A Smarter Way to Design Touchscreens

A Smarter Way to Design Touchscreens

Whether we’re on our smartphones, banking at an ATM, getting GPS directions in our cars, or playing video games, we’re interacting with touchscreens regularly in our everyday lives. Human/machine interfaces so far have been limited to “reading” the moves of our fingertips to take some corresponding action. In these cases, a capacitive touchscreen is sufficient. But what if the machines were able to sense the third dimension, or not only where we are touching but also how forcefully we are touching? This would not only provide a robustness to touch but other innovative ways to interact with the machines. To create this type of interaction, you need underlying technology that delivers force sensing, also referred to as 3D sensing.

By converting an input mechanical force into a change in resistance that can be measured by precision measurement devices, force sensing provides an expanded level of touch sensitivity. Mordor Intelligence estimates that the global force sensors market will grow to $2.47 billion by the end of 2020, up from $1.84 billion in 2014. Sensors have become increasingly important in automation and measurement applications. In addition, the growth of advanced electronic control systems along with better sensor accuracy, response time, reliability, and miniaturization are contributing to increasing demand for sensors in a variety of application areas, according to Mordor Intelligence.

ADC Delivers Touch Sensitivity

To incorporate force sensing in devices featuring human/machine interfaces, Maxim offers a reference design for an industrial smart force sensor. The MAXREFDES82# reference design essentially serves as a weigh scale and a touch interface with force sensing. It features four load cells on a clear plastic plate, measuring the force applied to each individual load cell as well as to the plate itself.

Once you turn on the smart force reference design (Figure 1), the sensor auto-calibrates and then is ready to measure force position and the force magnitude, explained Mulong Gao, senior member of the technical staff at Maxim. Gao developed this reference design, which won a 2016 ACE Award and was also named to EDN’s annual Hot 100 Products for 2016. Using the design, you can quickly create design prototypes and evaluate the reference design’s underlying IC, Maxim’s MAX11254.


Figure 1. Maxim’s smart force sensor, also known as MAXREFDES82#.   

The driving engine for this reference design that enables the force touch feature is Maxim’s MAX11254 analog-to-digital converter (ADC). The MAX11254 is a 24-bit, 6-channel, 64ksps delta-sigma ADC with an SPI interface. With its six channels, the IC delivers a high level of position accuracy as well as low-noise amplification, which allows its sensitivity to touch. The bridges on the reference design—which serve as the load cells on each corner—consume the most power on the board. One of the great features of this ADC is the integrated sequencer that allows you to activate and deactivate bridges before measuring them, which helps mitigate power consumption, explained Sohail Mirza, senior business manager, Core Product Group, at Maxim. The MAX11254 is available in a TQFN package for industrial applications and for small, portable designs, a WLP package. Figure 2 shows a block diagram of the smart force sensor reference design.

Figure 2. Smart force sensor reference design block diagram.

A Faster Way to Prototype a Variety of Industrial and Consumer Applications

Having a reference design like this can save time, effort, and cost that would otherwise go toward building a design prototype. With MAXREFDES82#, you can quickly evaluate how the underlying IC functions and performs. Verified and tested under typical application cases, the reference design can be customized for specific applications development. Weigh scale, industrial control, automation, smartphone/tablet, and even gaming controller applications are some examples.

Smart force sensor technology also provides more durability than capacitive sensing technology. Capacitive sensing technology involves a thin layer of glass embedded with capacitive sensing lines. Obviously, glass is prone to breakage and, in addition, such surfaces do not respond well when the user is wearing gloves, for example. Smart force sensor technology, on the other hand, can be used in rugged industrial environments and responds to users’ gloved fingers.  

Watch this MAXREFDES82# video to see the smart force sensor in action.