy of the second gear <NUM> and the wave generator <NUM> are guaranteed by the fifth bearing <NUM> and the sixth bearing <NUM>.

The flexspline pressing member (<NUM>) defines a third through hole <NUM> in its center. The post <NUM> passes through the third through hole <NUM> and the transmission shaft <NUM> and is connected to the central axis of the output member <NUM>. The output member <NUM> defines a fourth through hole in its center. The post <NUM> includes a threaded portion. The threaded portion passes through the fourth through hole and a nut is provided to engage with the threaded portion.

In the present embodiment, the outer casing <NUM> includes a first casing <NUM> and a second casing <NUM>. A chamber for mounting the harmonic drive <NUM> is formed between the first casing <NUM> and the second casing <NUM>. A first step groove <NUM> and a second step groove <NUM> are defined in positions where the first casing <NUM> and the second casing <NUM> join. The circular spline <NUM> is received in the first step groove <NUM> and the second step groove <NUM>. The first step groove <NUM> and the second step groove <NUM> clamp the circular spline <NUM> therein to hold the circular spline <NUM> in position.

As shown in <FIG>, in the present embodiment, a method for controlling a servomotor includes:.

The angular displacement of the output member <NUM> can be derived by the sensor that directly detects the rotational motion data of the post <NUM>. Since the post <NUM> is fixed to the output member <NUM>, and no other arrangements that cause transmission errors exist between the post <NUM> and the output member <NUM>, the rotational motion data detected by the sensor can directly reflect the actual motion of the output member <NUM>.

The angular displacement theoretically derived according to the initial motion planning ignores the backlash and the angular transmission accuracy existed in the electric motor <NUM>. The angular displacement that should be theoretically realized through the motion planning accords with the expected angular displacement that the motion planning expects.

In the embodiment, a spur gear reducer device is arranged between the electric motor <NUM> and the harmonic drive <NUM>. After the first stage speed reduction by the spur gears, a second stage speed reduction is done by the harmonic drive <NUM>. The above arrangement can further increase the transmission ratio, and the position of the electric motor can be adjusted by using gears of different sizes.

Claim 1:
A servomotor comprising:
a control circuit (<NUM>), an electric motor (<NUM>), a harmonic drive (<NUM>) and a sensor;
the control circuit (<NUM>) being electrically connected to the electric motor (<NUM>), and configured to output signals to control the electric motor (<NUM>) to rotate;
the harmonic drive (<NUM>) comprising an outer casing (<NUM>), a wave generator (<NUM>), a flex spline (<NUM>) and a circular spline (<NUM>) that are arranged within the outer casing (<NUM>), the electric motor (<NUM>) being arranged at one end of the outer casing (<NUM>), the wave generator (<NUM>) being driven by the electric motor (<NUM>), the flex spline (<NUM>) being sleeved on an exterior of the wave generator (<NUM>), the circular spline (<NUM>) being fixed within the outer casing (<NUM>), the flex spline (<NUM>) being located within the circular spline (<NUM>) and engaging with the circular spline (<NUM>);
the flex spline (<NUM>) being connected with an output member (<NUM>) that is configured to output power, and a post (<NUM>) arranged along a rotation axis of the output member (<NUM>); and
the sensor being arranged within the outer casing (<NUM>) and configured to detect angular displacement of the post (<NUM>);
characterised in that
the harmonic drive (<NUM>) further comprises a flex spline pressing member (<NUM>) and a bearing inner ring (<NUM>), the flex spline pressing member (<NUM>) comprises a position limiting portion (<NUM>) and a protruding portion (<NUM>), the flex spline (<NUM>) defines a first through hole (<NUM>) in a bottom thereof, the bearing inner ring (<NUM>) defines a second through hole (<NUM>), the protruding portion (<NUM>) of the flex spline pressing member (<NUM>) passes through the first through hole (<NUM>) and the second through hole (<NUM>) in sequence, and the flex spline (<NUM>) is arranged between the position limiting portion (<NUM>) of flex spline pressing member (<NUM>) and the bearing inner ring (<NUM>), and wherein the output member (<NUM>), the bearing inner ring (<NUM>), the flex spline (<NUM>) and the flex spline pressing member (<NUM>) are fixed together.