Patent Grant
8094427
The present invention relates generally to a system for precisely controlling the operational characteristics of a relay. More specifically, the present invention relates to a system for providing constant current to a relay despite variations in supply voltage and temperature.
Relays are generally operated under conditions that do not require precision timing. In some applications, however, precise control of the operational characteristics of a relay may be necessary. Important operational characteristics of a relay include the operate voltage, the release voltage, the operate time, and the release time. For a relay having normally open contacts, the operate voltage is the minimum relay coil voltage required to effect closure of the relay contacts following application of such operate voltage. The release voltage is the maximum relay coil voltage causing the relay contacts to remain closed before the contacts open when such voltage is decreased or removed. The operate time is the time elapsed from an application of the relay coil voltage until the contacts close. The release time is the time elapsed from removal of the relay coil voltage until the contacts open.
Operation of an electromagnetic relay is governed by physical properties such as the mass of moving parts, the frictional forces between components, the mechanical advantages of the design and the magnetic forces generated by a relay motor or solenoid which move a moveable mass to close the contacts. The mass of the moving parts, the component frictional forces and the mechanical advantages required to provide the requisite contact forces are generally unchanged by temperature. The magnetic forces generated by the relay motor or solenoid are directly proportional to the number of coil winding turns and the current flowing through those turns. The number of coil turns is fixed but the resistance of the coil winding material, and thus the coil current, varies with temperature according to the temperature coefficient of resistance of the winding material.
The operational characteristics of a relay are highly dependent on the coil current, which varies in accordance with coil resistance. Therefore, variations in temperature can cause substantial changes in the operational characteristics of relays and also present significant challenges in their design.
Aspects of the invention relate to a system for precisely controlling the operational characteristics of a relay. In one embodiment, the invention relates to a relay having performance characteristics that vary with a temperature of the relay, where the relay comprises a plurality of operational phases including a switching phase and a holding phase, and a relay control circuit configured to provide a preselected current to the relay at least during the switching phase and the holding phase, where the preselected current remains substantially constant despite a change in the temperature of the relay, where the relay is configured to transition from a non-energized position to an energized position during the switching phase, and where the relay is configured to maintain the energized position during the holding phase.
In another embodiment, the invention relates to a precisely controlled relay circuit including a relay having performance characteristics that vary with a temperature of the relay, where the relay comprises a plurality of operational phases including a switching phase and a holding phase, a relay control circuit configured to provide a preselected current to the relay at least during the switching phase and the holding phase, and a voltage source configured to provide a voltage to the relay control circuit, the voltage ranging from a minimum voltage to a maximum voltage, where the preselected current remains substantially constant despite a change in the voltage provided to the relay control circuit, where the relay is configured to transition from a non-energized position to an energized position during the switching phase, and where the relay is configured to maintain the energized position during the holding phase.
Variations in the ambient temperature and supply voltage can result in substantial changes in the operating parameters of a relay, especially the coil current. The standard material for winding a relay coil is copper magnet wire. For a temperature range of −55° C. to 85° C., corresponding to a temperature change of 130° C., the change in resistance caused by temperature can be as much as 60%. A typical 28 volt direct current (VDC) relay can have an operating voltage range that varies from 18 volts DC to 32 volts DC (or up to 40 VDC short term). This results in a maximum voltage range of up to 22 VDC (40 VDC minus 18 VDC) or a total change of approximately 50%. The combination of temperature and voltage variations, considered cumulatively, therefore often change the operating conditions or characteristics by more than 100%. Thus, typical relay circuits generally must accommodate wide varying operating conditions that often force undesirable compromises in their design.
To illustrate such variations in temperature and the resulting changes in the coil current, the operation of a typical 28 volt relay requiring approximately 2 watts of power to change the position of its contacts is analyzed. Table 1 illustrates the operational characteristics of a conventional 28 volt relay driven at approximately 32 volts over a range of temperatures.
In Table 1, the characteristics of the relay are shown at three temperature points including −40° C., 25° C., and 85° C. Also, in Table 1, the actual voltage applied to the coil is approximately 32 volts. The last row illustrates the coil current at the three temperature points. Table 2 illustrates the operational characteristics of the conventional 28 volt relay driven at approximately 18 volts over a range of temperatures.
In Table 2, the actual voltage applied to the coil is approximately 17 volts. The relay characteristics at the same temperature points of −40° C., 25° C., and 85° C. as in Table 1 are also depicted. Also, in Table 2, the smallest coil current is found at 85° C. and is 0.044 amps (A). In Table 1, the largest coil current is found at −40° C. and is 0.165 A. The ratio of the largest coil current to the smallest coil current is 3.75. Thus, the empirical data depicted in Table 1 and Table 2 illustrates a maximum current variation of 375% over a voltage range of 18 to 32 volts and temperature range of −40° C. to 85° C.
Turning now to the drawings, embodiments of relay control circuits precisely control the current provided to a relay despite changes in voltage and temperature. In many embodiments, the relay control circuits provide a constant current despite changes in voltage and temperature. In several embodiments, the relay control circuits include an adjustable linear voltage regulator and control resistor to provide the constant current. In other embodiments, other circuit components can be used to provide the constant current.
In one embodiment, the relay control circuits and controlled relays are used to control the distribution of power in an aircraft electrical system. Power can be distributed using various DC or AC (single, two or three phase) systems, or combinations thereof. In a number of embodiments, the relay has one load switch that switches a DC power source. In several embodiments, the DC power sources operate at 28 volts, 26 volts or 270 volts. In one embodiment, DC power sources operate in the range of 11 to 28 volts. In other embodiments, the relays include three load switches that switch AC power sources. In one embodiment, the AC power source operates at 115 volts and at a frequency of 400 hertz. In other embodiments, the relays controlled by the relay control circuits have a single load switch that can switch a DC power source or a single phase of an AC power source. In other embodiments, the power sources operate at other voltages and other frequencies. In one embodiment, the DC power sources can include batteries, auxiliary power units and/or external DC power sources. In one embodiment, the AC power sources can include generators, ram air turbines and/or external AC power sources.
In operation, the adjustable linear voltage regulator 214 maintains a relatively constant voltage across the output terminal 217 and adjustment input 215. By placing a control resistor 216 across the output terminal 217 and adjustment input terminal 215, the adjustable voltage regulator acts to provide a constant current, and therefore constant voltage drop, through the control resistor 216. In such case, where the resistance of the relay 204 varies in accordance with changes in temperature or applied voltage, the adjustable voltage regulator acts to compensate for those changes such that constant current is provided despite the variation.
In one embodiment, the adjustable voltage regulator is a LM117 positive adjustable voltage regulator made by Linear Technology Corporation of Milpitas, Calif. In such case, the regulator attempts to maintain a constant reference voltage of 1.25 volts across the output terminal and adjustable input terminal. In this case, a constant current of approximately 0.1 A is provided to the relay 204, when the voltage regulator 214 is turned on. When the resistance of the windings in the relay coil 204 changes in response to a change in temperature, the adjustable voltage regulator adjusts the voltage provided at its output terminal 217 to maintain the constant current of approximately 0.1 A. For example, when the temperature increases, the resistance of the relay coil 204 also increases. In such case, the voltage regulator 214 must increase the voltage at output terminal 217 in order to maintain the constant current and the reference voltage across the control resistor 216.
The use of an adjustable linear voltage regulator provides benefits not only in maintaining a constant current in the relay coil, but also in withstanding swings in the switching voltage supplied to the relay. For example, the voltage regulator is generally able to accept a wide range of swings in input voltage provided that such input voltage is greater than that of the regulator output voltage by at least a drop-out voltage. The drop-out voltage is generally a characteristic of the regulator.
In the illustrated embodiment, the control resistor is 12 ohms. In other embodiments, the control resistor can take other values. In a number of embodiments, the control resistor has a very low tolerance to minimize variation in current flowing through the resistor.
In other embodiments, other circuits capable of providing a constant current can be used to control the relay. In some embodiments, the other relay control circuits can tolerate wide voltage swings while providing the constant current.
The simulated relay 304 includes a resistor R2 coupled to the adjustment terminal of the regulator 314 and to the ground 323 via an inductor/coil L1. The inductor L1 is coupled in parallel by a third capacitor C3. The simulated relay 304 also includes a resistor R4 coupled to the adjustment terminal of the regulator 314 and the ground 323. The combination of R2, L1, C3 and R4 provides the electrical characteristics of a typical relay.
The transient suppression circuit 318 includes a diode 324 coupled in series with two zener diodes (326, 328). The cathode of zener diode 328 is coupled to ground 323. Zener diodes 326 and 328 are oriented in the same direction such that the cathode of zener 326 is coupled to the anode of zener 328. Diode 324 and zener 326 are connected in a back to back configuration such that the anode of diode 324 is coupled to the anode of zener diode 326. The cathode of diode 324 is coupled to the adjustment terminal of the regulator 314. The transient suppression circuit 318 handles reverse biased voltage spikes with the zener diodes and effectively discharges the simulated relay 304. More specifically, the transient suppression circuit 318 can discharge the energy stored in the coil L1.
In operation, the AC voltage source 312 provides a voltage signal of 32 volts after a 10 millisecond (ms) delay. The voltage signal is thus provided with a rise time of 10 ms and a fall time of 10 ms. In other embodiments, the AC voltage source can provide a voltage signal at another voltage level and with other timing characteristics.
In simulation testing of the embodiment described in
In a number of embodiments, the performance characteristics of a relay can be classified into multiple operational phases. In some embodiments, for example, a switching phase can be defined as the phase where the relay transitions from a non-energized state to an energized state. In one embodiment, the switching phase is a time period that corresponds to the operate time. In another embodiment, in a holding phase of the relay can be defined as the phase where the relay maintains the energized state. The non-energized state, as used herein, means the state of the relay when little or no voltage has been applied to the relay coil. The energized state, as used herein, means the switched state of the relay after a switching voltage, sufficient to effect a change in position of the relay contacts, has been applied to the relay.
An external control circuit 620 is coupled to the adjustment input 615 of the linear voltage regulator 614. In the embodiment illustrated in
In operation, the relay control circuit 602 can otherwise operate in the manner described for the embodiment of
The drain terminal of the MOSFET switch 804 is coupled to the relay 802. The source terminal of the switch 804 is coupled to the control resistor 806 and to an adjustment input, “ADJ”, of the linear voltage regulator 808. The control resistor 806 is coupled to an “OUT” terminal of the voltage regulator 808. An “IN” terminal of the voltage regulator 808 is coupled to the pre-regulator circuitry 810. The pre-regulator circuitry 810 is coupled to a voltage source 811. The voltage source 811 is also coupled to the control circuitry 812. The control circuitry 812 is coupled by signal source circuitry to the gate of MOSFET switch 804. An RC circuit couples the gate of the MOSFET switch 804 to the source. In several embodiments, the MOSFET switch 804 is P-channel power MOSFET.
In operation, the control circuitry 812 controls the MOSFET switch 804 by way of the signal source. When the signal source is driven to ground, the gate voltage of the switch 804 becomes a fraction of the voltage at the source terminal. The fraction can depend on resistor values in the illustrated voltage divider. In the embodiment illustrated in
The pre-regulator circuitry 810 conditions the voltage provided to the voltage regulator. In some embodiments, the pre-regulator circuitry includes transient suppression circuitry that suppresses transients, such as spikes in supply voltage.
In one embodiment, the adjustable voltage regulator is a LM317 positive adjustable voltage regulator made by Linear Technology Corporation of Milpitas, Calif. In the illustrated embodiment, the control resistor has a resistance of 9.1 ohms. In other embodiments, the control resistor can have a resistance value that is greater than or less than 9.1 ohms. In one embodiment, the MOSFET switch is a IRFR5410 P-channel power MOSFET made by International Rectifier Corporation of El Segundo, Calif. In one embodiment, relay 802 controls the flow of power between a secondary power source and a primary bus on an aircraft. In such case, the relay may have to react to a sudden loss of power within a short amount of time. In this instance, the precisely controlled relay circuit, being virtually resistant to variations in temperature and voltage, can react quickly to switch auxiliary power to the aircraft primary bus. In other embodiments, the relay control circuit is used to switch power between other power sources and buses, or between other components of power systems.
As temperature ranges from +25° C. to +85° C. (row five), the relay resistance varies approximately 20 percent, the relay current varies approximately 20 percent, the operating voltage varies approximately 20 percent, the release voltage varies approximately 20 percent, the operate time varies approximately 20 percent and the release time varies approximately 20 percent. Similarly, as temperature ranges from +25° C. to −55° C. (row six), the relay resistance varies approximately 30 percent, the relay current varies approximately 30 percent, the operating voltage varies approximately 30 percent, the release voltage varies approximately 30 percent, the operate time varies approximately 30 percent and the release time varies approximately 30 percent. Accordingly, the table of
In the table shown in
While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as an example of one embodiment thereof. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4318154 | DePuy | Mar 1982 | A |
| 5317475 | Siepmann | May 1994 | A |
| 20030218847 | Lesesky et al. | Nov 2003 | A1 |
| 20050135040 | Suzuki et al. | Jun 2005 | A1 |
| 20060139839 | Sato et al. | Jun 2006 | A1 |
| 20100177453 | Critchley et al. | Jul 2010 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20100177453 A1 | Jul 2010 | US |
