Abstract:
A microcomputer-based electronic control system for a full time four wheel drive torque transfer case monitors the relative slip between the front and rear output shafts of the transfer case, and generates a signal to engage an electromagnetic clutch for a predetermined time period in the event a predetermined slip threshold is exceeded. The control system continuously interrogates two Hall effect sensors positioned over toothed wheels on the front and rear outputs of the transfer case. Tooth counts are stored in two numerical stacks (one for each sensor), the depth of which may be adapted to the requirements of the particular design. Upon detecting a tooth count, the system adds the count to the sensor&#39;s current stack register and pushes all of the registers of the other stack down one register. The last register is pushed out or deleted from the stack memory. In this way, the system retains a continuous memory of tooth counts from one sensor for the last ‘n’ tooth counts of the other, and vice-versa. By comparing the sum of the tooth counts from each stack, the relative rotation of one shaft to the other shaft is directly ascertained independent of any time-based reference. Thus, the system detection relative rotational differences at all speeds down to zero rpms. In addition, because of an equal probability of missing tooth counts, the system works well above the maximum sampling rate of the system.

Description:
BACKGROUND OF THE INVENTION 
     a) Field of the Invention 
     The present invention relates generally to an electronic control for a vehicle four wheel drive system and, in particular, to an electronic control which includes means for detecting a predetermined slip (speed differential) between the front and rear wheels of the vehicle, and means for selectively engaging a clutch during excessive slip conditions to prevent relative slip between the front and rear wheels. 
     b) Description of Related Art 
     Four wheel drive systems for vehicles are becoming increasingly common. In the past, such systems typically included torque transfer case having an input shaft connected to the output of the vehicle driver for selecting front and rear output shafts connected to the front and rear differentials of the vehicle for driving the front and rear wheels respectively. Typically, such systems were provided with selective control means operable by the vehicle driver for selecting whether the vehicle is to be operated in either a two wheel or a four wheel drive mode. When operated in the four wheel drive mode, these systems did not provide for any speed differentiation between the front and rear wheels such that, on dry pavement, “hopping” of the front wheels would occur during turning of the vehicle, due to the normal overspinning of the front wheels. Consequently, it was recommended that the four-wheel mode be used only during wet, icy, or low traction road surface conditions. 
     Recently, certain vehicles have been provided with a “full time” four wheel drive system. In these systems, the torque transfer cases are typically provided with an interaxle differential for dividing torque between the vehicle front and rear differentials. The interaxle differential enables the front and rear wheels to rotate at different speeds, which occurs during turning of the vehicle, or in the event the front and rear wheels have different diameter tires. Also, in order to prevent excessive slipping between the front and rear wheels, these transfer cases typically include a selectively engageable clutch means which is operative to lock the interaxle differential upon sensing a predetermined slippage between the front and rear output shafts of the transfer case. 
     However, the automatic control systems for these selectively engageable clutches has many drawbacks that increase cost without provide accurate and efficient activation of the selectively engageable clutches. 
     Current torque coupling clutch control systems require separate algorithms to perform low-speed and high speed detection of wheel slip. Moreover, current control systems provide only periodic sampling of rotational speeds differences in the torque coupling transmission. 
     Current control systems also do not permit modularization of the control system, allowing processing and decision making to be broken up onto separate, smaller processors that would be required if all of the processing occurred at one central processor. 
     The need therefore exists for a torque coupling control system that overcomes the drawbacks inherent in the prior art, while providing a controller that performs low and high speed detection of wheel slip with a single algorithm that continuously assesses operation of the drivetrain. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a microcomputer-based electronic control system for automatically controlling a full-time four wheel drive torque transfer case or similar drivetrain device. The transfer case includes an input shaft coupled to the output of the vehicle transmission, and an interaxle planetary gear differential for dividing torque between a rear output shaft connected to the vehicle rear differential and a front output shaft connected to the vehicle front differential. An electromagnetic clutch is located in the transfer case and is adapted to selectively lock the planetary gear differential to prevent relative slip between the front and rear output shafts. 
     In accordance with the present invention, the electronic control utilizes a pair of Hall effect sensors for monitoring the speed of the front and rear output shafts. When a predetermined amount of slip is detected between the front and rear output shafts, the electronic control is operative to selectively engage the electromagnetic clutch for a predetermined time period. During this predetermined time period, the clutch is engaged to lock the differential and prevent slip between the front and rear output shafts. 
     The electronic control of the present invention includes several unique operating features. For example, the system of this invention is capable of detecting the relative rotational displacement of one shaft with respect to another at all speeds down to zero revolutions per minute. 
     The control system continuously interrogates two Hall effect sensors positioned over toothed wheels on the front and rear outputs of the transfer case. Tooth counts are stored in two numerical stacks (one for each sensor), the depth of which may be adapted to the requirements of the particular design. Upon detecting a tooth count, the system adds the count to the sensors current stack register and pushes all of the registers of the other stack down one register. The last register is pushed out or deleted from the stack memory. In this way, the system retains a continuous memory of tooth counts from one sensor for the last ‘n’ tooth counts of the other, and vice-versa. By comparing the sum of the tooth counts from each stack, the relative rotation of one shaft to the other shaft is directly ascertained independent of any time-based reference. Thus, the system detection relative rotational differences at all speeds down to zero rpms. In addition, because of an equal probability of missing tooth counts, the system works well above the maximum sampling rate of the system. 
     The system of this invention further permits modularization of the controller, thus allowing the processing and decision-making function to be broken up onto separate, smaller processors than would be required if all of the processing occurred on one central processor. This modularization may decrease the overall cost of the control system. 
     The above features, as well as other advantages of the present invention, will become readily apparent to one skilled in the art from reading the following detailed description in conjunction with the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a four wheel drive system having a torque transfer case which can be controlled with the electronic control of the present invention; 
     FIG. 2 is a schematic view illustrating the internal components of a transfer case which can utilize the electronic control of the present invention; 
     FIG. 3 is a flow diagram which illustrates the operation of the electronic control of the present invention in automatically controlling the transfer case of FIG.  2 . 
     FIG. 4, there is shown a flow diagram which will be utilized to explain the operation of the electronic control. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, there is shown a vehicle four wheel drive system which utilizes a torque transfer case  10  which can be controlled by the electronic control of the present invention. The transfer case  10  is secured to the rear of a transmission unit  11  (shown in phantom) which in turn is coupled to a drive engine  12  (also in phantom). The transmission  11  is provided with an output shaft which is coupled to an input shaft of the transfer case  10 . The transfer case  10  transfer torque to the front and rear output shafts. The rear output shaft is connected to the front end of a rear drive shaft  13  by means of a conventional universal joint coupling  14 . The rear end of the drive shaft  13  is coupled to an input shaft of a rear differential  15  by means of a universal joint coupling  16 . The rear differential  15  is adapted to divide torque from the drive shaft  13  between the rear wheels  15   a . The front output shaft which is connected to the front drive shaft  17  by means of a universal joint coupling  18 . The front drive shaft  17  has a front end connected to an input shaft of a front differential  19  by means of a universal joint coupling  20 . The front differential is adapted to divide torque received from the drive shaft  17  between the vehicle front wheels  19   a.    
     Referring now to FIG. 2, the internal components of the transfer case  10  are schematically shown, along with certain electrical connections to an electronic control which is represented in FIG. 2 as a block  24 . As shown in FIG. 2, the transfer case  10  includes an input shaft  25  coupled to the output shaft of the vehicle transmission  11  (shown in FIG.  1 ). The transfer case  10  also includes a rear output shaft  26  adapted to be connected to the rear drive shaft  13 , and a front output shaft  27  for connection to the front drive shaft  17 . The transfer case utilizes an interaxle planetary gear differential, generally indicated by the reference numeral  28 , for dividing torque between the rear output shaft  26  and the front output shaft  27 . Also, an electromagnetic friction clutch, generally represented by the reference numeral  29 , is provided for selectively locking the planetary gear differential to prevent any slip between the front and rear output shafts. 
     In particular, the input shaft  25  is secured to a planet carrier  31  which carries a plurality of circumferentially spaced and individually rotatable planet gears  32 . A sleeve member  33  is rotatably mounted about the input shaft  25  and has one end which carries a sun gear  34  of the planetary gear differential  28  and an opposite end which carries a first output gear  35 . The first output gear is connected to drive a second intermediate output gear  36  which in turn drives a third output gear  37  secured to the front output shaft  27 . A ring gear  38  of the planetary gear differential  28  is secured to the rear output shaft  26 . 
     The electromagnetic clutch assembly  29  includes a first group of clutch plates  39  which are secured for rotation with the ring gear  38 , and a second group of clutch plates  41  which are secured to the sleeve member  33  for rotation with the sun gear  34 . An annular clutch coil  42  is positioned adjacent the clutch plates  39  and  41  and is adapted to receive a clutch engagement signal from the electronic control  24 . The electromagnetic clutch  29  has a construction wherein, when a clutch engagement signal is generated to energize the coil  42 , the magnetic field generated by the energized coil  42  exerts a magnetic force to urge the clutch plates  39  and  41  into frictional engagement with one another to prevent relative rotation therebetween, thus locking the sun gear  34  and the ring gear  38  together. This prevents any relative slip between the front and rear output shafts. 
     The speed of the front output shaft is monitored by a speed sensor  44  which can be positioned adjacent the periphery of the teeth of the front output gear  37  or any suitable tone wheel. The speed sensor  44  generates a front output shaft speed signal to the electronic control  24 . Similarly, the speed of the rear output shaft  26  is monitored by a speed sensor  46  which can be positioned adjacent the periphery of the ring gear  38  or any suitable tone wheel, and can be adapted to sense a plurality of circumferential speed external teeth provided about the periphery of the ring gear. The speed sensor  46  generates a rear output shaft speed signal to the electronic control  24 . 
     As previously mentioned, the planetary gear differential  28  is provided for dividing torque between the rear output shaft  26  and the front output shaft  27 . Normally, the clutch coil  42  is not energized such that a predetermined slippage can occur between the front and rear output shafts to accommodate slightly different front and rear wheel speeds which occur during normal traction conditions such as when turning the vehicle. However, as will be discussed in more detail below, when slippage between the front and rear wheels exceeds a predetermined amount, the electronic control  24  will generate a clutch engagement signal which causes the planetary gear differential to lock and provide a direct drive connection between the input shaft  25  and the front and rear output shafts  26  and  27 . In particular, when the clutch coil  42  is energized, the ring gear  38  is locked relative to the sun gear  34  to prevent relative rotation therebetween. When the ring gear  38  is locked relative to the sun gear  32 , the planet gears  32  are prevented from rotating about their associated shafts, thereby preventing rotation of the planet carrier  31  relative to either the ring gear  38  or the sun gear  34 . 
     It should be noted that the transfer case illustrated in FIG. 2 is only one example of a transfer case which can utilize the electronic control of the present invention, and that other transfer cases which use a selectively engageable clutch means can be operated by the electronic control  24 . 
     The electronic control  24  is centered around a microcomputer  51  (see FIG.  3 ). The front speed sensor  44  is connected to the microcomputer  51  through an input circuit  52  which conditions the output signal from the sensor  44  prior to supplying the signal to the microcomputer  51 . Similarly, the rear speed sensor  46  is connected to the microcomputer  51  through an input conditioning circuit  53 . While various types of commercially available speed sensors could be used with the present invention, it has been found that it is preferable to use a Hall effect device as the speed sensing unit. The Hall effect device functions as a digital switch and provides an output signal which alternates between a high and low logic level as the associated gear teeth or other actuating teeth cause the magnetic field through the switch to vary as the associated component rotates. While a conventional variable reluctance speed sensor could be utilized, it has been found that this type of sensor requires extra input conditioning circuitry and does not provide a clean, square wave form at low shaft speeds which is desirable with the present invention. 
     As shown in FIG. 3, the front speed sensor  44  includes a Hall effect device  54  having output terminals  54 - 1 ,  54 - 2  and  54 - 3 , all of which are connected to the input circuit  52 . Typically, the terminal  54 - 1  is connected to a regulated power supply source of a predetermined magnitude, while the terminal  54 - 3  is connected to the circuit ground potential. The output signal of the Hall effect device is generated at the terminal  54 - 2 . In accordance with the present invention, a pair of internal resistors  55  and  56  are included in the speed sensor  44  and are connected across selected output terminals of the Hall effect device  54 . In particular, the resistor  55  is connected between the terminals  54 - 2  and  54 - 3 , while the resistor  56  is connected between the terminals  54 - 1  and  54 - 3 . Thus, the entire sensor  44 , including the Hall effect device  54 , and the resistors  55  and  56 , are located in the transfer case adjacent the periphery of the front output gear  37 , as shown in FIG.  2 . 
     Generally, the electronic control  24  is located within a separate housing which is external to the transfer case housing. For example, the electronic control can either be attached to an exterior portion of the transfer case or it can be located at another location on the vehicle. By incorporating the resistors  54  and  55  in the sensor  44 , the microcomputer can generate signals to the input circuit  52  which enable the condition of the wiring up to the sensor to be checked. Otherwise, without the resistors  55  and  56 , the diagnostic routine of the microcomputer  51  would not be able to distinguish between a fault in the sensor unit  44 , or the wiring between the sensor  44  and the electronic control  24 . 
     The rear speed sensor  46  has a construction similar to that of the front speed sensor  44 . In  15  particular, the rear speed sensor  46  includes a Hall effect device  57  having output terminals  57 - 1 ,  57 - 2  and  57 - 3  connected to the input circuit  53 . Also, an internal resistor  58  is connected between the terminal  57 - 2  and  57 - 3 , while a second internal resistor  59  is connected between the terminals  57 - 1  and  57 - 3 . 
     The microcomputer  51  is connected to generate a clutch engagement signal on a line  67 . The line  67  is connected to the gate of a transistor  68  through a resistor  69 . The source of the transistor  68  is connected to the circuit ground potential, while the drain of the transistor is connected to a terminal  42 - 1  of the clutch coil  42  by a line  70 . The other terminal  42 - 2  of the coil  42  is connected to the vehicle +B power supply. Normally, the transistor  68  is maintained in an off state by generating a low level signal near ground potential on the line  67 . When the transistor is off, the current flow through the clutch coil is sufficiently low such that the clutch is in its disengaged position. When a high level signal is generated on the line  67 , the transistor  68  is turned on to place the clutch terminal  42 - 1  near ground potential, and enable sufficient current flow through the clutch coil  42  to thereby engage the clutch. 
     A diode  71 , a capacitor  72 , and a zener diode  73  are provided to protect the transistor  68  from voltage spikes and current surges which can occur when the transistor  68  is turned on and off. In particular, the diode  71  has an anode connected to the clutch coil terminal  42 - 1  and a cathode connected to the clutch coil terminal  42 - 2 . The capacitor  72  is connected between the line  70  and the circuit ground potential, while the zener diode  73  has an anode connected to the circuit ground potential and a cathode connected to the line  70 . 
     A transistor  74  is responsive to the level of the signal on the line  70 . In particular, a resistor  75  is connected between the gate of the transistor  74  and the line  70 . A filter capacitor  76  is connected between the line  70  and the circuit ground potential, while a biasing resistor  77  is connected between the gate of the transistor  74  and the circuit ground potential. The drain of the transistor  74  is connected to a regulated +V power supply through a resistor  78 , while the source of the transistor  74  is connected to the circuit ground potential. 
     The level of the signal at the drain of the transistor  74  is supplied to the microcomputer  51  on a line  79 . In operation, the transistor  74  provides a means of checking the continuity of the clutch coil  42 . When the microcomputer is generating a low level signal on the line  67  such that the transistor  68  is off, the clutch is disengaged, and the line  70  will be at or near the +B voltage potential, providing that there is circuit continuity through the clutch coil  42 . When the line  70  is at the +B potential, the high level signal supplied to the gate of the transistor  54  will turn on the transistor  74  to place the line  79  near the circuit ground potential. In the event there is discontinuity in the clutch coil  42 , the level of the signal on the line  70  will not be sufficient to turn on the transistor  74 , such that the line  79  will be at or near the +V potential. Thus, by monitoring the level of the signal on the line  79  prior to engaging the clutch, the microcomputer can determine whether there is continuity through the clutch coil. 
     Referring now to FIGS. 4, there is shown a flow diagram which will be utilized to explain the operation of the electronic control. The operation of the control is initiated at an oval  100  and then the system polls the sensors  44 ,  46  at step  102 . If the rear sensor  46  detects a tooth count (step  104 ), the system pushes all of the registers of the front sensor&#39;s stack down one register (step  105 ) (the last register is pushed out or deleted), and the system adds the count to the rear sensor&#39;s current stack register (step  106 ). Likewise, if the front sensor  44  detects a tooth count (step  107 ), the system pushes all of the registers of the rear sensor&#39;s stack down one register (step  108 ) (the last register is pushed out or deleted), and the system adds the count to the front sensor&#39;s current stack register (step  109 ). 
     After a count is added to the front sensor&#39;s stack register (step  109 ), the system calculates the sum of the front stack register (step  110 ). Likewise, after a count is added to the rear sensor&#39;s stack register (step  106 ), the system calculates the sum of the rear stack register (step  112 ). Next, the system compares the sum of the tooth counts from each stack and calculates the relative rotational difference independent of any time-based reference (step  114 ). If a predetermined relative rotational difference is exceed (step  116 ), the system sends a signal to actuate the clutch pack of the transfer case  10 . 
     Thus, the system detection relative rotational differences at all speeds down to zero rpms. In addition, because of an equal probability of missing tooth counts, the system works well above the maximum sampling rate of the system. The system of this invention further permits modularization of the controller, thus allowing the processing and decision-making function to be broken up onto separate, smaller processors than would be required if all of the processing occurred on one central processor. This modularization may decrease the overall cost of the control system. 
     The present invention has been illustrated and described in its preferred embodiment. However, it will be appreciated that the above described embodiment of the electronic control be modified without departing from the scope of the attached claims. For example, while the above discussed control is utilized to selectively control the engagement of an electromagnetic clutch, it will be appreciated that the electro-magnetic clutch can be replaced with either a hydraulically or other fluid actuated clutch which in turn can be controlled by electrically actuated solenoid valves.