Abstract:
The present teachings generally include a method of controlling a relay. The method generally includes momentarily initiating a pull-in pulse when an input signal indicates a first state. A sense resistor controller is activated based on the pull-in pulse. A current flow is controlled to bypass a sense resistor and flow to the relay based on the activation of the sense resistor controller. The relay is controlled based on the current flow.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/884,904 filed on Jan. 15, 2007. The disclosure of the above application is incorporated herein by reference. 

   FIELD 
   The present disclosure relates to methods and systems for controlling current to mechanical relays. 
   BACKGROUND 
   Coils in mechanical relays generate heat. When a relay is activated, the relay needs large current to pull in the armature. Once the armature is pulled in, only a small current is needed to hold the armature in place. 
   Relay manufacturers design relays such that they can operate under various operating scenarios. It is known that coil resistance increases with temperature. Instead of taking into account the actual temperature, current supplied to operate the armature of the relay is operated at above normal requirements to ensure operation at all temperatures. In some cases, during normal operating conditions current supplied to operate the armature can be more than double the requirement (i.e., to accommodate for high ambient air temperatures). The excess energy is then dissipated as heat. This excess heat generated by the relay coil can cause thermal problems for other electrical components. For example, power distribution center modules (PDCs) for a vehicle can include more than twenty relays. The twenty relays can provide enough heat to affect the operation of other electrical components within the vehicle. 
   SUMMARY 
   The present teachings generally include a method of controlling a relay. The method generally includes momentarily initiating a pull-in pulse when an input signal indicates a first state. A sense resistor controller is activated based on the pull-in pulse. A current flow is controlled to bypass a sense resistor and flow to the relay based on the activation of the sense resistor controller. The relay is controlled based on the current flow. 
   Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 

   
     DRAWINGS 
     The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
       FIG. 1  is a block diagram of a vehicle including a power distribution center in accordance with various aspects of the present teachings. 
       FIG. 2  is a block diagram illustrating a relay driver system in accordance with various aspects of the present teachings. 
       FIG. 3  is an electrical schematic illustrating an example of various aspects of a relay driver system as shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   The following description is merely exemplary in nature and is not intended to limit the present teachings, their application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. As used herein, the term module, control module, component and/or device can refer to one or more of the following: an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated or group) and memory that executes one or more software or firmware programs, a combinational logic circuit and/or other suitable mechanical, electrical or electromechanical components that can provide the described functionality and/or combinations thereof. 
     FIG. 1  illustrates a vehicle generally at  10  that can include a power distribution module  12 . The power distribution module  12  can provide electrical energy from a vehicle battery  14  to various electrical systems  16  of the vehicle  10 . The power distribution module  12  can include one or more instances of a relay driver system  18  that can control an armature of a relay  20  according to various aspects of the present disclosure. 
   With reference to  FIG. 2  and in various aspects of the present teachings, the relay driver system  18  can control the flow of current to operate the relay  20 . In one aspect of the present teachings, the current flow can be controlled to provide a full battery voltage to the relay  20  during an initial pull-in period (i.e., moving an armature of the relay). In another aspect of the present teachings, after the pull-in period (i.e., a period in which the position of the armature is maintained), a voltage of the current flow is regulated such that a position of the armature of the relay  20  can be maintained without utilizing excess electrical energy and/or creating excess heat. 
   The relay driver system  18  shown in the example of  FIG. 2  can generally include a pull-in pulse generator  22 , a sense resistor controller  24 , a comparator  26 , a fast turn off system  28 , a logic gate  30 , a sense resistor  32 , and the relay  20 . The relay  20  can include a relay coil  34  and a main switch  36 . An input signal  38  can be commanded to the relay driver system  18 . Based on the input signal  38 , the relay driver system  18  can control an armature of the main switch  36  while minimizing the dissipation of heat. According to various aspects of the present teachings, the current can flow from the vehicle battery  14  through various paths of the relay driver system  18  to the relay  20 . 
   More particularly, the logic gate  30  can control the state of the main switch  36  to be ON or to be OFF. When the main switch  36  is ON, the flow of current can be regulated by the pull-in pulse generator  22 , the sense resistor  32 , the comparator  26 , the fast turn off system  28 , and/or any combinations thereof. At the beginning of relay operation, the pull-in pulse generator  22  can generate a pull-in pulse for a time at which it takes to pull in the relay armature. Based on the pull-in pulse, the sense resistor controller  24  can prevent the flow of current past the sense resistor  32  momentarily to allow full battery voltage to be applied to the relay coil  34  during the pull-in period. After the armature is pulled in, the sense resistor controller  24  can allow current to flow past the sense resistor  32  according to a first mode of operation. During the first mode of operation, the comparator  26  can compare the voltage drop across the sense resistor  32  to a reference voltage and/or hysteresis. Based on the voltage drop, the fast turn off system  28  can regulate the current flow past the relay coil  34  according to a freewheeling method as will be discussed in more detail below. 
   With reference to  FIG. 3 , an electrical schematic illustrates an example of various aspects of the relay driver system  18  shown in  FIG. 2 . The relay driver system  18  can include the relay coil  34  (L 1 ). The sense resistor  32  (R 3 ) can sense coil current. The main switch  36  can include a switch Q 5 . The switch Q 5  can control coil current. 
   The comparator  26  can include a pull-up resistor R 1 , a Zener diode Z 1 , a second resistor R 2 , a comparator U 1 B, a third resistor R 4 , a fourth resistor R 5 , and a capacitor C 1 . More particularly, the pull-up resistor R 1  can be required for operation of the comparator U 1 B. The Zener diode Z 1  and the second resistor R 2  can provide the comparator U 1 B with a voltage reference. The third resistor R 4 , the fourth resistor R 5 , and the capacitor C 1  can provide the comparator U 1 B with a hysteresis for comparison. The sense resistor controller  24  can include a first controlling transistor Q 1  and a second controlling transistor Q 2 . The controlling transistors Q 1  and Q 2  can be used to control the flow of current past the sense resistor R 3 . 
   The pull-in pulse generator  22  can include a comparator U 3 A, a resistor R 8 , a capacitor C 2 , and a logic gate U 2 A. As discussed above, the pull-in pulse generator can generate a pull-in pulse at the beginning of relay operation. The logic gate  30  can include an AND gate U 2 B, a Zener diode Z 3 , and a resistor R 7 . The AND gate U 2 B can allow the input signal  38  and an output of the comparator U 1 B to jointly control the main switch Q 5 . The Zener diode Z 3  can limit the output voltage of the comparator U 1 B to a logical range. The fast turn off system  28  can include a freewheeling diode D 1 , a fast turn off transistor Q 4 , a resistor R 6 , a switch Q 3 , and a Zener diode Z 2 . The freewheeling diode D 1  can be controlled by the fast turn off transistor Q 4 , the resistor R 6 , and the switch Q 3  to regulate current flow past the coil L 1 . The Zener diode Z 2  can be used for fast turn off as well as reverse battery protection. 
   As can be appreciated in light of the disclosure the relay driver system  18  can operate according to the following methods. When the input signal  38  is low, the logic gate U 2 B can shut the main switch Q 5  OFF. Thereby, preventing current flow through the sense resistor R 3  and/or the coil L 1 . The relay  20  ( FIG. 2 ) can be considered deactivated and the voltage drop across the sense resistor R 3  can be zero. The output of the comparator U 1 B can be high thus allowing the logic gate U 2 B to be ready to be controlled by the input signal  38 . 
   When the input signal  38  changes from low to high, the logic gate U 2 B can turn the main switch Q 5  ON. At the same time, the pull-in pulse generator  22  that can include the comparator U 3 A and logic gate U 2 A can generate a high pull-in pulse at point B. The pull-in pulse can turn on the sense resistor controller  24  that can include the second controlling transistor Q 2  and the first controlling transistor Q 1 . In this scenario, the current path can begin at Vbatt, and can flow to the controlling transistor Q 1 , to the coil L 1 , to the switch Q 5 , and on to the ground GND. The full battery voltage can be applied to the coil L 1 . The current of the coil L 1  begins to ramp up. 
   When the input signal  38  is high, the fast turn off transistor Q 4  and the switch Q 3  can be ON. The diode D 1  can be connected across the coil L 1  through the switch Q 3  and the sense resistor R 3 . The diode D 1  can be ready to perform a freewheeling function for the coil L 1 . More particularly, after the pull-in pulse ends, the second controlling transistor Q 2  and the first controlling transistor Q 1  can be turned OFF. The current passing through the coil L 1  can be shifted immediately from the first controlling current Q 1  to current from the sense resistor R 3 . The current flowing through the sense resistor R 3  can cause a voltage drop across the sense resistor R 3 . The voltage at point A (Va) can be below the low threshold of the comparator U 1 B. The output of U 1 B can become low. The low comparator output can turn the main switch Q 5  OFF through the logic gate U 2 B thereby, preventing coil current from flowing through the main switch Q 5 . Instead, the coil current can ramp down through a new path that can begin at the bottom of the coil L 1 , and can flow to the diode D 1 , to the switch Q 3 , to the sense resistor R 3  back to the top of the coil L 1 . This path can also be referred to as a freewheeling path. The voltage drop across the sense resistor R 3  ramps down with the coil current and voltage at point A (Va) becomes greater (i.e. closer and closer to Vbatt). 
   When the voltage at point A (Va) becomes higher than the high threshold of the comparator U 1 B, the output of the comparator U 1 B can become high. This high output of the comparator U 1 B can turn the main switch Q 5  ON through the logic gate U 2 B. The coil current can then begin to ramp up. For example, the coil current path can begin at Vbatt, and can flow to the sense resistor R 3 , to the coil L 1 , to the main switch Q 5 , and on to the ground GND. 
   While the coil current is ramping up, the voltage at point A (Va) can become lower and lower. When the voltage at point A (Va) becomes lower than the low threshold of the comparator U 1 B, the output of the comparator U 1 B can become low. This low comparator output can turn the main switch Q 5  OFF through the logic gate U 2 B. This method of regulating the voltage at point A (Va) can repeat. In this way, the coil current can be regulated at a constant level much lower than the pull-in current. When battery voltage changes, or the coil temperature changes, and/or both change, the coil current level does not change. 
   When the input signal changes from high to low, the fast turn off transistor Q 4  and the switch Q 3  can be turned OFF. The freewheeling path can be removed. At the same time, the main switch Q 5  can be turned OFF by the logic gate U 2 B. The coil current can decay to zero through a fast turn OFF path that can begin at the bottom of the coil L 1 , and can flow to the diode Z 2 , and on to the ground GND (i.e. the negative terminal of the vehicle battery), through the battery  14 , to the positive terminal of the battery  14 , to the sense resistor R 3 , to the top of the coil L 1 . The magnetic energy stored in the coil L 1  can be discharged at a high rate. The higher the Zener break-down voltage, the higher the discharge rate and the faster the turn off process. 
   While specific aspects have been described in this specification and illustrated in the drawings, it will be understood by those skilled in the art that various changes can be made and equivalents can be substituted for elements thereof without departing from the scope of the present teachings, as defined in the claims. Furthermore, the mixing and matching of features, elements and/or functions between various aspects of the present teachings may be expressly contemplated herein so that one skilled in the art will appreciate from the present teachings that features, elements and/or functions of one aspect of the present teachings may be incorporated into another aspect, as appropriate, unless described otherwise above. Moreover, many modifications may be made to adapt a particular situation, configuration or material to the present teachings without departing from the essential scope thereof. Therefore, it is intended that the present teachings not be limited to the particular aspects illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out the present teachings but that the scope of the present teachings will include many aspects and examples following within the foregoing description and the appended claims.