Abstract:
In a motor actuator, e.g. a valve actuator, comprising a reversible electrical motor ( 1 ), a drive circuit ( 2 ) is providing a stop function, when the torque of the motor exceeds a predetermined maximum torque. The drive circuit ( 2 ) is constructed as a two-terminal circuit connected in series with the reversible electrical motor ( 1 ), and the electrical power is supplied to the series connection with a mainly fixed voltage VCC and a polarity which provides the desired rotational direction of the motor ( 1 ). This provides the possibility of having the drive circuit ( 2 ) positioned close to the motor ( 1 ) and maintaining full torque control and stop function with two wire connections.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a motor actuator of the kind set forth in the preamble of claim  1 .  
         BACKGROUND ART  
         [0002]    In motor actuators of this kind, it is known to provide a torque control by sensing the current to the motor and disconnecting the supply to the motor when the measured current exceeds a predetermined value. A motor actuator of this kind is e.g. known from U.S. Pat. No. 4,455,012, in which it is further described that a timing circuit provides a possibility of having higher starting currents than the predetermined maximum current value. This described construction of the control system is intended to be positioned remotely from the controlled motor actuator, which is connected to the controller by two wires. If a feedback for the absolute position of the valve is needed, a potentiometer or digitizer is suggested, requiring two further wires for the connection to the controller.  
         DISCLOSURE OF THE INVENTION  
         [0003]    It is the object of the present invention to provide a motor actuator of the kind referred to above, with which it is possible to have the control of the motor actuator positioned close to the motor or even integrated with the motor, and still providing a torque control and stop function, and wherein the connection to a centralised control system is provided by two wires, and this object is achieved with a motor actuator of said kind, which according to the present invention also comprises the features set forth in the characterising clause of claim  1 . With this arrangement, the motor actuator can be fully remotely controlled with respect to its direction of rotation in a simple manner by simply applying a mainly fixed voltage with appropriate polarity to the two connecting wires, and having the drive circuit fully controlling the torque and the stop function of the motor actuator. Preferred embodiments of the motor actuator, the advantages of which over and above what will be evident for a man skilled in the art are revealed in the following detailed part of the present description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    In the following detailed -part of the present description, the invention will be explained in more detail with reference to the exemplary embodiments of a motor actuator according to the invention shown in the drawings, in which  
         [0005]    [0005]FIG. 1 shows a schematic block diagram of a preferred embodiment of the motor actuator,  
         [0006]    [0006]FIG. 2 shows the current limiting function in the motor actuator in FIG. 1, and  
         [0007]    [0007]FIGS. 3 and 4 show how the motor actuator is connected to a centralised controller in a simple on/off controller configuration in FIG. 3 and a more advanced microprocessor control with position control in FIG. 4. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0008]    The motor actuator shown in FIG. 1 comprises a reversible electrical motor  1  and a drive circuit  2 . The motor  1  receives its current from a supply VCC and is connected in series with the drive circuit  2 , which controls the motor as described in the following. The drive circuit  2  comprises a rectifier  3  securing that the current I in the drive circuit is always in the same direction independent of the polarity of the applied voltage VCC. The current I is controlled by an output circuit  13 . The drive circuit  2  controls the current I based on measurements of the regulated voltage VCC reg  and the measured current I indicated as I′ mot . A maximum current I limit  is set in a current limiter  4  and is compared to the measured current I′ mot . The output circuit  13  delivers a current I to the motor  1  which is limited by this maximum current I limit . As shown in FIG. 2, the current consumption of the motor  1  is dependent on the load on the motor or in other words the torque delivered by the motor and until a maximum current is reached, the current is as shown proportional to the load. After reaching the maximum current limit I limit , the drive control  12  controls the output circuit  13  in such a way that only this maximum current is delivered to the motor  1 .  
         [0009]    This means that the voltage VCC reg  is increasing and when this voltage reaches a stop reference voltage V stop     —     ref , as indicated in the stop voltage reference  14 , the stop circuit  5  is activated and deactivates the drive controller  12 , whereby the output circuit  13  is deactivated and no current is delivered to the motor. The reaching of the stop voltage corresponds to the load rising to a certain level, which would normally correspond to the motor actuator having reached a mechanical stop at one of the ends of its travel. Thus, the motor will be stopped when reaching such an end of travel. In order to be able to start up the motor  1 , a timer  7  is included, which upon application of the voltage VCC inhibits the functioning of the stop circuit  5  and the current limiting function  4  during a start-up period corresponding to the short surge current drawn by the motor  1  during such start-up.  
         [0010]    The motor  1  can be connected to e.g. a valve stem through a suitable gear mechanism in order to transform the high-speed low torque from the motor to a suitable low speed high torque needed for such a valve control.  
         [0011]    From the above, it can be seen that the output circuit  13  will be able to adjust the current  1 , which is approximately proportional to the torque of the motor, said control being performed through a control signal from the drive control  12 . The drive control  12  bases this control on the inputs from the current limiter  4  and the stop function  5  in such a way that the motor actuator is driven at a maximum but limited by the current limit I limit , as shown in FIG. 2. When the load on the motor is increasing, the current will increase correspondingly but will be limited by the maximum current I limit , whereby the maximum torque is limited leading to a controlled reaching of a possible end of travel position. The stop function  5  reacts on the voltage VCC reg , which is dependent on the voltage over the motor  1 , and uses a stop voltage reference  14  to provide a stop of the motor actuator when the motor voltage becomes sufficiently low corresponding to a stop of the motor and a corresponding high current in the motor limited by the current limiter  4 . The motor will draw a current higher than I limit  during start-up. In order to be able to start up the motor, the stop functions and the current limiter  4  are deactivated by the timer  7  during such a start-up interval. The timer  7  is activated when the voltage VCC is applied in order to activate the motor actuator.  
         [0012]    When a voltage VCC is applied to the motor actuator, the full voltage will from the start be applied to the drive circuit  2 , the motor ideally functioning as a short circuit when stopped and not generating any counter-electromotive force. After a short time period, the drive circuit  2  will be activated and the timer  7  will start and inhibit the stop function  5  and the current limiter  4 . Hereby the output circuit  13  will deliver the current necessary to start up the motor. As soon as the motor starts running, it will generate a counter electromotive force, whereby the applied voltage VCC will be shared between the motor  1  and the drive circuit  2 .  
         [0013]    After a certain time, set in the timer, the current limiter  4  and stop function  5  are allowed to function, and as long as everything proceeds as normal, the current in the motor will now be below I limit  set by the current limiter  4 . When the motor actuator meets a certain resistance, the current will be raised and this will continue until the maximum current I limit  is reached, whereupon a constant current equal to I limit  is supplied to the motor  1 . Due to the fact that the motor does not receive the necessary current for the load, the motor will reduce its revolutions and the voltage over the motor will fall, whereby the voltage VCC reg  will rise. When this voltage reaches the limit set in the stop voltage reference  14 , V Stop     —     ref , the stop function  5  will be activated and shut down for the current to the motor via the drive control  12  and the output circuit  13 . Thus, the drive circuit has secured a limitation of the torque and a soft stop at the end of travel. After this stopping, an insignificant current will naturally be present in the series connection of the motor  1  and the drive circuit  2  in order to supply power to the drive circuit  2 .  
         [0014]    The centralised controller  8  shown in FIG. 3 comprises several outputs Out  1 - n  for controlling several motor actuators. With the integrated motor  1  and drive circuit  2 , the centralised controller  8  can be a simple relay control applying the necessary voltage VCC to the individual motor actuators with the required polarity to obtain the desired rotation of the motor  1 .  
         [0015]    In FIG. 4 is shown a more sophisticated centralised controller  8  comprising a current sensor  9  measuring the current delivered to the motor actuator presently being supplied with a voltage VCC. The current sensor  9  measures this current and delivers a signal to a filter circuit  10 , which detects the ripple of the current and filters this ripple to obtain a sequence of impulses, the frequency of these impulses being proportional to the motor speed and the number of impulses thus being an indication for the relative position of the motor actuator. It is especially convenient to choose a motor  1  with an uneven number of poles, whereby the ripple will be more distinct and easier to detect by the filter  10 . The filter  10  is connected to a micro-controller  11 , which is able to use the impulses received from the filter to calculate the relative position of each of the individual motor actuators taking into account the parameters thereof, such as gear ratio on a possible gear mechanism on the motor, the number of poles on the motor, etc. In order to have secure control of the position of the individual motor actuator, it will naturally be necessary to provide a zero position detection by e.g. controlling the motor actuator to a well-defined end of travel position. After such a zero position setting, the absolute position of the motor actuator can be calculated by the microprocessor  11 , based on counting of impulses with applied positive and negative voltages, respectively.  
         [0016]    The above system described in connection with FIG. 4 will thus be able to provide a centralised registration and control of the relative position of several independent motor actuators without any extra input/output ports for positioning signals from the individual motor actuators.  
         [0017]    As can be seen in FIGS. 3 and 4, the motor actuators and their connections to the centralised controller  8  are the same in both situations only using two wires for the connection. Furthermore, the detection or measurement of the ripple current  9  and filtering  10  necessary for the position control of the motor actuators are positioned centrally in the centralised controller  8 , which makes the costs of producing such a system relatively low.  
         [0018]    Although the invention above has been described in connection with preferred embodiments thereof, it will be evident for a man skilled in the art that several modifications can be provided without deviating from the concept of the following claims.