Abstract:
A control system for the starter assembly of an engine includes a first field effect transistor (FET) electrically connected between an electrical power supply and the starter motor and a second FET electrically connected between the power supply and the solenoid. A control unit is electrically connected to the gate of each FET and is configured to selectively apply a voltage to each gate, wherein the FET provides a current to the respective starter motor and solenoid as a function of the applied voltage. The control unit can selectively apply the gate voltages for cold start, soft start, and start-stop operation of the engine, and in response to sensor signals received by the control unit, such as ring gear rotational speed.

Description:
FIELD 
       [0001]    This application relates to the field of vehicle starters, and more particularly, to solenoids and motor control for starter motor assemblies. 
       BACKGROUND 
       [0002]    Starter motor assemblies that assist in starting engines, such as engines in vehicles, are well known. A conventional starter motor assembly is shown in  FIG. 1 . The starter motor assembly  200  of  FIG. 1  includes a solenoid  210 , an electric motor  202 , and a drive mechanism  204 . The solenoid  210  includes a coil arrangement  211  that is energized by a battery upon the closing of an ignition switch. When the coil arrangement  211  is energized, a plunger  216  moves in a linear direction, causing a shift mechanism, such as shift lever  205 , to pivot, and forcing a pinion gear  206  into engagement with a ring gear of a vehicle engine (not shown). When the plunger  216  reaches a plunger stop, electrical contacts are closed connecting the electric motor  202  to the battery B ( FIG. 2 ). The energized electric motor  202  then rotates and provides an output torque to the drive mechanism  204 . The drive mechanism  204  transmits the torque of the electric motor through various drive components to the pinion gear  206  which is engaged with the ring gear of the vehicle engine. Accordingly, rotation of the electric motor  202  and pinion gear  206  results in cranking of the engine until the engine starts. 
         [0003]    Many starter motor assemblies, such as the starter motor assembly  200  of  FIG. 1  may be configured with a “soft-start” starter motor engagement system. The intent of a soft start starter motor engagement system is to provide limited power to the starter motor before the pinion gear engages the engine ring gear. Once the pinion gear meshes with the engine ring gear full electrical power is applied to the starter motor. If the pinion gear abuts into the ring gear during this “soft start”, the motor provides a small torque to turn the pinion gear and allow it to properly mesh into the ring gear before high current is applied. The configuration of the solenoid, shift yoke, electrical contacts, and motor drive are such that high current is not applied to the motor before the gears are properly meshed. Accordingly, milling of the pinion gear and the ring gear is prevented in a starter motor with a soft-start engagement system. 
         [0004]    Starters with a soft start engagement system typically include a coil arrangement with two distinct coils—a pull-in coil  212  and a hold in coil  214 . During operation of the starter, the closing of the ignition switch I (typically upon the operator turning a key) energizes both the pull-in coil  212  and the hold-in coil  214 , as reflected in the conventional starter circuit diagram of  FIG. 2 . The electric motor  202  is in series with the pull-in coil so that current flowing through the pull-in coil  212  at this time also reaches the motor, applying some limited power to the electric motor, and resulting in some low torque turning of the pinion gear. Energization of the pull-in coil  212  and hold-in coil  214  moves the solenoid shaft or plunger in an axial direction. The axial movement of the solenoid plunger moves the shift lever  205  and biases the pinion gear  206  toward engagement with the engine ring gear. ( FIG. 1 ) As the pinion moves toward engagement with the ring gear, it freely rotates. However, once the pinion abuts the ring gear, the rotational speed of the pinion gear is limited by frictional drag, which prevents further acceleration of the pinion gear. Thus, the pinion rotates into full mesh with the ring gear at a relatively slow rotational speed (relative to the normal cranking speed), which allows the pinion and ring gears to more easily mesh. 
         [0005]    Prior to the solenoid plunger reaching the plunger stop, a set of electrical contacts  220  is closed, thereby delivering full power to the electrical motor. Closing of the electrical contacts effectively short circuits the pull-in coil  212 , preventing thermal related failures of the pull-in coil. However, with the pull-in coil shorted, the hold-in coil  214  provides sufficient electromagnetic force to hold the plunger in place and maintain the electrical contacts in a closed position, thus allowing the delivery of full power to continue to the electric motor  202 . The fully powered electric motor  202  drives the pinion gear  206 , resulting in rotation of the engine ring gear, and thereby cranking the vehicle engine. 
         [0006]    After the engine fires (i.e., vehicle start), the operator of the vehicle opens the ignition switch I. The electrical circuit of the starter motor assembly is configured such that opening of the ignition switch causes current to flow through the hold-in coil and the pull-in coil in opposite directions as long as the contacts  220  are closed. The pull-in coil  212  and the hold-in coil  214  are configured such that the electromagnetic forces of the two coils  212 ,  214  cancel each other upon opening of the ignition switch, and a return spring  217  (and in some cases an over-travel spring  218 ) forces the plunger  216  back to its original un-energized position. As a result, the electrical contacts  220  that connected the electric motor  202  to the source of electrical power are opened, and the electric motor is de-energized. 
         [0007]    Wear due to gear milling can be a problem for starter gears. In most cases the engine is stopped so the ring gear is not rotating, but the pinion gear is rotating as it is advanced into engagement. In other cases the engine ring gear may be rotating. In these cases the pinion gear is at least initially rotating at a different speed, but even when rotating at the same speed as the ring gear milling still occurs until the gears are meshed. It is desirable to minimize gear milling that occurs in either case. It is also desirable for the pinion gear to be fully engaged to the ring gear before full torque is applied to the pinion gear to start the engine. 
         [0008]    In certain applications, the “soft start” starter assemblies are utilized in vehicles in which the engine is automatically stopped such as a traffic light, and then quickly restarted when the traffic light turns and the driver performs an operation to move the vehicle, such as releasing the brake pedal. In these cases, it is important that the engine re-start quickly and reliably. The speed of the engine restart can be reduced by ensuring that the starter pinion gear is meshed with the engine ring gear even before an engine start command is required. Since it is highly undesirable to maintain the starter pinion gear constantly meshed with the engine ring gear, it is necessary to provide a starter assembly that is capable of efficiently engaging the pinion gear to the ring gear, while still minimizing gear milling. 
       SUMMARY 
       [0009]    In one aspect, a control system for the starter assembly of an engine is provided that comprises a first field effect transistor (FET) electrically connected between an electrical power supply and the starter motor, and a second FET electrically connected between the power supply and the solenoid. A control unit is electrically connected to the gate of each FET and is configured to control a voltage applied to each gate so that the FET provides a variable voltage to the respective starter motor and solenoid. The control unit can selectively apply the gate voltages for cold start, soft start, and start-stop operation of the engine, and in response to sensor signals received by the control unit, such as ring gear rotational speed. 
         [0010]    In a further aspect, a single FET is electrically connected between the pull-in and hold-in coils of a solenoid of a starter assembly. A control unit is electrically connected to the gate of the FET and is operable to control the FET to control the electrical power supplied to the coils. In one embodiment the control unit can control the FET by pulse width modulation. The starter motor is connected in series with the pull-in coil so that electrical power is supplied to the motor through the FET and the pull-in coil. In one feature, the starter motor is also connected to the power supply through electrical contacts, while the solenoid plunger is coupled to a contact plate that is movable to close the electrical contacts, thereby shorting the pull-in coil so that the electrical power is supplied to the starter motor directly from the electrical power supply and not through the FET. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0011]      FIG. 1  is partial cross-sectional view of a conventional engine starter assembly. 
           [0012]      FIG. 2  is a circuit diagram of a conventional electrical circuit for the engine starter assembly shown in  FIG. 1 . 
           [0013]      FIG. 3  is a circuit diagram of an electrical circuit according to the present disclosure for the starter assembly of  FIG. 1 . 
           [0014]      FIG. 4  is a graph of spring force and solenoid current for the starter assembly shown in  FIG. 1 . 
           [0015]      FIG. 5  is a graph of a voltage applied to the solenoid in the circuit diagram of  FIG. 3 . 
           [0016]      FIG. 6  is a circuit diagram of an electrical circuit according to a further embodiment of the present disclosure for the starter assembly of  FIG. 1 . 
           [0017]      FIG. 7  is a graph of a voltage applied to the starter motor and solenoid in the circuit of  FIG. 6  for a normal cold start condition. 
           [0018]      FIG. 8  is a graph of the voltage applied to the starter motor and solenoid in the circuit of  FIG. 6  in a start-stop condition. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    In one aspect of the present disclosure, the starter circuit for energizing the coils  212 ,  214  of the starter solenoid  210  is modified from the conventional circuit depicted in  FIG. 2 . In particular, the modified starter circuit  10  illustrated in  FIG. 3  integrates with an engine control unit (ECU)  20  and replaces the ignition switch I (which may, for instance, constitute an ignition solenoid that is actuated by a user-operated key switch) with a field effect transistor (FET)  30 . The ECU  20  controls the voltage V G  provided to the gate G of the FET  30  so that the FET  30  can provide a variable effective voltage to the solenoid  210 . In one embodiment, the FET provides a variable voltage through pulse-width modulation in which the ECU rapidly turns the voltage V G  on and off, with the dwell between on and off states establishing the FET voltage. The inductance of the circuit in  FIG. 3  smoothes the switched voltage to an effective voltage provided to the solenoid coils and motor. The engine control unit can be of conventional design, incorporating a microprocessor capable of executing stored commands or stored programs to sense engine conditions and control the operation of the engine and other components. 
         [0020]    It is known that the magnetic force generated by the two coils  212 ,  214  is a function of the current provided to the coils. The axial movement of the plunger due to the coil magnetic forces is resisted by the spring force of the return spring  217  until the contacts  220  are closed, and then by the combination of the return spring and over-travel spring  218  thereafter. The coil magnetic force and spring forces increase as the plunger is retracted further into the solenoid, as shown in  FIG. 4 . As reflected in  FIG. 4 , the resistive spring force increases incrementally at plunger position X 1  by the amount of pre-load of the over-travel spring  218 . It is at this point that full battery voltage is supplied to the motor to drive the pinion gear at its operational speed. At this point X 1  the pinion gear should be substantially meshed with the ring gear. Thus, prior to the plunger reaching position X 1  the pinion gear should be meshed with the ring gear to avoid unnecessary gear milling. 
         [0021]    In the conventional non stop-start circuit of  FIG. 2 , once the ignition switch I is closed the solenoid  210  uniformly drives the plunger  216  to shift the pinion gear and close the contacts  220 . In the conventional non stop-start circuit, full battery voltage can be applied to the starter motor driving the pinion before the pinion gear is fully meshed with the ring gear. Under certain conditions, it is desirable to delay fully energizing the pinion motor  202  until the pinion gear is fully meshed with the engine ring gear. This concern is addressed by the circuit of  FIG. 3  in which the ECU controls FET  30  to control the voltage V S  to more accurately determine when the contacts  220  are closed to fully energize the starter motor  202 . Thus, as shown in the graph of  FIG. 5 , the voltage V S  is initially zero, corresponding to a de-energized state of the solenoid  210 . When a start signal is received by the ECU  20 , the ECU modulates the voltage V G  applied to the gate of the FET  30  corresponding to a solenoid voltage V S  of V 1 . At this voltage the current through the solenoid coils  212 ,  214  drives the plunger  216  to shift the pinion gear, overcoming the spring force of the return spring  216 . Once the pinion gear is initially meshed with the ring gear, it is desirable to slow down the advance of the plunger toward the contacts  220  to allow the gears to be fully meshed before full motor power is applied. Thus, the ECU is configured to modulate the voltage V G  to the FET  30  to reduce the voltage V S  to V 2 , as reflected in  FIG. 5 . The ECU may be provided with a signal from a sensor  50  that indicates when the pinion and ring gears mesh. The reduced voltage V S , and thus reduced current i S , to the solenoid cause the plunger to advance more slowly to the contacts  220  while simultaneously advancing the pinion gear to fully mesh with the ring gear. At some point in the travel of the plunger the contacts  220  are closed and the motor  202  is directly connected to the power supply B to drive the starter motor at its full operational speed. 
         [0022]    In an alternative approach, the sensor  50  may be a ring gear speed sensor. In certain circumstances, it is desirable to engage the pinion gear to the ring gear while the ring gear is still rotating, albeit decelerating. If the ring gear is rotating too fast the pinion gear cannot mesh and it is unnecessary, and even damaging, to rotate the pinion gear at full speed. The ECU  20  can implement the same protocol shown in the graph of  FIG. 5  except that the start signal is based on the ring gear speed. The ECU can be configured to determine a differential speed between the pinion gear (if it is rotating) and the ring gear, and to compare that differential speed to a stored threshold value. The “start signal” of  FIG. 5  thus corresponds to a determination that the differential speed is below the threshold value. Alternatively the ECU can compare the ring gear speed, as determined by the sensor  50 , and compare that to a speed threshold value, with the “start signal” again corresponding to the ring gear speed falling below the threshold. 
         [0023]    As shown in the circuit diagram of  FIG. 3 , the FET  30  controls the current provided to both the pull-in coil  212  and the hold-in coil  214 . In addition, until the pull-in coil is short circuited by closure of the contacts  220 , the pull-in coil variably feeds current to the motor  202  by virtue of their series connection. Once the contacts  220  are closed, the pull-in coil is short-circuited and the motor  202  is fed directly by the power supply or battery B, rather than through the FET  30 . The hold-in coil  214 , however, remains energized to hold the solenoid plunger in the contact closure position. 
         [0024]    In another embodiment, the contacts  220  are replaced by an FET  40  connected between the starter motor  202  and the power supply B, and controlled by the ECU  20 , as shown in the circuit diagram of  FIG. 6 . In this configuration, the solenoid plunger operates only to shift the pinion gear into engagement with the ring gear. The voltage V S  provided to the solenoid  210 ′ is also controlled by the ECU  20  through the FET  30 . It can be appreciated that the two FETs  30 ,  40  replace the ignition switch I of the starter system shown in  FIG. 2  and provide a control capability absent in the prior system. The ECU can control the two FETs according to a variety of protocols. In a normal cold start condition, the ECU  30  can modulate the voltage signals V G  to the gates of the corresponding FETs  30 ,  40  to provide full battery voltage V 1  to the solenoid and starter motor, as reflected in  FIG. 7 . 
         [0025]    The ECU  20  can receive signals from sensors  50 , which can include a ring gear speed sensor. The ECU can poll the sensor  50  to determine whether the engine is operating—i.e., whether the ring gear is rotating. If it is not, then the ECU can direct implementation of the normal cold start protocol of  FIG. 7 . If the ring gear is rotating the ECU can implement the protocol depicted in  FIG. 8 . According to this protocol, the ECU initially controls the FET  40  to provide a voltage V M  at a lower initial value V 2  to the starter motor  202  to limit the motor torque. Since the pinion gear is not yet meshed with the ring gear, a higher driving torque would cause the pinion gear to mill against the ring gear, hence the lower initial torque. The lower torque mode continues while the ECU  20  evaluates the ring gear speed signal from the sensor  50 . As explained above, the ECU can determine whether the difference between ring gear and pinion gear rotational speeds falls below a predetermined threshold (or whether the ring gear speed itself falls below a threshold), at which point the ECU  20  applies a voltage to the gate of the FET  30  for the solenoid. The energized solenoid advances the plunger and thus the pinion gear until it meshes with the ring gear. Once the gears are meshed the ECU can deenergize the starter motor until an engine restart signal is received by the ECU. The solenoid remains energized so that the starter gear remains meshed with the ring gear. Once an engine restart is commanded the ECU can apply a new voltage to the motor ECU  40  so supply the greater battery voltage V 1  to the motor to drive the motor at its operational speed for starting the engine. Once the engine is restarted the ECU can drop the voltage V G  to the FETs  30 ,  40  to deenergize the solenoid and starter motor. 
         [0026]    It can be appreciated that the use of ECU commanded FETs  30 ,  40  to supply controllable voltage to the solenoid  210  and starter motor  202  provides a great deal of flexibility to the engine start/restart protocols, particularly with the addition of condition sensors  50 , such as a ring gear speed sensor. The ECU can evaluate various engine conditions to determine which protocol is appropriate to implement. Other sensors may be added that are specific to the starter system, such as position or proximity sensors to determine the location of the solenoid plunger, or force sensors to measure solenoid and/or spring forces. The use of FETs allows calibration of the voltage and current supplied to the solenoid and starter motor to minimize response time while reducing gear milling.