Vishay Intertechnology Inc. (Vishay) is a leading company with many years of experience in the field of optoelectronics, especially in optical sensing. Their rich optoelectronics portfolio includes IR receivers, IR emitters, IrDA transceivers, optocouplers, optical sensors, photodetectors, and much more. However, the scope of this article is Vishay’s VCNL series of integrated Proximity-Sensing (PS) and Ambient Light-Sensing (ALS) solutions and their implementation into a robust force-sensing Human-Machine Interface (HMI) application with haptic feedback.
Using Force Sensing for HMI Applications
Before diving deeper into technicalities, let’s first discuss why we need force sensing in HMI applications when capacitive touch is the de facto standard in this area.
Current trends in the automotive industry are geared towards using big capacitive TFT displays with appealing graphical interfaces. Capacitive touch-sensing technology relies on measuring charge redistribution in the presence of an interacting object (finger) to determine its location. Although the technology has matured over time, it is not yet perfect: the material of the interacting object greatly affects the display’s ability to detect touch events reliably, so wearing gloves or using other insulating materials could be an issue. Another downside of using large capacitive TFT displays is their ever-increasing size: it becomes difficult not to touch them by accident, so false triggering became a real problem in modern HMI applications. Using a mechanical button as a wake-up command could potentially solve this problem, but such buttons are prone to wear and have a limited number of switching cycles.
The proposed force-sensing solution is based on the optical proximity detection principle, using an infrared light emitter (IR LED) and a photodetector (PD) to measure the distance of a reflective object. An external force applied to the reflective object, carefully positioned above the emitter-sensor pair at a specific distance, causes its displacement, affecting the intensity of the reflected light. The intensity of the light reflection can be measured by the PD and converted into action.
A simple but reliable force-sensing HMI can be realized by enclosing all the elements in a casing with a semi-flexible cover. Such an HMI provides not only the detection of a touch event but also the measurement of the applied force, enabling reliable suppression of false triggering in the event of accidental contact. Most importantly, the material of the force-exerting object does not affect the detection reliability, enabling operation under different conditions (dirt, humidity).
Integrated Solutions for Simplified Implementation
The proposed solution must be cost-effective and simple enough to make its way into real-world applications. Therefore, we should carefully consider its implementation: using conventional, off-the-shelf discrete components could introduce too much drift and noise into the signal chain, reducing the resolution and sensitivity. Similarly, using specialized low-noise components would negatively affect production costs. In summary, any design with discrete components would increase the complexity and costs, making the solution unfeasible. This is where Vishay’s integrated optical sensing solutions come into play.
Vishay’s VCNL series of automotive-qualified (AEC-Q101) integrated PS and ALS solutions provide all the necessary components on a chip, along with high-resolution 12/16-bit ADC converters and a signal conditioning circuit, offering orders of magnitude greater performance and precision than any discrete solution in a similar range price. Devices such as the VCNL3030X01 and the VCNL4030X01 are slightly better choices for developing force-sensing applications due to their integrated IR LEDs. The VCNL3036X01 and the VCNL4035X01 offer three integrated LED drivers instead of the IR LEDs, making them a good choice for both force and gesture-sensing applications (when combined with external LEDs). Thanks to their high resolution and perfectly matched and calibrated internal components, these devices provide highly accurate measurements on the order of a few tens of micrometers (provided no stray ambient light affects the PD). The VCNL sensors series represent the complete force-sensing solution, offering price and simplicity close to a mechanical button.
Adding Haptic Feedback to the Design
Haptic (sometimes called force-feedback) actuators provide touch activation feedback in the form of tactile information, which is very important for non-mechanical, contactless HMI designs, such as capacitive TFT and optical force sensing. A haptic actuator is often designed as a tiny motor that drives an unbalanced weight attached to its shaft (a vibration motor). However, solenoid platforms are among the leading foundation technologies for haptic products. Such actuators provide high acceleration, enabling reliable tactile feedback in a wide range of conditions (e.g., background vibrations from the engine).
In addition to the VCNL optical sensing solutions, Vishay offers custom magnetics for implementing haptic feedback, such as the compact IHPT-1411AF-AB0 solenoid with a static “U” core and a moving “I” bar. This compact solenoid measures only 35.6 x 34.95 mm in size, with mounting holes for easy integration within a touch screen or a touch button application. The actuator can drive a load of 0.5 kg up to an acceleration of 6 g using 12 V and 5 ms pulses, making it the perfect choice for applications exposed to constant background vibrations.
As mentioned before, the VCNL series include an ASIC with a feature-rich DSP section, allowing seamless operation when used with haptic actuators. The integrated DSP offers functions such as proximity cancellation, internal averaging, threshold-driven interrupts, selectable measurement rate, and more, providing developers with tools for easy implementation of haptic feedback and precise system calibration even on low-cost, low-end MCU processing platforms. More in-depth technical information about high-resolution detection, calibration, and design-in, can be found in Vishay’s Application Notes.
Although not the complete replacement for a nice GUI, these proximity sensors can be used in various automotive applications, including a wake-up command for the main display, seat tilt toggle switch, push-button on a dashboard, and similar applications. A force-sensing HMI can also be very useful for the broadest range of consumer and industrial applications outside the car: unlike a mechanical switch, optical force-sensing HMIs do not wear over time due to contactless construction, withstanding an unlimited number of switching cycles.
Combined with a haptic feedback actuator, the VCNL series can be used to develop HMIs that work reliably, consume less power, and last longer, providing satisfying tactile feedback during operation. As part of their comprehensive support for the VCNL series, Vishay also offers a stand-alone force-sensing demonstrator based on the VCNL4030X01 sensor. The demonstrator can be used as a proof of concept and a reference for custom designs. If you are interested in developing your own optical force sensor HMI with haptic feedback, do not hesitate to contact our experts or your nearest EBV sales representative and ask for support.
Force Sensing Demonstrator