Power Kit adds haptic feedback to capacitive touch
2014/01/13
Motion control in miniature is the aim of the HapTouch BoosterPack, which aims to add vibration feedback to user interfaces.
Developed by Texas Instruments and Farnell, it is based around two TI chips: the DRV2603 (and here) resonant motion driver and newly-released MSP430TCH5E ‘haptics-enabled microcontroller’, as TI calls it.
The driver includes a four-mosfet full-bridge driver for linear resonant actuators – which are essentially loudspeaker voice coils driving a mass, rather than a cone.
These things have high Q and a temperature-dependant production-dependent narrow resonant peak. “Typically they have a window no wider than +/-2.5Hz in which sufficient resonance is achieved,” said TI.
To make matters worse when a profile of vibration is required, resonant frequency can also be slightly amplitude-dependent.
TI decreases the chance of missing resonance by sensing coil back-EMF immediately after applying the first pulse of its drive signal.
An on-board DSP – called the auto-resonance engine by TI – immediately calculates the resonant frequency and adjusts the output pulse repetition frequency accordingly. “This dynamic tracking will ensure more consistent vibration. Consistent vibration is much harder to achieve with a non-auto-resonance drivers,” said the firm.
Variation in transducer gain is also automatically compensated for with, according to TI, resultant acceleration within a few percent compared with a 2:1 range when the firm’s earlier non-feedback DRV8601 was employed in tests.
Any PWM frequency presented to the 2603 between 10 and 250kHz will cause it to provide the correct output frequency.
Having calculated the actuator’s dynamic parameters, the chip can also offer dynamic braking – actively supressing ringing when the control signal is removed by applying an anti-phase waveform.
Supply range is 2.5 to 5.2V; inputs are 1.8V compatible and 5V tolerant; and the chip will also work with conventional motor-based eccentric rotating mass actuators.
Although this chip can be operated by anything that can send it a PWM waveform, TI has put together the SLAU543 library of haptic waveforms, and the MSP430TCH5E microcontroller to execute the library. So specifically are they paired that the library will run on no other MSP430. “If executed on another MSP430,” said TI, “the application will build, but the API functions will return an error”
The library includes 122 different haptics effects, and these can be sequenced and repeated with varying delays to give a wide range of tactile sensations with both eccentric rotating mass and linear resonant actuators. On top of this, the library also includes an audio-to-haptics feature, “which creates a haptics experience based on an audio input”, says TI.
It is based on the TouchSense TS2200 library from haptics firm Immersion. “Using the MSP430TCH5E solution allows implementation of haptics without patent infringement and without royalties,” said TI. Its audio interface is also based on an Immersion product, called Audio2Haptics.
The library is not pre-programmed into the device. It must be downloaded and programmed into each TCH5E.
Together, in response to commands over an I2C interface, the microcontroller and DRV2603 driver can act as a stand-alone vibration generator.
However, another feature of the MSP430TCH5E is that it is designed to execute TI’s capacitive touch library (SLAA490) at the same time as the haptics library, to produce vibrations in response to touch buttons, sliders, or wheels.
This is where the HapTouch BoosterPack (SLAA616) comes in. It is a reference design for applications in which both haptic and touch libraries run on the microcontroller while it feeds the auto-tuning DRV2603 resonant driver.
http://www.element14.com/community/docs/DOC-65011?ICID=knode-TIlaunchpad-space#anchor4
Looking very much like a games controller HapTouch “allows users to add vibrational feedback to all capacitive touch buttons, sliders and wheels, for use in mobile computing and gaming devices, industrial control panels, point-of-sale terminals and many other applications”, said Farnell.
It comes with a software development kit and, to work, has to be plugged into TI’s MSP-EXP430G2 LaunchPad board.
“Developers can also leverage the PC-based MSP430 Capacitive Touch Pro graphical user interface tool to evaluate, diagnose and tune capacitive touch button, slider and wheel designs in real time, featuring user-configurable ranging, recording and printing options,” said Farnell.
TI is not the only company to pair capacitive touch and haptic feedback. Atmel is another, and has an application note with an informative section on vibrator positioning in portable devices.
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