Abstract:
A clutch actuator for an electromechanical clutch having a solenoid actuating coil initially provides power to the solenoid at a high rate by using a high duty cycle pulse with a modulated controller. When the initial engagement of the clutch elements is sensed by a decrease in current, the duty cycle of the pulse with modulator is reduced and thereafter increased in a control fashion to accomplish a soft start.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     N/A 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None 
     REFERENCE TO A “SEQUENCE LISTING” 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the actuation of electromagnetic clutches and more particularly to a controller for such clutches that reduces the stresses associated with engagement of the clutches by providing a progressive or soft start. 
     2. Description of Related Art 
     Electromagnetic clutches are used in a variety of applications, including coupling large and small engines and motors to equipment operated by the engines or motors. Especially in the case of relatively small engines and motors, the price of clutch controllers is a significant factor in the implementation of such controllers. However, small engine applications also benefit significantly from controlling the abrupt engagement of clutches since such engagement may increase wear, resulting in undesirable operating characteristics such as jerking, or cause the engine to stall if the clutch is engaged abruptly. 
     There have, in the past, been some efforts made towards reducing the abruptness of clutch engagement. Such methods have taken various forms, including mechanical arrangements that suffer from the disadvantage that they are complex and expensive, and electrical arrangements that have provided less than optimal results. Since an electromagnetic, clutch requires a clutch controller for controlling power applied to the clutch, it would be desirable to combine such controller with method and apparatus for providing for gradual engagement of the clutch in a single unit. This invention provides such method and device. 
     In almost all instances, an electromagnetic clutch includes a coil or solenoid through which a current is passed to actuate the clutch and an at least partially ferrous core that is arranged to be drawn into the coil when current is supplied to the coil. The core is mechanically connected to the clutch mechanism so that when power is applied to the coil and the core is drawn into the coil, the clutch mechanism is moved from a disengaged to an engaged position. This invention controls the actuation of the clutch by controlling the current passing through the coil to provide for a gradual engagement of the clutch rather than an abrupt engagement. This invention relies on the characteristic of a solenoid type of clutch actuator that the inductance of a solenoid increases as the core is drawn into the body of the solenoid. Since, the core is mechanically connected to the clutch, movement of the core is directly related to the position and therefore the state of the clutch and by taking advantage of this, the present invention permits the position of the clutch to be determined from the increase in the inductance of the coil that occurs as the core is drawn into the coil. 
     Because the current flowing through the coil will tend to increase with time, according to a well-known relationship, the actual current through a coil as a function of time can be predicted relatively accurately. Where the inductance of the coil increases quickly enough as the core moves into the coil, the current through the coil will decrease rather than increase as a function of time, and by monitoring the current through the coil and recognizing this decrease in current as the clutch begins to engage, the present invention provides a method and apparatus for controlling the engagement of the clutch to provide a soft start. 
     It is desirable to provide a clutch controller that automatically adjusts for different clutch models. Clutches come in many different sizes, larger clutches requiring more current than smaller clutches, in prior art controllers, predetermined absolute current set points have been used to control the operation of the clutches. For example, a controller might initiate a ramp at a starting point of 1.2 amps for three amp clutch, and a starting point of 2 amps for a 5 amp clutch. 
     As clutches wear, more current is required to activate them. Consequently, if a fixed current is employed at the beginning of the ramp portion of the clutch activation, the clutch may disengage. Since absolute current set points always produce the same ramp current profile regardless of wear, controllers using this technique may be unreliable. 
     Another problem of known controllers is that the current ramp increases the current slowly from a preset value to 100%. In practice, the clutch is fully engaged at a value somewhat less than 100% and continuing the ramp past this value may cause clutch slippage and overheating. 
     A still further problem associated with known clutches is that clutches do not always engage squarely, especially if they are worn. If a clutch pulls in obliquely, a current sensor may indicate a false engagement when one portion of the clutch plate touches the opposite clutch plate. 
     While a variety of methods for controlling the current passing through the clutch may suggest themselves to those skilled in the art, and in accordance with the invention, it is preferred to control the current through the use of a pulse width modulator which can be adjusted to provide a controlled amount of current to the coil of the clutch and thereby to accomplish a soft start. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with a presently preferred embodiment of the invention, current through the coil of a clutch actuator is initially sent to a high value by establishing a high duty cycle for a pulse width modulated controller. When a decrease in current through the clutch is sensed, thereby indicating that engagement of the clutch has begun, the duty cycle of the pulse width modulator is reduced quickly and thereafter increased in a controlled fashion to accomplish a soft start. 
     In accordance with another aspect of the invention, if desired, once the clutch is fully engaged, the current through the coil may be reduced to a holding value that is somewhat less than the current required to actuate the clutch by adjusting the duty cycle of the pulse width modulated control power to a holding value. This feature reduces solenoid coil heat dissipation, thereby enabling the use of a higher power solenoid than would be possible without this control. 
     While the novel aspects of the invention are set forth with particularity in the appended claims, the invention itself together with further objects and advantages thereof may be more readily comprehended by reference to the following detailed description thereof taken in conjunction with the accompanying drawing in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         FIG. 1  is a diagrammatic view of an arrangement for actuating an electric clutch utilizing the self start clutch controller of this invention; 
         FIG. 2  is a diagrammatic view of the idealized engagement of a clutch of the type to which the invention relates; 
         FIG. 3  is a diagrammatic view of the typical engagement of a clutch of the type to which this invention relates; 
         FIG. 4  is a graphical representation of the current flowing through a clutch solenoid in accordance with one aspect of this invention; 
         FIG. 5  is a graphical representation of the current through a solenoid in accordance with another aspect of this invention; 
         FIG. 6  is a schematic diagram of a clutch controller in accordance with this invention; 
         FIG. 7  is a software block diagram of the softstart algorithm for the clutch controller shown in  FIG. 6 ; and 
         FIG. 8  is a software block diagram of the current control PWM algorithm for the clutch controller of  FIG. 6 . 
         FIG. 9  is a state diagram of the current control set point algorithm for the clutch controller of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a diagrammatic view of a clutch arrangement utilizing a clutch controller in accordance with this invention. A clutch  10  includes an input connector  12  for connecting clutch  10  to clutch controller  18  by way of first and second electrical conductors  14  and  16 . Conductors  14  and  16  are connected to output terminals  20  and  22  of clutch controller  18 . Clutch controller  18  also includes input terminals  24  and  26 . Input terminal  28  is conventionally connected to ground while input terminal  24  is connected to a source of 12 volt DC power such as a battery  30  by way of a power switch  28 . When power switch  28  is closed, clutch controller  18  applies power to clutch  10  by way of connector  12  as will be described in more detail below. 
       FIGS. 2 and 3  are diagrammatic illustrations showing clutch engagement under ideal and typical circumstances. As shown in FIGS.  2 ( a ) and  3 ( a ), when disengaged, the clutch driven side and the clutch output side are spaced apart so that no power is transferred between them and moreover the outside clutch plate is ideally disposed parallel to the driven side clutch plate. As the clutch is drawn in, and in an ideal clutch, the output side clutch plate remains parallel to the driven side clutch plate as shown in  FIG. 2(   b ) and engages substantially simultaneously over the entire surface. 
     In practice, as shown in  FIG. 3 , while it is often possible to maintain the driven side and output side clutch plates essentially parallel when the clutch is disengaged, when the clutch is engaged, the output side clutch plate may contact the driven side clutch plate obliquely as shown in  FIG. 3(   b ) and subsequently move into the position shown in  FIG. 2(   b ). Some clutch manufactures use a permanent magnet brake on the back side of the clutch plate which further exaggerates this problem. This invention allows for this common effect. 
       FIGS. 4 and 5  are graphical representations of the current applied to a clutch solenoid in accordance with first and second embodiments of the invention. Referring to  FIG. 4 , the current is shown on a vertical axis against time shown on the horizontal axis. When the clutch is engaged, for example when switch  28  as shown in  FIG. 1  is closed, the current begins to increase with time at a rate determined primarily by the inductance of the clutch solenoid. As the current increases, the clutch controller monitors the current and detects a local maximum at 50 where the current begins to decrease. Normally, this maximum occurs at the point where the clutch plate begins to move toward the driven side of the clutch, just before contact is first made between the driven side and the output side of the clutch. When the current through the clutch solenoid decreases to 95% of the maximum current, the start of clutch engagement is declared. At this point the maximum current is stored as “reference current”, and power to the clutch solenoid is removed and the current begins to decrease with time at a rate again determined primarily by the inductance of the solenoid. 
     When the current falls to 40% of the reference currents current is again supplied to the clutch solenoid but at a controlled rate to facilitate a smooth engagement of the clutch. Preferably, the controlled rate is a linear ramp but other controlled increases in current are also contemplated. When the current reaches 66% of the reference current, the controlled ramp is terminated and current is applied to the coil at a rate limited only by the coil inductance. At this point, the clutch is fully engaged. 
       FIG. 5  shows a graph of the current through a clutch solenoid vs. time in accordance with another embodiment of the invention having an additional feature adapted to detect and compensate for uneven engagement of the clutch plates as shown in  FIG. 3 . The wave form of  FIG. 5  compensates for partial pull in and which would otherwise be detected as full pull in and causing the current through the clutch solenoid to be reduced and the clutch to either disengage or, drag along the output disc edge until the ramp current increases to a point where the clutch disc pulls in fully which results in a delayed and abrupt engagement. 
     As shown in  FIG. 5 , when the maximum current is detected, a set point is established at 95% of the maximum current to detect the beginning of clutch engagement. When the current falls below 95% of the maximum current, the maximum current is saved as “reference current”, a new set point is established at 1.2 times the reference current and power is continuously applied to the clutch until the new threshold is reached whereupon power to the clutch solenoid is removed and the current begins to decrease with time at a rate again determined primarily by the inductance of the solenoid to a point equal to 40% of the reference current. At this point, the current ramp up proceeds as already described in connection with  FIG. 4 . 
     This second embodiment allows for the initial reduction of current caused by the sort of uneven initial contact illustrated in  FIG. 3(   b ) and it continues to apply current until a higher threshold is reached thereby ensuring that actual clutch contact has occurred. 
       FIG. 6  is a schematic diagram of a clutch controller in accordance with a presently preferred embodiment of the invention. A power source such as a 12 V DC power source is connected to an input terminal  102 . Terminal  102  is connected by way of a diode  104  to an input terminal  106  of a voltage regulator  108 . Regulator  108  has a ground terminal  110  and an output terminal  112  that provides an operating voltage for example 4.7 V to the other elements of the clutch controller as will be discussed in more detail below. A filter capacitor  114  filters the output of voltage regulator  108  and the filtered output is available on terminal  116 . 
     Input terminal  102  is also connected to the source terminal  120  of field effect transistor  122 . Drain  124  of FET  122  is connected to a first clutch solenoid terminal  126 . The other end of the clutch solenoid is connected to terminal  128  which is connected to ground through low resistance resistor  132  which may have resistance of approximately 0.1 ohm. Resistor  132  is connected in such a way that both the ON and OFF current through the clutch solenoid may be measured by sensing the voltage drop across resistor  132 . Ground is connected to output terminal  130 . A snubber diode  134  is connected between terminal  126  and ground to provide a path for the clutch solenoid recirculating current during the PWM off period. 
     Gate electrode  136  of FET  124  is clamped to a maximum gate-source voltage of approximately 10V by zener diode  138 . Gate terminal  136  is connected to the collector of gate drive transistor  140  by current limiting resistor  142  which may have a value of approximately 390 ohms. The zener diode, preferably a 20 V zener diode  144  is connected between the collector and the emitter of transistor  140  to limit the voltage applied across transistor  140  during a “load-dump” transient. Load-dump transients can occur when the 12V battery is suddenly disconnected from a running engine&#39;s charging system. Zener diode  144  also forces FET  120  ON during the load dump, both to keep FET  144 &#39;s drain-source voltage within safe limits and to help to suppress the load-dump by providing a load via the clutch. Collector  146  of transistor  140  is connected to the 12 volt source through resistor  148  which is preferably a 1.5 K. ohm resistor. Base  150  of transistor  140  is connected to an output of microcontroller  160  by a series resistor  162 . Base resistor  164  is connected between the base  150  and ground and preferably has a value of approximately 2 K. ohms 
     The current through the clutch solenoid coil is sensed as a voltage drop across resistor  132  which is connected through a filter comprising a series resistor  170  and a capacitor  173  to a non-inverting input  172  of a comparator  174 . Preferably, resistor  170  has a value of approximately 2 K. ohms. An inverting input  176  of comparator  174  is connected to ground through a series resistor  178  which preferably has a value of about 1000 ohms. A feedback resistor  180  is connected between output  182  of comparator  174  and inverting input  176 . The output of comparator  174  is connected to an input  190  of controller  160  through a filter comprising a series resistor  192  which preferably has a value of approximately 2 K. ohms and a capacitor  194  which preferably has a value of 0.01 μF. 
     The filtered current signal is connected to the inverting input  198  of a comparator  200  whose non-inverting input  202  is connected to a voltage divider comprising a first resistor  204  which preferably has a value of approximately 20 K. ohms and a second resistor  206  which preferably has a value of approximately 10 K. ohms. A filter capacitor  208  is connected in parallel with resistor  206 . Comparator  200  provides a signal at output  210  when the current through the clutch solenoid exceeds a predetermined value set by the ratio of resistors  204  and  206 . The current overload signal is applied to input  212  of controller  160  which is preferably an interrupt input. 
     The clutch controller uses a high side driver with the FET  122  switching the voltage provided to the clutch at terminal  126  and senses the current in the return path at terminal  128 . In the case of an external short circuit to ground, the return path is bypassed. In this case the FET  122  could see a dangerously high current while the sense circuit measured zero current. 
     The FET drain-source saturation voltage is dependent on the current and the FET R DSON  of 0.06 ohms. If the current is normal (&lt;5 A), the FET will saturate to less than 0.3V across its drain-source. As the current increases the saturation voltage increases. Therefore, by monitoring the saturation voltage the approximate current through the FET can be sensed. 
     A saturation detector comparator  220  has a non-inverting input  234  connected to a first voltage divider comprising resistors  222  and  224  connected between the FET drain terminal  124  and ground, and a second inverting input  235  connected to a second voltage divider comprising resistors to  226  and  228  connected between FET source terminal  120  and ground. Zener diodes  230  and  232  limit the voltage is produced by the two voltage dividers to safe values but do not otherwise affect the comparison. Preferably, resistor  222  has a value of approximately 75 K. ohms, resistor  224  has a value of approximately 10 K. ohms, resistor  226  has a value of approximately 100 K. ohms, and resistor  228  has a value of approximately 10 K. ohms. 
     Comparator  220  preferably has a feedback resistor  232  which may have a value of 1 meg. ohm connected between its noninverting input  234  and its output  236  to provide a degree of hysteresis for the saturation detector. Output  236  of saturation detector  220  is connected to an input  240  of microcontroller  160 . 
     Neglecting hysteresis resistor  232 , the resistor ratios are set up for a comparator transition with the FET source  120  at 12V and the drain  124  at 9.27V. Therefore, if the drain is above 9.27V the comparator output  220  is high, below 9.27 it is low. This gives a drain-source maximum of 2.73V—this threshold was set high to ensure that there would be no false trips, it could be reduced significantly. 
       FIG. 7  is a flowchart showing how the software in microcontroller  160  operates to implement the invention. On boot up, either upon the initial application of power or upon the system being reset, an initialization routine as shown at 40 is performed. The current set point is set to zero and the reference current is set to zero. A 50 ms delay occurs at  42  and the current set point is set to 100% at 44. 
     After the current set point and reference current have been initialized, the software enters a loop. The current is measured and compared to the reference current. If the current is greater than the reference current then the reference current is updated to equal the current as shown in boxes  46  and  48 . This assures that the reference current will continue to increase as long as the present current or the current current, as it were, continues to increase. If the current is not greater than the reference current then the software determines whether it has fallen to less than 95% of the reference current in step  50 . If it has not, the software loops back to box  46 . If it has, the reference current is frozen at this maximum level and drive current remains applied to the solenoid and tested in block  52  until the current equals the reference current times 1.2 at which time the current set point is set to the reference current times 0.4 in box  54 . At this point, the controlled soft start current ramp is initiated and the current is tested in block  56  until it reaches the reference current times 0.66. As long as the current remains below this value the current is increased by the ramp value in block  58  to maintain a constant controlled increase to accomplish the soft start function. Once the current reaches 0.66 times the reference current the current set point is set to 100% in block  60 , full current is restored to the solenoid and the routine ends at  62 . It will be seen that the software routine follows the current wave form shown in  FIG. 5 . The simpler wave form shown in  FIG. 4  can be accomplished by eliminating flow chart blocks  52  and  54 . 
       FIG. 8  is a software block diagram showing the manner in which the controller shown in  FIG. 6  controls the current through the clutch solenoid. The current is sampled by measuring a voltage across resistor  132  at a rate of 50 kHz. The analog to digital conversion occurs within controller  160 . The current is averaged every 50 samples, that is approximately 1,000 times per second in block  66  and the average current is tested against the current set point minus hysteresis in block  68 . If the current is below the current set point FET  123  is turned on in block  70  and the saturation detector  220  is tested in block  72 . If the current is higher than the saturation current and the over current timeout has expired as tested at block  74  then the FET is latched off in block  76 . As long as the saturation current is not exceeded or is exceeded only for a short time the routine terminates in block  78 . Returning to block  68 , if the current is greater than the current set point minus hysteresis and continues to increase until it is greater than the current set point as tested in block  80 , the FET is turned off in block  82 , the over current timeout is reset in block  84  and the routine terminates in block  78 . If the current is not greater than the current set point as tested in block  80  then the routine terminates at block  78 . 
     While the invention has been described in connection with certain presently preferred embodiments thereof, those skilled in the art will recognize that many modifications and changes may be made therein without departing from the true spirit and scope of the invention which accordingly is intended to be defined solely by the appended claims.