Abstract:
A motor control system for controlling a plurality of motors for moving an object in respectively different axial directions that includes a plurality of target devices, each target device including a (pulse width modulation) PWM signal generator that generates a PWM signal for driving a motor using a triangular signal, and a host device for supplying a synchronization signal to each of the target devices, wherein each target device is arranged with a corresponding motor, and each PWM signal generator resets the triangular signal in response to the synchronization signal.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a motor control system for controlling a plurality of motors. In particular, the present invention relates to a motor control system for controlling motors to cause an object to move in the directions of different axes. 
   DESCRIPTION OF THE PRIOR ART 
   Machine tools such as electric discharge machines and machining centers are comprised of motors for moving an object in respective axial directions, an analysis section for analyzing NC code to obtain a path along which the object will move, and a position/velocity/current control section for controlling position, velocity and drive current for motors based on the path. The object is, for example, a tool attached to a head, or a workpiece fixed to a table. Machine tools are generally provided with a numerical control unit (NC) that is physically separate. The analysis section is contained in the numerical control unit, and NC code is sent to the analysis section as an NC program or in a format input by the operator. Many conventional machine tools are implemented with either of the following typical hardware structures: 
   (1) A structure where one analysis section is provided as a host device and the position/velocity/current control sections and servo amps are arranged as target devices for respective motors; and 
   (2) A structure where the analysis section and the position/velocity/current control section are the same host device and current commands are sent from the host device to the servo amps for the respective motors. 
   In the case of the first structure (1), signals can be transmitted from the analysis section to each of the position/velocity/current control sections using comparatively low speed wiring such as a serial bus, which means that it is possible to eliminate a lot of wiring. 
   In the case of the second structure (2), since current commands to the servo amps for each motor are determined using the output of the single host device, it is easy to synchronize a plurality of motors and it is possible to realize high speed high precision drive. U.S. Patent Publication No. 2004-160206 and U.S. Pat. No. 7,026,778 disclose a device relating to motor control using the second structure (2). 
   However, in the case of the first structure (1), it is difficult to synchronize target devices as position/velocity/current control sections that are arranged for respective motors. Specifically, as the synchronous drive of motors is also difficult, the first structure (1) is unsuitable for machine tools that require high-speed high precision performance. Also, in the case of the second structure (2), since the current control having the heaviest computational load is processed using a processor of the single host device, the processing load on the processor is increased if a machine tool needs many control axes. Therefore, when controlling a number of motors it becomes necessary to handle the setting of the motor execution cycle to a low frequency etc., and high-speed drive is complicated. 
   SUMMARY OF THE INVENTION 
   The present invention attends to the above-mentioned problems. In general, the invention provides a motor control system with reduced wiring for control signals for driving a plurality of motors at a high speed and with high precision. 
   According to one aspect of the present invention, a motor control system for controlling a plurality of motors for moving an object in respectively different axial directions includes a plurality of target devices, each target device including a Pulse Width Modulation (PWM) signal generator that generates a PWM signal for driving a motor using a triangular signal, and a host device for supplying a synchronization signal to each of the target devices, wherein the target device is arranged for each motor, and the PWM signal generator resets the triangular signal in response to the synchronization signal. 
   Preferably, each PWM signal generator has a counter for repeatedly counting clock intervals in order to generate the triangular signal, and the host device has a counter for repeatedly counting clock intervals at a cycle that is the same as or an integer number of times that of the counter in the PWM signal generator and generates a synchronization signal when the count of the counter in the PWM signal generator reaches a specified value. 
   It is preferable for a clock input to the counter in the host device to have the same frequency as a clock input to each of the counters in the PWM signal generators, but the clock input to the host device counter can also have a frequency of an integer number of times the frequency of the clock input to the counter in each triangular wave generator. 
   Also, preferably, each target device further includes a reset signal generator that generates a reset signal in response to the synchronization signal, and the triangular signal generator resets a triangular signal in response to the reset signal. 
   According to another aspect of the present invention a motor control system for controlling a first motor and a second motor for moving an object in respectively different axial directions includes a position and velocity controller that generates first and second velocity commands based on first and second position errors between first and second position commands and first and second position feedbacks, and that generates first and second torque commands based on first and second velocity errors between first and second velocity commands and first and second velocity feedbacks, respectively. The motor control system further includes a first servo amp that supplies a first drive current to the first motor and generates a first current feedback, a first PWM signal generator that supplies a first PWM signal to the first servo amp, a first current controller that supplies a first three phase voltage command to the first PWM signal generator based on the first current feedback and the first torque command, a second servo amp that supplies a second drive current to the second motor and generates a second current feedback, a second PWM signal generator that supplies a second PWM signal to the second servo amp, a second current controller that supplies a second three phase voltage command to the second PWM signal generator based on the second current feedback and the second torque command, and first and second serial buses that transmit the first and second torque commands to the first and second controllers, respectively. 
   Preferably, the first PWM signal generator has a first counter that generates a first count signal in order to generate a triangular signal, the second PWM signal generator has a second counter that generates a second count signal in order to generate a triangular signal, and the position and velocity controller includes a synchronization signal generator that generates a synchronization signal that synchronizes the first and second counters. 
   According to still another aspect of the present invention a motor control system for controlling a first motor and a second motor for moving an object in respectively different axial directions includes a first PWM signal generator having a first counter that repeatedly counts clock intervals and generating a first PWM signal for the first motor, a second PWM signal generator having a second counter that repeatedly counts clock intervals and generating a second PWM signal for the second motor, and a synchronization signal generator that generates a synchronization signal that synchronizes the first and second counters. 
   As each triangular signal generator repeatedly resets the triangular signal in response to a synchronization signal generated by the host device, it is possible to suppress undesirable variation in phase of triangular signals. Accordingly, it is possible to achieve phase synchronization of a plurality of motors, and it is possible to realize a motor control system having high speed and high performance. 
   Also, by having a structure in which the positional control and the velocity control, which have a light processing load, are carried out in the host device, and in which the current control processing, that has a heavy processing load, is carried out in each target device, it is possible to connect the host device and each target device using comparatively low-speed wiring such as a serial bus, thereby resulting in reduced wiring in the motor control system. 
   Other novel features of the invention will be described in the following description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a motor control system of the present invention. 
       FIG. 2A  is a timing chart showing a count signal within the host device. 
       FIG. 2B  is a timing chart showing a synchronization signal within the host device. 
       FIG. 2C  is a timing chart showing reset signals within the target devices. 
       FIG. 2D  is a timing chart showing a count signal within the target devices. 
       FIG. 2E  is a timing chart showing a triangular signal within the target devices. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An exemplary embodiment of a motor control system of the present invention will now be described with reference to the drawings. 
   As shown in  FIG. 1 , a motor control system  100  comprises a host device  2 , a hub  3 , and an X-axis controller  4   x  and a Y-axis controller  4   y  as target devices. In order to simplify the description, with this embodiment the motor control system  100  controls two servo motors  51   x  and  51   y  for moving an object in two directions of the X-axis and the Y-axis. However, the control axes are not thus limited. When an additional control axis becomes necessary, it is possible to add a motor and a target device similar to the X-axis controller  4   x.    
   A human interface  1  is a unit provided with a computer, for example a personal computer provided with a keyboard, mouse, display etc. In this exemplary embodiment, the human interface  1  and the host device  2  are contained in a numerical control device (“NC”) that is physically separated from the machine tool. An operator of the machine tool can input various types of data and commands etc via the human interface  1 , which are then supplied to the host device  2 . 
   The host device  2  is comprised of an analyzer and position/velocity controller  21 , memory  22  and a serial to parallel converter  23 . The analyzer and position/velocity controller  21  is constituted by a single CPU etc. The analyzer and position/velocity controller  21  is provided internally with a data/program storage section (memory) (not shown), and prior to machining an NC program is stored in the storage section. The analyzer and position/velocity controller  21  obtains path information for moving the object that is to be operated on by the machine tool by analyzing the NC code defined in the NC program. Further, the analyzer and position/velocity controller  21  generates position commands for the motors  51   x ,  51   y , respectively, based on the path information. Position feedbacks for the x-axis and the y-axis are supplied to the position/velocity controller  21  from position detectors appropriately provided in the servo motors  51   x  and  51   y . Velocity commands for servo motors  51   x  and  51   y  are generated based on position errors between position commands and position feedbacks. Further, torque commands for servo motors  51   x  and  51   y  are generated based on errors between velocity commands and velocity feedbacks which can be differential values of the position feedbacks. 
   Torque commands are temporarily stored in the memory  22 , then converted from parallel signals to serial signals by the serial to parallel converter  23  and supplied to the X-axis controller  4   x  and the Y-axis controller  4   y . In this way, the position/velocity control processing, which has a light processing load, are executed by a single processing unit, namely the analyzer and position/velocity controller  21 . The current control processing, which has a high processing load, is respectively executed by the X-axis controller  4   x  and the Y-axis controller  4   y . In doing this, it is possible to wire between the host device  2  and the X-axis controller  4   x  and the Y-axis controller  4   y  using comparatively low velocity serial buses  32 , respectively. It is therefore possible to reduce the number of wires in the motor control system. The hub  3  is a distributor for transmitting torque commands generated by the host device  2  to the X-axis controller  4   x  and the Y-axis controller  4   y.    
   The X-axis controller  4   x  comprises a serial to parallel converter  41   x , a memory  42   x , a current controller  43   x , a PWM signal generator  44   x , a servo amp  45   x , an encoder counter  46   x , and a reset signal generator  47   x . The serial to parallel converter  41   x  converts torque commands transmitted from the host device  2  via the hub  3  and serial bus  32  from a serial signal to a parallel signal. The converted torque command is temporarily stored in the memory  42   x . The current controller  43   x  is constituted by a single CPU. Position feedback for the servo motor  51   x  is generated by the encoder counter  46   x  and current feedback is generated by the servo amp  45   x , and both feedbacks are supplied to the current controller  43   x . The current controller  43   x  reads out a torque command stored in the memory  42   x , and generates a three phase voltage command based on the position feedback, the current feedback and the torque command. The PWM signal generator  44   x  has a triangular signal generator  441   x  that generates a triangular signal of a specified period, and the PWM signal generator generates a PWM signal based on the triangular signal and the three phase voltage command. The servo amp  45   x  generates a drive current based on the PWM signal, and supplies the drive current to the servo motor  51   x . The servo motor  51   x  is equipped with a rotary encoder  52   x . The rotary encoder  52   x  supplies angular position of the servo motor  51   x  to the encoder counter  46   x  as a pulse signal. The triangular signal generator  441   x  has a clock generator (not shown) and a counter  442   x  which counts the clock intervals to generate a count signal, as shown in  FIG. 2D . The triangular signal generator  441   x  generates a triangular signal, as shown in  FIG. 2E , using the count signal from counter  442   x . The counter  442   x  can be an up down counter, or can be a counter for simply repeatedly counting from zero to an upper limit value. 
   The Y-axis controller  4   y  is realized using similar elements as the X-axis controller  4   x , as described above, and so its description is therefore not repeated here. 
   The invention achieves matching of the periods of triangular signals generated by the triangular signal generators  441   x  and  441   y  and driving of the servo motors  51   x ,  51   y  in synchronism. For this reason, clock generators inside the triangular signal generators  441   x  and  441   y  generate clock intervals at the same frequency or at frequencies an integer number of times different. However, in actual fact the clock generators are separate, which means that the respective clocks will become asynchronous with the passage of time. As a result, variation occurs in the count signals of the counters  442   x  and  442   y , resulting in phase shifting in the triangular signals generated by the triangular signal generators  441   x  and  441   y . If this type of situation arises, each motor is in an asynchronous state and it is not possible to perform high precision control of the motors. 
   Therefore, a synchronization signal generator  211  for generating a synchronization signal for resetting each triangular signal generator at the same time is provided within the position/velocity controller  21 . The synchronization signal generator  211  has a counter  212  for repeatedly counting clocks at a period that is the same as or an integer number of times that of the counters  442   x  and  442   y .  FIG. 2A  shows a count signal generated by the counter  212 . The counter  212  counts clock intervals output from a clock generator (not shown) provided either internally or externally. The counter  212  can be an up down counter, or can be an up counter for simply repeatedly counting from zero to an upper limit value. The synchronization signal generator  211  generates a signal PH_CLK, namely a synchronization signal, every time the count of the counter  212  reaches a specified value. The signal PH_CLK is a rectangular signal switching between “Hi” and “Low” when the count signal in  FIG. 2A  becomes “0”. For example, the signal PH_CLK switches from “Low” to “Hi” at time t 1 , and switched from “Hi” to “Low” at time t 2 . The signal PH_CLK is supplied via the hub  3 , serial bus  32  and the serial to parallel converter  41   x  to the reset signal generators  47   x  and  47   y . The PWM signal generators  44   x ,  44   y  reset the respective count signals to zero in accordance with this synchronization signal PH_CLK, and the count starts again. In this manner, phase shifting of the triangular signals generated in the triangular signal generators  441   x  and  441   y  is periodically corrected, and synchronization of each triangular signal is achieved. 
   In order to simplify the description, the following description is given mainly using elements contained in the X-axis controller  4   x , but the same signal processing is also carried out in the Y-axis controller  4   y.    
   The reset signal generator  47   x  generates a pulse signal SYNC_COM as a reset signal, in response to the rising edge of the signal PH_CLK, as shown in  FIG. 2C . The signal SYNC_COM is generated at, for example, time t 1  and time t 3 , namely every one cycle of signal PH_CLK. The signal SYNC_COM is supplied to the triangular signal generator  441   x . The triangular signal generator  441   x  resets the count signal to zero in response to the rising edge of the signal SYNC_COM, and again starts generating the count signal. In this way, the triangular signal generator  441   x  resets the triangular signal in response to the reset signal. For example, reset of the count signal is carried out at time t 1  and time t 3 . From time t 1  to time t 3 , even if phase shifting occurs between triangular signals generated in the triangular signal generators  441   x  and  441   y  reset of the count signal in response to the rising edge of the signal SYNC_COM is repeatedly carried out, which means that it is possible to suppress phase shifting between each triangular signal. Accordingly, synchronization of the servo motors  51   x  and  51   y  is achieved, and a motor control system having high speed and high performance is realized. 
   The present invention is not intended to be limited to the disclosed form. It is clear that many improvements and variations are possible with reference to the above description. For example, as illustrated in  FIGS. 2C and 2D , count signals of counters  442   x  and  442   y  are reset to zero in response to a rising edge of the signal SYNC_COM. Alternatively, they may be reset to zero in response to a rising edge of the signal PH_CLK. The illustrated embodiment was selected to explain the essence and practical application of the invention. 
   While the present invention has been particularly described with reference to the various figures and embodiments, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the invention. There may be many other ways to implement the invention. Many changes and modifications may be made to the invention, by one having ordinary skill in the art, without departing from the spirit and scope of the invention.