Getting a Grip on Force-Sensing Technology

An analysis of the Quantum Tunneling Composite materials touch development kit from Peratech.

Developments in human machine interfaces (HMIs) have come a long way in the past ten years. The distant memory of apprehensively trading in a classic Blackberry keypad for a capacitive touch smartphone is the only recall of this revolutionary user interface. Several major iterations of the user experience followed suit. We were wowed by using standard gestures to manipulate pictures. The ubiquity and simplicity of the touch screen user interface has changed society: toddlers now try to pinch and swipe images in a magazine and wonder why nothing happens. Our kids refer to malfunctioning devices as glitchy, and not broken — as if a software update might magically replace a lightbulb.

As we have ingrained touch interfaces into our daily lives, we’ve placed increasing demands on them to advance how we interact with our analog world. In an attempt to incorporate more functional control with multi-use mechanical switches, we ended up making the user interface rather clunky. Also, in our drive to make user experiences ever more creative and intuitive, we ran into a problem with touch: it’s flat, and it doesn’t actually measure physical contact. So began our disillusionment with capacitive touch solutions to be the smart interface between us and a comfortingly-analog world.

We went back to keyboards and tried to package physical interactions and haptics into interfaces to give people back this tactile sensory experience they were missing. We struggled. The technologies at our disposal were at their respective limits of capability. We made glorious attempts at next generation interfaces — many of which failed because we could not overcome the unnatural feel of capacitive sensing. We needed to find a way to overcome this flatland of capacitive touch while preserving its simple elegance. We also needed a way to build prototypes with new technologies that weren't going to take one too many product lifecycles and a bank full of cash. 

Quantum Tunneling Composite (QTC®)

The Quantum Tunneling Composite (QTC®) material developed by Peratech is able to measure the degree of force applied, in addition to traditional 2D X, Y position sensing. Most notable is the fact that such a sensor can be created using screen-printable ink, derived from semiconducting nanoparticles mixed into a polymer binder.

QTC is an insulator until you press on it — then the more pressure you apply, the more conductive it becomes, making it ideal for force sensing. Unlike the involved techniques used to fabricate touch panels and trackpads today, creating a QTC-based sensor is a relatively straightforward process. Tests show that the sensors work even in extreme temperatures and humid environments.

QTC-based sensors can be created behind many different surfaces — whether flat, curved or flexible — such as metal, plastic, thin glass and even wood veneers. This highlights another benefit of a QTC sensor: the sensing surface is behind the external surface of the device, so products don’t require an opening that could result in water and dust ingress. Compared to traditional touch-screen stack-ups, use of a QTC sensor can reduce the necessary layers of ITO and improves clarity and light transmission from the display. 

 

Figure 1. Peratech SP200-10 single-point sensor.

Peratech provides a number of standard single- and multi-touch sensors for use in a variety of different applications. An example of a single-touch sensor is the SP200 — see Figure 1. This has an operating force ranging from 0.1 to 20 N and a lifetime durability in excess of 1 million cycles and can operate over a wide temperature range from –40 to +100 °C. 

 

Figure 2. Construction of an SP200-10 sensor.

Figure 2 illustrates the construction of the sensor, with an exploded component view. In terms of electrical characteristics, the sensor is modeled as a variable resistor, and Figure 3 shows a graph of how the QTC single-point sensor’s resistance changes with applied pressure. 

 

Figure 3. Resistance vs applied pressure curve of SP200 single-point sensor.

Interfacing one of these sensors to an embedded microcontroller design is very straightforward, using the sensor as part of a simple voltage divider circuit feeding into an analog to digital port of an MCU — see Figure 4. Note that when no force is applied to the sensor the voltage divider is effectively an open circuit, keeping power consumption to a minimum, and contributing to achieving an ultra-low power design. 

 

Figure 4. Interfacing a single-point QTC sensor to an MCU.

An embedded application, typically running on an MCU, will read the ADC (analog to digital converter), calculate the applied force from the resistance/force characteristic model and pass this information to the HMI application.

 

Figure 5. QTC 3D matrix sensor.

A QTC matrix sensor is also available that can detect and report multiple 3D touches. Constructed as a series of row and column electrodes, each intersection or ‘junction’ of the matrix has an area of QTC ink deposited on one layer of the electrode matrix. An example of a 3D matrix sensor is shown in Figure 5.

 

Figure 6. QTC 3D matrix sensor.

Figure 6 illustrates how the matrix can be scanned sequentially by the host MCU, first taking each line high and the rest low, and reading the value at the ADC. The matrix rows provide the digital drive and the columns the analog sense. Repeating this process until the matrix has been completely scanned will give a full frame of matrix data, which can then be processed by the HMI application. The size of the matrix and the degree of resolution required for the end application will determine the MCU’s I/O and processing requirements. 

 

Figure 7. Contents of Peratech’s Touch Development Kit. 

For developers wishing to learn more about QTC force-sensing technologies, an evaluation kit is available. The Touch Development Kit (TDK) — see Figure 7 — comprises 5 mm and 10 mm single-point sensors, and a QTC opaque multi-force matrix sensor. They can all connect to a host MCU controller (based on an STMicro MCU), termed the Touch Processing Unit.

Sensor data can be visualized in 2 and 3 dimensions by Peratech’s PTSuite software running on a Microsoft Windows computer. The PTSuite software is available for download from the Peratech website using the username and password provided in the TDK. 

 

Figure 8. Touch Processing Unit.

A sample image from PTSuite can be seen in Figure 9. In this example, an adult right hand is pressed, with moderate pressure, on the matrix sensor, with the colors indicating the amount of pressure. The pressure pattern extends around immediate junctions. Thumb and finger pressure and contact pressure area are roughly equal for all. 

 

Figure 9. QTC matrix sensing moderate pressure from all five fingers.

Figure 10 shows a heavy press of the thumb only, while Figure 11 shows the result of pressing down heavily using just the nail of the index finger. 

 

Figure 10. QTC matrix sensing board pattern from thumb pressing heavily downwards.

 

Figure 11. QTC matrix sensing a single-contact point of the fingernail of the index finger.

Conclusion

This article has covered a small number of the capabilities offered by QTC force-sensing technology. The range of potential applications is as broad as the markets they might cover. For example, a custom-printed QTC sensor could be printed into the handle of a variable-speed electric drill/screwdriver. There would no longer be a need for a spring loaded linear potentiometer, with all the associated reliability issues and the potential for dust or moisture ingress.

As hand pressure around the handle increased, so would the drill speed. Looking back at the smartphone example at the start of this article, perhaps pressure sensors printed on the inside edge of the case could be used in place of the +/- volume control, again removing two switch sensors that could potentially become an ingress point for dust and moisture.

For more information, visit www.peratech.com.

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