초록 ▼

An improved method and apparatus for opening and closing a high performance shape memory alloy (SMA) fuel injector that minimizes performance variations due to changes in ambient temperature. When opening the injector to commence fuel injection, a relatively high voltage is applied to an SMA element

An improved method and apparatus for opening and closing a high performance shape memory alloy (SMA) fuel injector that minimizes performance variations due to changes in ambient temperature. When opening the injector to commence fuel injection, a relatively high voltage is applied to an SMA element of the injector to quickly heat the element to its high temperature state, quickly opening the valve. The current level is monitored as a measure of the element resistivity, and when the determined resistivity indicates that the high temperature state transition is complete, the voltage is reduced to a hold value, sufficient to continue resistivity measurement. When the determined resistivity indicates that the element is beginning to transition back to the ambient temperature state, the high voltage is re-applied to begin a new control cycle. To close the injector, the electric heating is discontinued, and fuel circulating through the injector cools the element to its ambient temperature state. The variability in opening response is reduced because the high voltage is significantly higher than a voltage that would thermally damage the SMA element if sustained, and the resistivity feedback during application of the high voltage is used to control the timing of the current reduction. The variability in closing response is reduced by the repeated switching between high and low voltage control, which controls the temperature of the element, and thereby maintains the element in a state of readiness to return to the ambient temperature state. Additionally, the repeated switching between high and low voltage control limits the peak temperature of the SMA element, and prevents damage to the element if the circulating fuel fails to adequately cool the element. Finally, the use of current measurement in the control circuit permits reliable determination of the resistivity of the SMA element without requiring external thermal sensors.

대표청구항 ▼

[ We claim:] [1.] A control method for activating an actuator having a SMA element that assumes a deactivated state at ambient temperature and an activated state at an elevated temperature, the control method comprising the steps of:when activation of said actuator is desired:(a) applying a high vol

[ We claim:] [1.] A control method for activating an actuator having a SMA element that assumes a deactivated state at ambient temperature and an activated state at an elevated temperature, the control method comprising the steps of:when activation of said actuator is desired:(a) applying a high voltage to the SMA element to initiate a transition from said deactivated state to said activated state, and monitoring a current through said element during the application of said high voltage;(b) when the monitored current reaches a first threshold corresponding to a resistivity of said SMA element that occurs when said SMA element has attained said activated state, lowering the voltage applied to said SMA element, and monitoring the current through the element during application of said lowered voltage; and(c) when the monitored current during the application of said lowered voltage reaches a second threshold corresponding to a resistivity of said SMA element that occurs prior to an onset of transition to said deactivated state, re-applying said high voltage and cyclically lowering said applied voltage in accordance with steps (b) and (c), so as to maintain the SMA element in a state of readiness to return to said deactivated state; andwhen deactivation of said actuator is desired, removing said applied voltage, and cooling said SMA element, so as to return said SMA element to said deactivated state.