Method and apparatus to drive a linear resonant actuator at its resonant frequency

Abstract

A method for driving a Linear Resonant Actuator (LRA) is provided. During a first off interval, the back-emf of the LRA is measured. During a first off interval, a timer is started when the back-emf reaches a predetermined threshold, and after a predetermined delay has lapsed following the back-emf reaching the predetermined threshold during the first off interval, the LRA is driven over a drive interval having a length and drive strength. A second off interval is entered following the drive interval, and during the second off interval, the back-emf of the LRA is measured. During the second off interval, the timer is stopped when the back-emf reaches the predetermined threshold. The value from the timer that corresponds to the duration between the back-emf reaching the predetermined threshold during the first off interval and the back-emf reaching the predetermined threshold during the second off interval determines the length.

1 . An apparatus comprising:
 an input circuit that is configured to receive an input signal;   a signal generator that is coupled to the input circuit and that is configured to output a drive signal that is based at least in part on the input signal, wherein the drive signal includes a plurality of positive drive intervals, a plurality of negative drive intervals, and a plurality of off intervals; and   a driver that is coupled to the signal generator and the input circuit, that is configured to drive a linear resonant actuator (LRA), and that is configured to receive the drive signal, wherein the input circuit is configured to measure the back electromotive force (back-emf) of the LRA during the plurality of off periods of the drive signal, and wherein the signal generator adjusts the duration of at least one of the positive and negative intervals based at least in part on the measured back-emf so as to drive the LRA substantially at its resonant frequency.   
     
     
         2 . The apparatus of  claim 1 , wherein the apparatus has an operational mode and an initialization mode, wherein each period of drive signal during the operational mode includes at least one positive drive interval or at least one negative drive interval between two consecutive off intervals, and wherein the measurement circuit is configured to determine whether the back-emf has reached a predetermined threshold in each of the two consecutive off intervals, and the controller is configured to measure the duration between the back-emf reaching the predetermined threshold for the two consecutive off intervals. 
     
     
         3 . The apparatus of  claim 2 , wherein the signal generator further comprises:
 an analog-to-digital converter (ADC) that is coupled to the input circuit; and   a controller that is coupled between the ADC and the driver.   
     
     
         4 . The apparatus of  claim 3 , wherein the measurement circuit further comprises:
 a sense amplifier that is coupled to the driver; and   a multiplexer that is coupled to the sense amplifier, the ADC, and the controller, wherein the multiplexer is configured to receive the input signal, and wherein the controller is configured to control the multiplexer.   
     
     
         5 . The apparatus of  claim 4 , wherein the driver is a class AB driver. 
     
     
         6 . The apparatus of  claim 4 , wherein the driver is a class D driver having:
 a gate drive circuit that is coupled to the controller; and   an H-bridge that is coupled to the gate drive circuit and the sense amplifier.   
     
     
         7 . The apparatus of  claim 6 , wherein the ADC receives a supply voltage as a reference voltage. 
     
     
         8 . The apparatus of  claim 6 , wherein the multiplexer receives a supply voltage as an input, and wherein the controller further comprises:
 a logic circuit that is coupled to the ADC; and   a compensator that is coupled between the logic circuit and the gate drive circuit.   
     
     
         9 . A method comprising:
 during a first off interval, measuring the back-emf of an LRA;   during a first off interval, starting a timer when the back-emf reaches a predetermined threshold;   after a predetermined delay has lapsed following the back-emf reaching the predetermined threshold during the first off interval, driving the LRA over a drive interval having a length with a drive strength, wherein the drive strength is based at least in part on an input signal and the back-emf;   entering a second off interval following the drive interval;   during the second off interval, measuring the back-emf of the LRA;   during the second off interval, stopping the timer when the back-emf reaches the predetermined threshold; and   setting a value from the timer that corresponds to the duration between the back-emf reaching the predetermined threshold during the first off interval and the back-emf reaching the predetermined threshold during the second off interval minus a delay period_as the length of the drive interval.   
     
     
         10 . The method of  claim 9 , wherein the predetermined threshold further comprises a first predetermined threshold, and wherein the drive interval further comprises a first drive interval, and wherein the method further comprises the step of performing initialization by:
 retrieving the length;   determining whether the back-emf magnitude of the LRA is less than a second predetermined threshold; and   if the back-emf magnitude is less than the second predetermined threshold, driving the LRA over a second drive interval of one-half of the length.   
     
     
         11 . The method of  claim 10 , wherein the first off interval further comprises a first delay interval following the time when the back-emf reaches the predetermined threshold, and wherein the second off interval further comprises a second delay interval following the time when the back-emf reaches the predetermined threshold. 
     
     
         12 . The method of  claim 11 , wherein the first and second delay intervals are about 200 μs. 
     
     
         13 . An apparatus comprising:
 a touch panel;   a touch panel controller that is coupled to a touch panel, wherein the touch panel includes an interface;   an LRA; and   a haptics driver having an operational mode and an initialization mode, wherein the haptics driver includes:
 an input circuit that is coupled to receive an input signal from the interface; 
 a signal generator that is coupled to the input circuit and that is configured to output a drive signal that is based at least in part on the input signal, wherein the drive signal includes a plurality of positive drive intervals, a plurality of negative drive intervals, and a plurality of off intervals; and 
 a LRA driver that is coupled to the signal generator, the input circuit, and the LRA, wherein the LRA driver is configured to receive the drive signal from the signal generator, and wherein the input circuit is configured to measure the back-emf of the LRA during the plurality of off periods of the drive signal, and wherein each period of drive signal during the operational mode includes at least one positive drive interval or at least one negative drive interval between two consecutive off intervals, and wherein the measurement circuit is configured to determine whether the back-emf has reached a predetermined threshold in each of the two consecutive off intervals, and the controller is configured to measure the duration between the back-emf reaching the predetermined threshold for the two consecutive off intervals, and wherein the signal generator adjusts the duration of at least one of the positive and negative intervals based at least in part on the duration between the back-emf reaching the predetermined threshold for the two consecutive off intervals so as to drive the LRA substantially at its resonant frequency. 
   
     
     
         14 . The apparatus of  claim 13 , wherein the signal generator further comprises:
 an ADC that is coupled to the input circuit; and   a controller that is coupled between the ADC and the LRA driver.   
     
     
         15 . The apparatus of  claim 14 , wherein the measurement circuit further comprises:
 a sense amplifier that is coupled to the LRA driver; and   a multiplexer that is coupled to the sense amplifier, the ADC, and the controller, wherein the multiplexer is configured to receive the input signal, and wherein the controller is configured to control the multiplexer.   
     
     
         16 . The apparatus of  claim 15 , wherein the ADC further comprises a first ADC, and wherein the interface further comprises a first interface, and wherein the touch panel controller further comprises:
 a second interface that is coupled to the touch panel;   a second ADC that is coupled to the second interface; and   a pre-processing circuit that is coupled to ADC and the first interface.   
     
     
         17 . The apparatus of  claim 16 , wherein the apparatus further comprises a host processor that is coupled to the second interface. 
     
     
         18 . The apparatus of  claim 17 , wherein the LRA driver further comprises a class AB driver. 
     
     
         19 . The apparatus of  claim 17 , wherein the LRA driver further comprises:
 a gate drive circuit that is coupled to the controller; and   an H-bridge that is coupled between the gate drive circuit and the LRA.   
     
     
         20 . The apparatus of  claim 19 , wherein the haptics driver further comprises a filter that is coupled between the second interface and the multiplexer.