Transducer Driver Circuit for Mid-Air Haptic Systems
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Solution Overview
Problem
Existing driver circuits for ultrasonic transducers in mid-air haptic systems face issues with excessive peak current requirements from the power supply, current variations with the number of simultaneously switching transducers, direct drive from high voltage power supplies causing crosstalk, unwanted modulation of transducer outputs, and inefficient discharging of transducer charge into ground.
Innovation Solution
The solution involves adding local decoupling capacitance with constant current charging, isolating PWM switching from energy accumulation, and reusing charge to reduce average current drawn from the power supply, allowing for simultaneous switching of multiple transducers without power supply limitations, and implementing trickle charging to utilize the full 40 kHz cycle for charge transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If multiple transducers switch simultaneously to generate haptic feedback, then the audio output power is maximized, but the peak current requirement from the power supply increases excessively
Solution Approach 1:
The patent divides the power supply system into multiple independent voltage rails (e.g., VDD1, VDD2, VDD3) that can independently drive different groups of transducers. This segmentation allows the total current demand to be distributed across multiple power supply channels, reducing the peak current burden on any single power supply while enabling simultaneous operation of multiple transducers for maximum audio output.
2Productivity
If the power supply sources high peak current for simultaneous transducer switching, then all transducers can switch at the same time, but the power supply voltage varies significantly causing crosstalk and modulation
Solution Approach 1:
The patent segments the transducer array into multiple independent groups, each driven by its own voltage rail with dedicated power supply. This isolation prevents voltage fluctuations from one group from affecting other groups, eliminating crosstalk and unwanted modulation while allowing all transducers to switch simultaneously without compromising power supply stability.
Solution Approach 2:
The patent introduces independent voltage rails as intermediary layers between the power supply and transducers. These voltage rails act as buffers that isolate the transducers from direct power supply fluctuations, maintaining stable driving voltages even when multiple transducers switch simultaneously, thereby preventing crosstalk and output modulation.
3Power
If direct voltage drive is used to maximize audio power output, then the transducer is driven to high voltage, but the capacitive load requires significant switching current
Solution Approach 1:
The patent segments the capacitive load of multiple transducers into separate groups, each connected to its own voltage rail. This segmentation distributes the total switching current demand across multiple independent power supply channels, reducing the peak current requirement for each individual power supply while maintaining the high voltage drive necessary for maximum audio power output from each transducer.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly reduces peak current requirements, eliminates power supply voltage variations, prevents crosstalk, and enhances efficiency by allowing any number of transducers to switch simultaneously, improving audio output stability and control point implementation.
Implementation Method 1
Since the transducer has a significant capacitance, typically 2 nF (or more) then the level shifting circuit must be capable of sourcing (sinking) a significant current for a very short time when the transducer switches between 0V and 20V
Implementation Method 2
The ultrasonic transducers are typically driven at a fixed frequency (often 40 kHz) corresponding to the resonant frequency of the transducer
Data Source
AI summary
A circuit for driving a transducer in a mid-air haptic system includes a voltage source, a voltage sink, a current source, a trickle capacitor, a storage capacitor, a haptic system transducer, a first switch, a second switch, and a third switch. Using these components, a portion of the charge required for switching a transducer is sourced from the decoupling capacitance. When the switching completes, additional charge is transferred immediately from the power supply back into the decoupling capacitance. This acts to lower the peak current by fully utilizing 100% of a switching waveform for transfer of charge from the power supply to capacitors local to the transducer.