Patent Publication Number: US-2019176877-A1

Title: Actuator system and abnormality detector

Description:
TECHNICAL FIELD 
     The present invention relates to an actuator system and an abnormality detector. 
     BACKGROUND ART 
     An actuator indispensable to the operation of a device has a redundant configuration, for example a duplexed system, in order to improve availability. 
     In Patent Literature 1, there is disclosed a motor vehicle steering system characterized in that: the motor vehicle steering system includes a steering mechanism that is mechanically discontinuous with a steering member and has a plurality of steering actuators to drive a steering shaft, one or more reaction force actuators to give a steering reaction force to the steering member, and first control means and second control means to control a currently-controlled steering actuator and a currently-controlled reaction force actuator respectively in the steering actuators and the reaction force actuators and has a plurality of control systems that allow data communication between both the control means by wired means and wireless means and control the currently-controlled steering actuator and the currently-controlled reaction force actuator on the basis of the data communicated by the wired means or the wireless means; and each of the control systems has disconnection judgment means to judge whether or not the wired means is disconnected and communication switching means to allow data communication by the wired means when the disconnection judgment means does not judge the wired means to be disconnected and switch from the wired means to the wireless means so as to allow the data communication when the disconnection judgment means judges the wired means to be disconnected. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2004-338563 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the invention described in Patent Literature 1, although a communication path by wired means and wireless means is made redundant, the occurrence of a failure in the wireless means is not assumed and abnormality of a redundant configuration cannot be detected. 
     Solution to Problem 
     The actuator system according to a first embodiment of the present invention has an actuator drivable in two directions, first and second actuator control sections to output control signals to the actuator, and an abnormality detection section to control the first and second actuator control sections and the abnormality detection section detects abnormality by making the first actuator control section output a first control signal to drive the actuator in a first direction and making the second actuator control section output a second control signal to drive the actuator in a second direction that is paired with the first direction. 
     The abnormality detector according to a second embodiment of the present invention is an abnormality detector used in an actuator system having an actuator drivable in two directions and first and second actuator control sections to output control signals to the actuator, the abnormality detector has an abnormality detection section to control the first and second actuator control sections, and the abnormality detection section detects abnormality by making the first actuator control section output a first control signal to drive the actuator in a first direction and making the second actuator control section output a second control signal to drive the actuator in a second direction that is paired with the first direction. 
     Advantageous Effects of Invention 
     The present invention makes it possible to detect abnormality of a redundant configuration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view showing an appearance of an electric power steering system. 
         FIG. 2  is a view showing a detailed configuration of an electric power steering system. 
         FIG. 3  is a flowchart showing the operation of an abnormality detection section. 
         FIG. 4  is a flowchart showing the operation of a first ECU and a second ECU. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 
     An embodiment of an electric power steering system  100  is explained hereunder in reference to  FIGS. 1 to 4 . 
     Configuration 
       FIG. 1  is a view showing an appearance of an electric power steering system  100 . The electric power steering system  100  assists steering of a steering wheel  201  operated by a user. The electric power steering system  100  has a motor  1 , a steering angle sensor  4 , an inverter  5 , an ECU  6 , a steering torque sensor  9 , and an abnormality detector  300 . 
     A steering shaft  202  is connected to the steering wheel  201 . A pinion shaft  204  is connected to an end of the steering shaft  202  on the side opposite to the steering wheel  201  with a torsion bar  203  interposed. The torsion bar  203  is installed between the steering shaft  202  and the pinion shaft  204 . The steering torque sensor  9  detects a steering torque of a user on the basis of a torsional amount of the torsion bar  203 . A pinion  205  attached to the tip of the pinion shaft  204  is engaged with rack teeth  206 . The rack teeth  206  are attached to a rack bar  207  and steerable wheels  208  are attached to both the ends of the rack bar  207 . The rack bar  207  is driven in a linear direction by receiving a driving force from the motor  1  through a decelerator  209 . In other words, the motor  1  assists the movement of the rack bar  207  when a user operates the steering wheel  201 . The assist of the steering of the steering wheel  201  by the motor  1  is referred to as “steering assist” hereunder. 
     The steering angle sensor  4  detects a steering angle of the steering wheel  201  by a user by detecting rotation of the steering shaft  202 . The steering torque sensor  9  detects a steering torque of a user on the basis of a torsional amount of the torsion bar  203 . Since a steering torque is detected by a torsion between the steering shaft  202  and the pinion shaft  204 , the steering torque is detected not only when the steering wheel  201  is steered by a user but also when the rotation of the motor  1  is transmitted to the pinion shaft  204 . When a user does not hold the steering wheel  201 , however, the steering shaft  202  follows easily to the movement of the pinion shaft  204  and hence a steering torque is hardly detected. 
     The steering angle sensor  4  detects a steering angle of the steering wheel  201  and outputs the steering angle to the abnormality detector  300  and the ECU  6 . The ECU  6  operates the motor  1  through the inverter  5  on the basis of outputs of the steering angle sensor  4  and the abnormality detector  300 . The steering torque sensor  9  outputs a detected torque to the ECU  6  and the abnormality detector  300 . 
       FIG. 2  is a view showing a detailed configuration of an electric power steering system  100 . The configuration of the electric power steering system  100  is made redundant and includes a first group A and a second group B and both the groups operate similarly. The electric power steering system  100  has the first group A, the second group B, and an abnormality detector  300 . 
     The first group A includes a first motor  11 , a first motor rotation sensor  12 , a first motor torque sensor  13 , a first steering angle sensor  14 , a first inverter  15 , a first ECU  16 , and a first steering torque sensor  19 . The second group B includes a second motor  21 , a second motor rotation sensor  22 , a second motor torque sensor  23 , a second steering angle sensor  24 , a second inverter  25 , a second ECU  26 , and a second steering torque sensor  29 . The first ECU  16  has a first signal generator  17  and a first abnormal group judgment section  18 . The second ECU  26  has a second signal generator  27  and a second abnormal group judgment section  28 . 
     The first motor  11  and the second motor  21  are two virtual motors and both the physical configurations are comprehended in the motor  1 . The first motor  11  and the second motor  21  share a rotor and a stator but do not share wiring wires. The first motor  11  and the second motor  21  have different wiring wire positions. The first motor  11  rotates on the basis of a control signal inputted from the first inverter  15 . The second motor  21  rotates on the basis of a control signal inputted from the second inverter  25 . The first motor  11  and the second motor  21 , however, are virtual motors as stated earlier and are physically the single motor  1 . If sinusoidal waves having an identical voltage amplitude, an identical frequency, and phases different by 180 degrees are inputted into the first motor  11  and the second motor  21  simultaneously therefore, both the torques compete with each other and no torques are generated in the motor  1 . 
     Both the first motor rotation sensor  12  and the second motor rotation motor  22  are motor rotation sensors incorporated in the motor  1  and detect a rotation of the motor  1 . The mounting positions of the first motor rotation sensor  12  and the second motor rotation sensor  22  are different but the measuring target is the same and hence a measurement result of the other sensor can be calculated from numerical conversion depending on the difference of the mounting positions. The first motor rotation sensor  12  and the second motor rotation motor  22  are collectively referred to as a motor rotation sensor  2  hereunder. 
     Both the first motor torque sensor  13  and the second motor torque sensor  23  are torque sensors incorporated into the motor  1  and detect a rotation torque of the motor  1 . The first motor torque sensor  13  and the second motor torque sensor  23  are collectively referred to as a motor torque sensor  3  hereunder. 
     The first steering angle sensor  14  and the second steering angle sensor  24  are independent sensors constituting a steering angle sensor  4 . Both the first steering angle sensor  14  and the second steering angle sensor  24  detect a steering angle of a steering wheel  201 . The steering angle sensor  4  outputs a detected steering angle to the abnormality detector  300  and an ECU  6 . The first steering torque sensor  19  and the second steering torque sensor  29  are independent sensors constituting a steering torque sensor  9 . Both the first steering torque sensor  19  and the second steering torque sensor  29  detect steering torques on the basis of a torsional amount of a torsion bar  203  shown in  FIG. 1 . 
     The first inverter  15  and the second inverter  25  are independent inverters constituting an inverter  5 . The first inverter  15  inputs a control signal, for example a sinusoidal wave having predetermined voltage amplitude, frequency, and phase, to the first motor  11  on the basis of an operation command, for example a PWM signal, outputted from the first ECU  16 . The second inverter  25  inputs a control signal, for example a sinusoidal wave having predetermined voltage amplitude, frequency, and phase, to the second motor  21  on the basis of an operation command, for example a PWM signal, outputted from the second ECU  26 . The inverter  5  feeds back an electric current value to the ECU  6  for feedback control. 
     The first ECU  16  and the second ECU  26  are independent ECUs constituting the ECU  6 . The ECU  6 : generates PWM signals on the basis of two kinds of operation commands that are received from the abnormality detector  300  and will be described later; and outputs the PWM signals to the inverter  5 . The ECU  16  and the ECU  26  have a time synchronization function and the timers of both the ECUs are synchronized appropriately. As a result, the ECU  16  and the ECU  26  can execute processing requiring consistency of timing as it will be described later. 
     The first ECU  16  has a CPU, a ROM, a RAM, and a PWM signal aenerating circuit, those being not shown in the figure. The first ECU  16  functions as the first sianal generator  17  and the first abnormal aroup judgment section  18  by making the CPU execute a program stored in the ROM with data stored in the RAM. The first signal aenerator  17  outputs a PWM signal to the first inverter  15  in consideration of an electric current value fed back from the first inverter  15 . The first abnormal group judgment section  18  judges which of the first group A and the second group B has an abnormality. The operation of the first abnormal group judgment section  18  will be described later. 
     The second ECU  26  has a CPU, a ROM, a RAM, and a PWM signal generating circuit, those being not shown in the figure. The second ECU  26  functions as the second signal aenerator  27  and the second abnormal group judgment section  28  by making the CPU execute a program stored in the ROM with data stored in the RAM. The second sianal generator  27  outputs a PWM signal to the second inverter  25  in consideration of an electric current value fed back from the second inverter  25 . The second abnormal group judgment section  28  judges which of the first group A and the second group B has an abnormality. The operation of the second abnormal group judgment section  28  will be described later. 
     The abnormality detector  300  is an ECU having an abnormality detection section  30 . The abnormality detector  300  has a CPU, a ROM, and a RAM, those being not shown in the figure, and executes the processing that will be described later as the abnormality detection section  30  by making the CPU execute a program stored in the ROM with data stored in the RAM. Outputs of the motor torque sensor  3  and the steering angle sensor  4  are inputted into the abnormality detector  300 . The abnormality detector  300  outputs abnormality detection commands and abnormal group judgment commands that will be described later to the ECU  6 . 
     Outline of Operation 
     The processing by the abnormality detection section  30  is roughly classified into abnormality detection processing of detecting the occurrence of a problem and abnormal group judgment processing of judging which group has an abnormality. Firstly, the abnormality detection processing is explained. 
     Firstly, the abnormality detection section  30 : judges whether or not steering assist is effective, in other words whether or not assist for the steering of a steering wheel  201  by the motor  1  is effective; and outputs abnormality detection commands to the ECU  6  and starts the abnormality detection processing when the assist is judged to be ineffective. whether or not steering assist is effective can be judged on the basis of outputs of various sensors and for example outputs of the first motor rotation sensor  12 , the first motor torque sensor  13 , the first steering angle sensor  14 , the first steering torque sensor  19 , the second motor rotation sensor  22 , the second motor torque sensor  23 , the second steering angle sensor  24 , and the second steering torque sensor  29  can be used. Judgment may be done on the basis of an output of only one sensor in the above sensors or outputs of multiple sensors. 
     The ECU  6  that has received abnormality detection commands makes the signal generator  7  output predetermined PWM signals. A control signal which the first inverter  15  outputs by a PWM signal is referred to as a “first control signal” and a control signal which the second inverter  25  outputs by a PWM signal is referred to as a “second control sianal” hereunder. The first control signal and the second control sianal are sinusoidal waves having an identical voltage amplitude, an identical frequency, and phases different by 180 degrees. The first control signal is outputted by the first inverter  15  simultaneously with the output of the second control signal by the second inverter  25 . The simultaneity can be materialized as follows for example. That is, information of the time when processing should be executed is inputted into the first ECU  16  and the second ECU  26  from the abnormality detection section  30  and the first ECU  16  and the second ECU  26  output PWM signals at the time. This is because the first ECU  16  and the second ECU  26  have synchronized time information as stated earlier. 
     The first motor  11  to which the first control sianal is inputted and the second motor  21  to which the second control signal is inputted generate rotation torques respectively. The first motor  11  and the second motor  21  are physically the motor  1  as stated earlier and the motor  1  operates on the basis of the magnitudes of a torque generated by the first motor  11  and a torque generated by the second motor  21 . Here, the torques generated by the first motor  11  and the second motor  21  should be at least larger than a loss caused by friction at the bearing part of the motor  1 , a so-called friction torque. In the motor  1 , both the torques are identical over the entire period if there is no problem in the electric power steering system  100  and hence the torques countervail each other. In other words, the torque generated by the first motor  11  and the torque generated by the second motor  21  offset each other, the motor  1  comes to be in an equilibrium state, and a rotation torque is not generated. If there is some sort of problem in the electric power steering system  100  and both the torques are not identical, however, in other words if both the torques lose balance, a rotation torque is generated in either direction and is detected by the motor torque sensor  3 . For example, when the coil resistances of the wiring wires constituting the first motor  11  and the second motor  21  are different by aging too, both the torques do not countervail each other and a torque is detected. Here, in the case where the coil resistances of the first motor  11  and the second motor  21  are different by aging too, a torque is detected likewise. 
     The abnormality detection section  30 : monitors the output of the motor torque sensor  3 ; judges that an abnormality is detected when the output of the motor torque sensor  3  exceeds a predetermined value, in other word when the motor  1  is not in an equilibrium state; and judges that an abnormality does not occur when the output of the motor torque sensor  3  does not exceed a predetermined value, in other word when the motor  1  is in an equilibrium state. Meanwhile, the abnormality detection section  30  may also judge whether or not the motor  1  is in an equilibrium state by using an output of any one of the motor rotation sensor  2 , the steering angle sensor  4 , and the steering torque sensor  9  in place of the motor torque sensor  3 . For example, it is also possible to: detect at least one of the rotational speed, the rotation amount, and the rotation acceleration of the motor  1  on the basis of an output of the motor rotation sensor  2  or the steering angle sensor  4 ; and judge whether or not the motor  1  is in an equilibrium state on the basis of the detection result. Further, it is also possible to: detect the rotation torque of the motor  1  on the basis of an output of the steering torque sensor  9 ; and judge whether or not the motor  1  is in an equilibrium state on the basis of the detection result. Otherwise, it is also possible to: detect a stroke of the rack bar  207 ; and judge whether or not the motor  1  is in an equilibrium state on the basis of the detection result. That is abnormality detection processing. Successively, abnormal croup judgment processing is explained. 
     The outline of the abnormal group judgment processing is as follows. 
     The abnormality detection section  30  that has judged an abnormality to occur through abnormality detection processing outputs an abnormal group judgment command to the ECU  6 . The first ECU  16  and the second ECU  26  that have received the abnormal group judgment command output PWM signals for staggering the output timings of a first control signal and a second control signal and outputting the first control signal and the second control sianal to the first inverter  15  and the second inverter  25 , respectively. Here, the first ECU  16  and the second ECU  26  may also output PWM sianals for changing a ratio of the magnitudes of a first control signal and a second control signal, in other words a ratio of the magnitudes of a voltage amplitude, from 1:1 to 2:1 or 1:0, in place of staggering the output timings of the first control signal and the second control signal and outputting the first control signal and the second control signal. In the case where a ratio is 1:0, however, if a first group A is judged to have no abnormality on the basis of the first control signal, either a second group B is assumed to have an abnormality or a ratio is set at 0:1 and only the second control signal is outputted again. The first abnormal group judgment section  18  and the second abnormal aroup judgment section  28  judge which of the first group A and the second group B has an abnormality on the basis of the outputted PWM signals and the output of the motor torque sensor  3 . Then the first abnormal group judgment section  18  and the second abnormal group judgment section  28  output the judgment result to the abnormality detector  300 . 
     A concrete example of the abnormal group judgment processing is as follows for example. 
     The first ECU  16  and the second ECU  26  output PWM signals so that a first control signal may be outputted in advance and a second control signal may start to be outputted when the first control signal is outputted for a predetermined period of time. On this occasion, only the first motor  11  operates on the basis of the first control sianal until the second control sianal starts to be outputted and hence a torque corresponding to the operation of the first motor  11  is expected to be detected by the motor torque sensor  3 . The first abnormal group judgment section  18  and the second abnormal group judgment section  28  judge that the first group A has an abnormality when the torque detected by the motor torque sensor  3  is different from an assumed torque by a predetermined value or more. When the first abnormal group judgment section  18  and the second abnormal group judgment section  28  judge that the first group A has an abnormality, the first abnormal group judgment section  18  and the second abnormal group judgment section  28  notify the result to the abnormality detector  300 . When the first abnormal group judgment section  18  and the second abnormal group judgment section  28  do not judge that the first group A has an abnormality, the first abnormal group judgment section  18  and the second abnormal group judgment section  28  output PWM signals for switching the output timings of a first control signal and a second control signal and outputting the first control signal and the second control signal to the first inverter  15  and the second inverter  25 , respectively. In other words, in contrast to the above example, PWM signals are outputted so that the second control signal may be outputted in advance and the first control signal may start to be outputted when the second control sianal is outputted for a predetermined period of time. Then similarly to the above case, the first abnormal Group judgment section  18  and the second abnormal group judgment section  28  judge whether or not the second group B has an abnormality and, when the first abnormal group judgment section  18  and the second abnormal group judgment section  28  judges that the second group B has an abnormality, the first abnormal group judgment section  18  and the second abnormal group judgment section  28  notify the result to the abnormality detector  300 . 
     The abnormality detector  300 , when a judgment result is outputted from the ECU  6 , stops a processing group that is judged to have an abnormality, in other words the first group A or the second group B. Then subsequently steering assist is executed by using only the processing group not stopped. 
     Flowchart 
       FIG. 3  is a flowchart showing operations of an abnormality detection section  30 . A subject that executes the steps explained below is a CPU in an abnormality detector  300 . The abnormality detection section  30  operates: at the startup time of a vehicle incorporating an electric power steering system  100 , in other words when the power source of electrical components in a vehicle is turned on; and subsequently when the vehicle stops for a predetermined period of time. 
     At Step S 501 , the CPU judges whether or not steering assist, in other words assist for the steering of a steering wheel  201  by a motor  1 , is effective. Whether or not steering assist is effective can be judged from an output of a steering angle sensor  4  and the operating conditions of the motor  1 . The execution of a program showing operations in the flowchart of  FIG. 3  is finished when the steering assist is judged to be effective and the process advances to Step S 502  when the steering assist is judged to be ineffective. 
     At Step S 502 , the CPU transmits an abnormality detection command to an ECU  6 . At subsequent Step S 503 , whether or not an output of a sensor value of the motor  1  is a predetermined value or more is judged. The sensor value is a value of anyone of a first motor torque sensor  13 , a second motor torque sensor  23 , a first motor rotation sensor  12 , and a second motor rotation sensor  22 . The process advances to Step S 504  when the output of a sensor value is judged to be the predetermined value or more and the execution of the program showing operations in the flowchart of  FIG. 3  is finished when the output of a sensor value is judged to be less than the predetermined value. Here, the affirmative judgment at the step means that an abnormality is detected. 
     At subsequent Step S 504 , the CPU transmits an abnormal group judgment command to the ECU  6 . At subsequent Step S 505 , the CPU receives a judgment result from the ECU  6 . At subsequent Step S 506 , a group judged to have an abnormality, namely the first group A or the second group B, is stopped on the basis of the judgment result received at Step S 505 . When either of the groups is judged not to have an abnormality, however, neither of the groups is stopped. After the above steps, the execution of the program showing the operations in the flowchart of  FIG. 3  is finished. 
       FIG. 4  is a flowchart showing operations of a first ECU  16  and a second ECU  26 . A subject that executes the steps explained below is a CPU of each of the first ECU  16  and the second ECU  26 . The first ECU  16  and the second ECU  26  execute the program showing operations in the flowchart of  FIG. 4  every predetermined period of time. The program, however, maybe executed by being triggered by the reception of some sort of command from an abnormality detector  300 . 
     At Step S 601 , whether or not an abnormality detection command is received from the abnormality detector  300  is iudged. The process advances to Step S 602  when the abnormality detection command is judged to be received and the process advances to Step S 603  when the abnormality detection command is judged not to be received. At Step S 602 , the first ECU  16  and the second ECU  26  output PWM signals so that a first control signal may be outputted from a first inverter  15  and a second control signal maybe outputted from a second inverter  25  simultaneously and the process advances to Step S 603 . 
     At Step S 603 , whether or not an abnormal group judgment command is received from the abnormality detector  300  is judged. The process advances to Step S 604  when the abnormal group judgment command is iudged to be received and the execution of the program shown in the flowchart of  FIG. 4  is finished when the abnormal group judgment command is judged not to be received. 
     At Step S 604 , the first ECU  16  and the second ECU  26  stagger the timings of a first control signal and a second control signal from each other and output the first control signal and the second control signal to the first inverter  15  and the second inverter  25  respectively. For example, PWM signals are outputted so that the first control signal may be outputted in advance and the second control signal may start to be outputted when the first control signal is outputted for a predetermined period of time. Here, it is also possible to: change a ratio of amplitudes in place of staggering output timings; and output the first control signal and the second control signal as stated earlier. At subsequent Step S 605 , a difference between a torque detected by a motor torque sensor  3  and an assumed torque is evaluated and the process advances to Step S 606  when the difference between the detected torque and the assumed torque is judged to be only less than a predetermined value and the process advances to Step S 608  when the difference between the detected torque and the assumed torque is judged to be the predetermined value or more. 
     At Step S 606 , the first ECU  16  and the second ECU  26  output PWM signals so that the outputs may be reversed from the outputs at Step s 604  chronologically, in other words so that the second control signal may be outputted in advance and the first control signal may start to be outputted when the second control signal is outputted for a predetermined period of time. At subsequent Step S 607 , a difference between a torque detected by the motor torque sensor  3  and an assumed torque is evaluated and the process advances to Step S 609  when the difference between the detected torque and the assumed torque is judged to be only less than a predetermined value and the process advances to Step S 610  when the difference between the detected torque and the assumed torque is judged to be the predetermined value or more. 
     At Step S 608  that is executed when negative judgment is given at Step S 605 , a first group A is judged to have an abnormality, the judgment result is outputted to the abnormality detector  300 , and the execution of the program showing the operations in the flowchart of  FIG. 4  is finished. 
     At Step S 609  that is executed when affirmative judgment is given at Step S 607 , both the first group A and a second group B are judged to have no abnormalities, the judgment result is outputted to the abnormality detector  300 , and the execution of the program showing the operations in the flowchart of  FIG. 4  is finished. 
     At Step S 610  that is executed when negative judgment is given at Step S 607 , the second group B is judged to have an abnormality, the judgment result is outputted to the abnormality detector  300 , and the execution of the program showing the operations in the flowchart of  FIG. 4  is finished. 
     According to the first embodiment stated above, the following operational advantages are obtained. 
     (1) An actuator system, for example an electric power steering system  100  has: a motor  1  drivable in two directions; first and second actuator control sections, for example a first inverter  15  and a second inverter  25 , to output control signals to the motor  1 ; and an abnormality detection section  30  to control the inverter  5  through an ECU  6 . The abnormality detection section  30  detects an abnormality by making the first inverter  15  output a first control signal to drive the motor  1  in a first direction, for example in a clockwise direction, and making the second inverter  25  output a second control signal to drive the motor  1  in a second direction that is paired with the first direction, for example in a counterclockwise direction. 
     As a result, by using a redundant configuration and executing conflicting operations and by evaluating a result of offsetting the respective actions, whether or not the redundant configuration is operating appropriately or whether or not there is some sort of problem in the configuration can be detected. That is, whether or not a redundant configuration has an abnormality can be detected on the basis of a single operation command. 
     (2) An abnormality detection section  30  detects an abnormality on the basis of an equilibrium state of a motor  1  when first and second control signals are outputted. As a result, whether or not an abnormality exits can be detected easily by simply judging whether or not the motor  1  is in the equilibrium state. 
     (3) An abnormality detection section  30  makes a first inverter  15  and a second inverter  25  output signals of the same magnitude at the same timing respectively as a first control sianal and a second control signal. As a result, whether or not an abnormality exits can be detected by whether or not a sum of the output torques of a first motor  11  and a second motor  21  is always zero. Further, a transient state can also be evaluated. 
     (4) An abnormality detection section  30  makes a first inverter  15  and a second inverter  25  output amplitude signals of phases reverse to each other as a first control signal and a second control signal respectively. As a result, in the two signals outputted to a first motor  11  and a second motor  21 , one signal can be generated easily on the basis of the other signal. 
     (5) A torque generated in a motor  1  by a first control signal and a torque generated in the motor  1  by a second control signal are larger than a friction torque of the motor  1 . As a result, the motor  1  operates when some kind of problem occurs in a first group A or a second group B and hence an abnormality can be detected by detecting the movement of the motor  1 . 
     ( 6 ) An electric power steering system  100  is incorporated into a vehicle and an abnormality detection section  30  makes a first inverter  15  and a second inverter  25  output a first control signal and a second control signal when the vehicle is started, in other words when the power source of electrical components is turned on. As a result, an abnormality can be detected before the vehicle is started. 
     (7) An abnormality detection section  30  can: detect at least one of a rotation torque, a rotational speed, a rotation amount, and a rotational acceleration of a motor  1  and judge the equilibrium state of the motor  1  on the basis of an output of a motor torque sensor  3  to detect a rotation torque of the motor  1 , a motor rotation sensor  2  to detect rotation of the motor  1 , or a steering angle sensor  4  to detect a steering angle based on steering of a steering wheel  201  installed in a vehicle by a user and rotation of the motor  1 ; and judge an equilibrium state of the motor  1 . As a result, an equilibrium state can be judged even when a user does not hold a steering wheel  201 . 
     (8) An abnormality detection section  30  can detect a rotation torque of a motor  1  and judge an equilibrium state of the motor  1  on the basis of an output of a steering torque sensor  9  to detect a steering torque generated by steering of a steering wheel  201  installed in a vehicle and rotation of the motor  1 . As a result, an equilibrium state can be judged by using the steering torque having a higher sensitivity than a motor rotation senor  2  and a steering angle sensor  4  when a user holds the steering wheel  201 . 
     (9) An abnormality detection section  30  specifies which of a first group A including a first inverter  15  or a second aroup B including a second inverter  25  has an abnormality by staggering timings of outputting a first control sianal and a second control signal or differentiating the maanitudes of the first control signal and the second control signal when the abnormality detection section  30  detects the abnormality. As a result, not only whether or not an abnormality exists but also which of the first group A and the second group B has an abnormality can be identified. 
     (10) An abnormality detection section  30  stops a group including an inverter  5  identified as having an abnormality, namely a first group A or a second group B. As a result, it is possible to operate an electric power steering system  100  by using only a croup where an abnormality is not detected. 
     (11) An abnormality detector  300  is used in an electric power steering system  100  having a motor  1  drivable in two directions and first and second actuator control sections, for example a first inverter  15  and a second inverter  25 , to output control signals to the motor  1 . The abnormality detector  300  has an abnormality detection section  30  to control the first inverter  15  and the second inverter  25  through an ECU  6 . The abnormality detection section  30  detects an abnormality by making the first inverter  15  output a first control signal to drive the motor  1  in a first direction, for example in a clockwise direction, and making the second inverter  25  output a second control signal to drive the motor  1  in a second direction that is paired with the first direction, for example in a counterclockwise direction. 
     The embodiments stated above may be modified as follows. 
     Modified Example 1 
     A period of time when a first ECU  16  and a second ECU  26  output PWM signals: is a predetermined period of time or shorter, which is sufficiently shorter than a period of time spent until rated revolutions are reached; and may be an extent of time allowing so-called inching operation. Even when either of processing groups has an abnormality, it is possible to suppress the amount of rotation of a steering wheel  201  and reduce an uncomfortable feeling of a user. 
     Modified Example 2 
     An abnormality detector  300  may make control signals divided into several pieces and outputted to a motor  1  by making a first ECU  16  and a second ECU  26  divide each of PWM sianals into several pieces and output the divided pieces. Even when either of processing groups has an abnormality, since a static friction force is larger than a dynamic friction force, it is possible to suppress the amount of rotation of a steering wheel  201  and reduce an uncomfortable feeling of a user. 
     Modified Example 3 
     A first ECU  16  and a second ECU  26  may physically be a single ECU. Otherwise, an abnormality detection section  30  may be configured integrally with the first ECU  16  or the second ECU  26 . 
     Modified Example 4 
     An abnormality detector  300  may output a judgment result to another abnormality detector installed in an identical vehicle when the judgment result is outputted from an ECU  6 . 
     Modified Example 5 
     In an electric power steering system  100 , a motor may not be redundant. In other words, an electric power steering system.  100  may be configured so that only one set of wiring wires may be prepared and voltages inputted from a first inverter  15  and a second inverter  25  may be inputted into the identical wiring wires. 
     Modified Example 6 
     Although a first ECU  16  and a second ECU  26  use synchronized timers in order to ensure the simultaneity of processing, it is also possible to ensure the simultaneity of processing by outputting PWM signals after a predetermined period of time has elapsed after an abnormality detection command is received from an abnormality detector  300 . 
     Modified Example 7 
     An abnormality detection method explained in the first embodiment can also be applied to another actuator steerable in two directions, for example a drive motor or the like. 
     Modified Example 8 
     A first ECU  16  and a second ECU  26  that have received abnormal group judgment commands may output control signals and judge an abnormality as follows. That is, firstly the first ECU  16  may make a first control signal outputted and judge an abnormality of a first group A on the basis of an output of a motor torque sensor  3  and successively the second ECU  26  may make a second control signal outputted and judge an abnormality of a second group B on the basis of an output of the motor torque sensor  3 . 
     Modified Example 9 
     A first ECU  16  and a second ECU  26  that have received abnormality detection commands may stop the output of PWM signals immediately when a motor torque sensor  3  detects a torque after the first ECU  16  and the second ECU  26  output the PWM sianals to an inverter  5 . The purpose is to prevent a user from being given an uncomfortable feeling by generating a torque more than necessary because the torque is detected and thus that an equilibrium state does not exist is detected. 
     Although a program is stored in a ROM not shown in the figures, the program may also be stored in a nonvolatile memory. Further, it is also possible that an abnormality detector  300  and an ECU  6  have an input and output interface not shown in the figures and a program may be read from another device through a medium available to the input and output interface when necessary. The medium cited here means, for example, a storage medium or communication medium detachable to an input and output interface, namely a wired, wireless, or light network, or a carrier wave or a digital signal propagating the network. Furthermore, a part or the whole of a function achieved by a program may be achieved through a hardware circuit or a FPGA. 
     The embodiments and modified examples stated above may be combined respectively. 
     Various embodiments and modified examples have been explained above but the present invention is not limited to those contents. Other embodiments that are conceivable within the technological thought of the present invention are also included in the scope of the present invention. 
     The disclosure of the following priority right basic application is incorporated herein by reference in its entirety: 
     Japanese Patent Application No. 2016-191118 (filed on Sep. 29, 2016) 
     LIST OF REFERENCE SIGNS 
       1  Motor 
       2  Motor rotation sensor 
       3  Motor torque sensor 
       4  Steering angle sensor 
       5  Inverter 
       9  Steering torque sensor 
       12  First motor rotation sensor 
       13  First motor torque sensor 
       14  First steering angle sensor 
       15  First inverter 
       17  First signal generator 
       18  First abnormal group judgment section 
       19  First steering torque sensor 
       22  Second motor rotation sensor 
       23  Second motor torque sensor 
       24  Second steering angle sensor 
       25  Second inverter 
       27  Second signal generator 
       28  Second abnormal group judgment section 
       29  Second steering torque sensor 
       30  Abnormality detection section 
       100  Electric power steering system 
       300  Abnormality detector