More Than a Touch Less Costly and Complicated



How sensing MCUs are simplifying capacitive touch while taming expenses

What is the first thing that comes to mind when you hear the phrase “capacitive touch”? I’d be willing to bet that the first thing that popped into your head was a touch screen. You may have thought about your smartphone, that tablet computer you wish you had, or a fancy infotainment system in a new car. You’d be right on all counts, but what you would be missing are the many products you use every day that also implement capacitive touch—just in a simplified way.

Figure 1: A thermostat with basic capacitive touch sensing

Today’s marketplace puts an emphasis on aesthetic quality, so the same technology used to implement a capacitive touch screen in a smartphone is trickling down to products such as new home appliances, industrial control panels, and electronic locks. Long gone are the days of beige computers and corded telephones. Capacitive touch frees designers and marketing professionals to rethink a product’s user interface, shape, and mechanical construction while improving both aesthetics and functionality.

Figure 1 shows an example of a thermostat with a simple capacitive touch interface. Because there’s no need for mechanical buttons, the front of the thermostat can be completely seamless, with no moving parts or cutouts for push-buttons. Taking out the moving parts and using a one-piece enclosure can improve long-term reliability and electrostatic discharge (ESD) tolerance, respectively.

HMI Flexibility
Capacitive touch sensing is of particular interest for industrial human-machine interfaces (HMIs) because of the flexibility it offers. Industrial HMI manufacturers often have several versions of a product; each version has a different feature set and price point. For example, one product in a family might require just six buttons, while another product in that same family requires 12 buttons. When using traditional mechanical switchgear, a unique mechanical enclosure with the right cutouts and mounting points for the mechanical switches is often necessary for each version of the product.

With capacitive touch sensing, it is possible to have a single printed circuit board (PCB) design with all of the required sensors to support the superset requirements. Multiple versions of a product that may only support a subset of the HMI can use the same PCB and software by changing the overlay graphics that identify the button locations.

Mix to Match Industrial Control Needs
Sometimes, the right answer for an industrial control panel is to use a combination of interface technologies. It’s possible to intermix mechanical and capacitive touch controls. Designers can specify mechanical switches where clear tactile feedback is desired (for example, using twist-lock switches for controlling safety-critical functions such as an e-stop), while using capacitive touch buttons to cost effectively add many multipurpose soft keys for configuration and setup. Figure 2 shows an industrial control panel that mixes interface technologies based on the controlled function.

Figure 2: An industrial control panel with mixed switch technologies

Capacitive Touch Sensors 101
So, what is a capacitive touch sensor anyway? A capacitive touch sensor consists of a conductive structure or set of structures from which an electric field is projected out through an insulating dielectric overlay material into the free space just above the overlay. When users touch the overlay material above the sensing element, they influence the capacitance of that element by a very small amount. These tiny changes in electrode capacitance are then measured through some type of acquisition method (referred to as capacitance-to-digital conversion) and post-processed to determine the state of the sensor. Capacitance-to-digital conversion involves translating the sensor’s capacitance into a measurable quantity—such as a time, current, or voltage—that varies proportionally with the capacitance.

Figure 3 shows the typical mechanical stackup of a capacitive touch sensor. The overlay dielectric material is almost always the external housing of the product itself, and it serves several purposes. First and foremost, the overlay must isolate users from the conductive sensing element. This isolation provides electrical safety and is the primary method of protecting the sensing integrated circuit (IC) from potentially damaging ESD events. More often than not, the overlay will also include the mounting points for the sensor. It is common to implement the sensing elements as copper structures on a PCB and bond that PCB directly to the overlay material with an adhesive. This rigid bonding method eliminates air gaps in the stackup between the sensors and the point of touch, which improves the reliability of the user interface by ensuring an optimal and consistent dielectric constant and dielectric thickness between users and the sensor.

Figure 3: A thermostat with basic capacitive touch sensing

Touch Measurement Topologies
There are two capacitive touch measurement topologies: self-capacitance and mutual capacitance. Self-capacitance measurement involves looking for an increase in the self-capacitance of a single conductive sensing electrode. When users approach a sensor, they add a small shunt capacitance to the existing electrode capacitance. In this topology, a touch will add approximately 0.1pF to 1pF of capacitance to the sensing electrode. Since the untouched self-capacitance of a typical button is in the range of 15pF to 30pF, a touch increases the electrode’s capacitance anywhere from a few percent to less than 1%. Self-capacitance is an ideal topology for basic applications with a few buttons.

Mutual capacitance, on the other hand, involves looking for a decrease in the mutual capacitance between two different conductive sensing electrodes, which are commonly called a transmitter and a receiver. This topology enables a higher key density with a given number of pins, since receivers and transmitters can be shared in a matrix fashion. When users approach a transmitter/receiver pair, they reduce the capacitive coupling between the two electrodes. The sensor geometry defines the base mutual capacitance of a button. It is typically quite small (on the order of several picofarads). However, users touching the overlay above the sensor can have a significant impact on this coupling, changing it by several percent.

In both the self and mutual capacitance cases, the change in capacitance that is being measured can be quite small—on the order of 10-13 farads. If the capacitance being measured is charged to 1V, that’s just 10-13 coulombs—or a few hundred thousand electrons!  Measuring such small changes in a capacitance is clearly more complex than simply checking the state of a digital input pin that is controlled by a mechanical switch. As a result, swapping out a mechanical switch or push-button for a capacitive touch button has not always been a trouble-free process. There are increased upfront software development and qualification costs, as well as recurring IC costs.

Cost Barriers Addressed
To address these limitations and increase the adoption of capacitive touch buttons, TI launched its CapTIvate™ touch technology and development ecosystem in 2015. The CapTIvate Design Center is a drag-and-drop sensor creation environment with built-in visualization tools, tuning tools, data logging, documentation, and source-code generation. Figure 4 shows the CapTIvate Design Center views. CapTIvate technology lowers software development costs and reduces time to market for designers.

Figure 4: CapTIvate Design Center sensor canvas and data-visualization views (click to enlarge)

The ability to get started without having to write a line of code or become an expert in capacitive sensing has removed the upfront development cost barrier. However, adding a touch sensing microcontroller (MCU) has still been cost-prohibitive for a simple product with just a few buttons. If you’re designing a simple product with only one or two buttons, you may have had a hard time justifying the added expense of touch sensing.

Figure 5: MSP430 CapTIvate Touch Keypad BoosterPack for the MSP430FR2522 MCU

Now, TI is removing the IC cost barrier by expanding the CapTIvate touch sensing MCU portfolio to include MSP430FR2512 and MSP430FR2522 touch sensing MCUs. The MSP430FR2512 MCU can support as many as four capacitive touch buttons. It’s a good fit for products with simple user interfaces such as elevator call buttons, computer monitors, and TVs.

If you need more than four buttons for something like a wireless speaker or electronic access control product, the MSP430FR2522 MCU supports up to eight self-capacitance buttons or 16 mutual capacitance buttons. The MSP430™ CapTIvate Touch Keypad BoosterPack, pictured in Figure 5, makes development with the MSP430FR2522 a snap.

The new MSP430FR2512 and MSP430FR2522 MCUs have the same differentiating capacitive sensing performance as the rest of the CapTIvate portfolio, just in a smaller package and at an appealing price.

 


Walter Schnoor  is a system applications engineer at Texas Instruments, Inc. He received a bachelor’s degree in electrical and computer engineering from Calvin College. He has more than five years of experience in MCU-based capacitive touch sensing technology, IC development flow, and technical applications support. Schnoor has a passion for developing industry-leading technology that solves customer problems and gets customers to production faster with less effort. At Texas Instruments, he is responsible for new capacitive touch technology development and integration. He also contributes to ecosystem development, including reference designs, software, hardware, and documentation.

 

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