Abstract:
System for braking either linear or rotary electric motor having no permanent magnets comprising at least one servo amplifier and an electronic circuit for generating a pulsed control signal for controlling the braking of the motor. The electronic circuit of the present invention may be responsive to any emergency such as loss of the line power, a motion controller failure, or actuation of a limit switch by a driven member. A method of braking is also disclosed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The present invention relates to the field of dynamic brakes for electric motors, and, more particularly, to an electronic braking circuit system and method for providing an electromotive braking force for an electric motor having no permanent magnets, for example, in an emergency situation such as a motion controller failure, or actuation of a limit switch by a driven member of a linear motor.  
         [0003]     2. Related Art  
         [0004]     Braking of electric motors having permanent magnets typically is accomplished by shorting the motor coils, for example, through a resistor thereby applying a large load to the electromotive force generated in the motor&#39;s coils in the presence of permanent magnets. Of course, such a solution is not possible with motors having no permanent magnets. For example, according to U.S. Pat. No. 3,832,613, a permanent magnet motor is simply controlled by a dynamic brake  162  which takes advantage of the generator action of the motor to create a reverse torque for braking. Dynamic brakes for DC motors are also described in U.S. Pat. Nos. 3,786,329 and 4,767,970.  
         [0005]     A method and system for braking an electric motor having no permanent magnets is described in U.S. Pat. No. 5,828,195. However, the braking system according to this patent incorporates position tracking sensors, which, inter alia, significantly increases the cost of the system. In addition, for braking a linear motor the sensors have to be mounted on the moving member, which requires special fixtures and expensive flex cables. The accuracy of the location of the sensor in relation to the specific motor parts is very critical, because a small displacement from the optimum position can significantly affect the braking performance or even cause acceleration of the motor.  
         [0006]     It can be seen from above, that there exists in the art a requirement for a system and method, of braking without using any position tracking devices, for braking an electric motor having no permanent magnets.  
         [0007]     Thus, it is one object of the present invention to provide a sensorless braking system for utilization in braking electric motors having no permanent magnets.  
       SUMMARY OF THE INVENTION  
       [0008]     The problems and failures of the prior art are solved by the principles of the present invention, an electronic sensorless braking system which applies pulsed current to at least one phase coil of the motor for reversing and thereby braking motor movement (in the case of a linear motor) or rotation (in the case of a rotational motor).  
         [0009]     In a first general aspect, the present invention provides a system for braking either a linear or rotary electric motor having no permanent magnets and having either a stator or a rotor with equally spaced teeth comprising: 
        at least one servo amplifier, having an output and current error signal;     at least one motor phase coil connected said output;     at least one electronic circuit responsive to said current error signal, and producing a current command required for braking; and     means for said servo amplifier to use said current command to produce a current necessary for braking in said motor phase coil.        
 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]      FIG. 1  depicts a schematic view of an embodiment of a system, in accordance with the present invention;  
         [0015]      FIG. 2  depicts a schematic view of a second embodiment of a system wherein a digital signal processor (DSP calculates the current error signal, in accordance with the present invention;  
         [0016]      FIG. 3  depicts a method, or flowchart, in accordance with the present invention; and  
         [0017]      FIG. 4  depicts the current error signal, the current command, and the current in the motor phase coil as a function of time during the braking process, in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Although certain embodiments of the present invention will be shown and described in detail, it should be understood that various changes and modifications may be made without departing from the scope of the appended claims. The scope of the present invention will in no way be limited to the number of constituting components, the materials thereof, the shapes thereof, the relative arrangement thereof, etc., and are disclosed simply as an example of an embodiment. The features and advantages of the present invention are illustrated in detail in the accompanying drawings, wherein like reference numerals refer to like elements throughout the drawings.  
         [0019]     As a preface to the detailed description, it should be noted that, as used in this specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.  
         [0020]     The present invention is directed to the braking of an electric motor without permanent magnets in an emergency through the generation of an electromotive force opposite to the direction of motor motion. This braking is employed in the event motor motion must be rapidly ceased as a result of an emergency. Examples include but are not limited to motion controller system failure, limit switch actuation, line power failure, or any other emergency event which should trigger the braking force.  
         [0021]     The present invention will be described in the context of an uncoupled variable reluctance linear motor used for moving a member from one end of a linear track to another. As used herein, the term uncoupled describes the arrangement where each phase of the motor produces magnetic flux that is for practical purposes not coupled to other phases. Such linear motors are typically utilized in robotic systems, for example, pick and place systems, in which a first linear motor moves a beam mounted to a sliding device along one axis and a second linear motor moves a carriage along the beam on an orthogonal axis so as to provide Cartesian movement of robotics end effector mounted on the carriage over a desired work area. The present invention is not limited to this use. It may also be used to brake many electric motor types, which have no permanent magnets. The present invention also is adaptable to rotary motors.  
         [0022]     The first number of each element in a given figure represents the figure number where said element first appears. For a given element, the same element number is used in subsequent figures in which said element appears.  
         [0023]      FIGS. 1 and 2  show a servo system controlled via a motion controller  110 . The motion controller  110 , using data from an encoder (not shown), provides a control signal (current command  116 ) to achieve a desired motor movement. In the event of an emergency, such as a motion controller system failure, a braking system is required for braking the motor. This braking system is the subject of this invention.  
         [0024]      FIG. 1  further shows all signals used to generate the pulsing output current in a motor phase coil  120  necessary for braking. It also shows how the current error signal  122  is calculated: by subtraction of the current sense signal  124  from the current command signal  116 . This is the definition of the current error signal  122  used in the following detailed description. A dotted line indicates how the brake request signal  118  disconnects the motion controller  110  from the servo amplifier and connects the current command  116  from the output of the electronic circuit responsive to the current error signal  112  to the servo amplifier input.  
         [0025]     With regards to  FIG. 2 , in an embodiment, the current sense signal  124  is converted to digital signal by an analog to digital converter  226  connected to the digital signal processor (DSP)  220 . The Brake Signal request  118  is also connected to the DSP  220 . It initiates the braking process according to the flow chart shown in  FIG. 3 . DSP  220  generates the current command signal  116  in response to the current error signal  124 . The embodiment uses a PWM servo amplifier power stage  224 , and the DSP  220  converts the current command signal  116  to the power stage control signal(s)  222 .  
         [0026]     Referring now to  FIGS. 3 and 4 , if there is an event which triggers an emergency brake request signal, control is switched from the normal operation of the motion control procedure, Step  312 , to the braking procedure, Step  340 , to provide a control signal for braking. The first step of braking procedure  340  is to determine if the brake request was just initiated, Step  314 , if so then produce a lower value of current command, Step  316  and wait for greater of rise time or the fall time, Step  318 . The next step is to calculate the current error, Step  320 . If the calculated current error is greater than or equal to zero then a lower level current command  116  is produced, Step  330 , else if the current error is less than zero a higher level current command  116  is produced, Step  324 . This procedure includes waiting for a specific period of time after each current command  116  change. This time is required in order for the circuit to avoid responding to the effect of the rise time  410  or fall time  412  of the current in the motor phase coils  120  on the current error signal  122 . It has to be at least as long as the rise time  410  when current command  116  changes to the higher value  414 , Step  328 , or at least as long as the fall time  412  of this current when current command  116  changes to the lower value  412 , Step  334 . This time can be found experimentally for each motor.  
         [0027]      FIG. 4  illustrates the effect of the current rise time  410  and fall time  412  on the current error signal during the braking process. In this illustration, the current error  122  starts out above zero, and the current command  116  and the current in the motor phase coil are at their lower values. The current error signal eventually falls below zero and the current command  116  changes from a lower value  416  to a higher value  414 . In response to this, the phase current in the motor coil starts to increase. During the current rise time  410 , the current error signal  122  is positive. One may wait at least as long as the rise time  410  before responding to the change of current error signal in order to avoid continuous oscillation of the current command  116 . The current error signal returns to below zero when the current in the phase coil reaches its higher value.  
         [0028]     When the current error signal  122  becomes greater than zero, the current command  116  changes from a higher value  414  to a lower value  416 . In response to this the phase current in the motor coil starts to decrease. During the current fall time  412 , current error signal  122  is negative. One may wait at least as long as the fall time  412  before responding to the change of current error signal in order to avoid continuous oscillation of the current command  116 . The current error signal returns to above zero when the current in the phase coil reaches its lower value.  
         [0029]     The above description provides a basic description of the operation of the braking system. Numerous variations are appropriate to achieve variations in performance, reliability, size, ease of manufacturing, cost, and other design features. Moreover, with experimentation, different versions may better suit other specific applications. Other embodiments and applications of the present invention may come to mind by reading the above description of the present invention. Moreover, the scope of the present invention should only be deemed to be limited by the claims that follow.