BOS1901 system overview
Piezoelectric actuators convert force applied on them to voltage, and voltage applied on their terminals to displacement. This behavior can be used to sense pressure applied by a user to a device, but also to provide haptic feedback. BOS1901 actuator driver supports both these capabilities in a single product.
This document explains how to use BOS1901 to sense when the user applies enough force on a piezoelectric actuator in order to then trigger a haptic feedback response using the same chip. A typical click button application sequence is given.
1 System Overview
Since BOS1901 features both sensing and feedback driving, only one circuit is required to fully control the piezoelectric actuator. A typical solution will thus require three components: the piezoelectric actuator, the driver, and the microcontroller (MCU) or application processor.
Figure 1 System components using piezoelectric actuator
1.1 Piezoelectric Actuator
The actuator converts voltage to displacement, and displacement to voltage. Through its integration in a device, it can sense on much pressure the user is exerting on the device surface, and it can produce vibrations of various kind to be felt by the user. It is the component that will be in direct interaction with the human user.
When compressed, it converts internal stress to charges and voltage. This voltage can be measured and processed to determine how the user is interacting with the device, whether is it pushing a button, a screen or any other surface, or releasing it.
When a high voltage is applied on its terminals, it tends to deform. Through use of proper voltage waveforms, some effects and vibration can be played to give the user the impression of clicking a button or feeling a texture for example.
1.2 BOS1901 Driver IC
Electrical circuits must be used to measure the voltage on the actuator terminals and to generate the high voltage required to deform it and create a force.
BOS1901 was designed to drive piezoelectric actuators in haptics application, where the signal frequency content is typically below 300 Hz. It was designed to optimize power and be small enough to be integrated into battery-operated mobile devices. It features both sensing and feedback capabilities thereby reducing significantly the bill-of-material.
When sensing, it can read the voltage generated across the actuator and send it to the MCU via the SPI interface. When playing a feedback to be felt by the user, it drives the high voltage required for the actuator to vibrate. Any waveform shape, amplitude and frequency may be played to the BOS1901. Data points are sent via the SPI interface to the internal FIFO to be used at the predefined sampling rate.
The SPI interface operates in full-duplex, allowing to write operations and read information at the same time.
1.3 MCU or Application Processor
Although BOS1901 provides voltage measurement and driving capabilities it does not embed detection algorithms. These algorithms reside in the MCU or application processor connected to the BOS1901 by the SPI interface.
This allows any application to use the appropriate complexity for the sensing algorithm, and to change the feedback waveforms given the context and interaction with the user.
The MCU determines the operation mode of the BOS1901, handles its register configuration and initialization, processes the voltage readings to realize the appropriate sensing algorithm, and generates the waveforms to be played to it. All application-specific code is running on the MCU. BOS1901 is acting mostly as a driver and sensor circuit.