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Touch Sensing for Industrial Automation

By European Editors

Contributed By Digi-Key's European Editors

One of the main driving forces for the Internet of Things (IoT) is the simplification of the network. Sensor nodes with wireless links can connect up millions of devices to provide much higher levels of monitoring and be controlled remotely, all without human intervention. Sometimes though, human input is still required. Monitoring can provide the data, but ‘boots on the ground’ are often needed to make adjustments and check on the operation of the equipment.

Equipment on the factory floor can be in a challenging environment with dirt and water posing a risk to the safety of operators, making a sealed interface necessary. This drives the need for a touch-enabled interface, which can be implemented in several different ways.

Implementation

There are many different ways to build a physical interface for the IoT environment that don’t use failure prone traditional keyboards or mechanical buttons. These range from a sealed touchscreen with an IP65 rating, to smaller screens designed to fit a specific piece of equipment, down to completely sealed reconfigurable capacitive buttons.

The simplest solution is a standalone touchscreen such as the CT104 from Crouzet. This 4.3 inch screen provides a resistive touchscreen with a resolution of 480 x 272 pixels and a brightness of 400 cd/m². The unit uses a 600 MHz 32-bit ARM® Cortex®-A8 processor with 128 Mbytes of RAM.  To make it easy to fit into an IoT implementation, there are pre-configured drivers for Modbus RTU, Modbus TCP/IP (Ethernet) and SLin/SLout industrial protocols. Its 2 and 4 wire COM connectors allow other equipment and sensors to easily be added to the system. 

Image of Crouzet CT104 complete resistive touchscreen

Figure 1: The CT104 is a complete resistive touchscreen for human machine interface (HMI) applications.

For the industrial environment, resistive touchscreens provide a balance of performance and reliability. The screens are coated with transparent lines of indium tin oxide in a grid that identifies the point of touch through a change in resistance. They can be used with gloved hands in hostile environments, and the greater the number of wires used in the grid (4, 5 or 8), the greater the resolution. The data capture speed determines the responsiveness, another key factor for a user interface.

The AD7873 from Analog Devices is a 12-bit successive approximation analog-to-digital converter (ADC) with a synchronous serial interface and low on resistance switches for driving 4-wire resistive touchscreens.

When operated from a 3 or 5 volt supply and provided with a 2 MHz clock,  it has a throughput rate of 125 ksample/s. It includes temperature measurement to assist in compensating for the change in resistance, a key factor in an industrial environment. It also has integrated touch pressure measurement, and has an on-board 2.5 V reference that can be used for the auxiliary input, battery monitor, and temperature measurement modes.

An external voltage reference can also be applied from 1 V to VCC, creating an analog input range from 0 V to VREF for measuring any one of the X, Y, and Z panel coordinates or the chip temperature. The multiplexer is configured with low resistance switches that allow an unselected ADC input channel to provide power and an accompanying pin to provide ground for an external device. However, for some measurements, the on resistance of the switches could present a source of error. Using a differential input to the converter and a differential reference architecture avoids this problem.

The part is available in a 16-lead, 0.15 inch quarter size outline package (QSOP), a 16-lead, thin shrink small outline package (TSSOP), and a 16-lead, lead frame chip scale package (LFCSP).

Diagram of Analog Devices AD7873 in a resistive touchscreen design

Figure 2: Using the AD7873 in a resistive touchscreen design.

Figure 2 shows a typical connection diagram for the AD7873 in a touchscreen control application. The value of the reference voltage sets the input range of the converter and the conversion result is output MSB first, followed by the remaining 11 bits and three trailing zeros, depending upon the number of clocks used per conversion.

Capacitive technologies also have a role to play in providing an effective user interface for the IoT. Although the full touchscreen doesn’t work well with gloves or dirt, capacitive technology can be used for buttons to avoid these problems. Coupled with a simple monochrome LCD such as the LK162A from Matrix Orbital, a reconfigurable button system can be easily implemented using the PSoC family of devices from Cypress Semiconductor.

A single-chip programmable controller such as the CY8C20336A is designed to replace multiple traditional microcontroller unit (MCU)-based components to reduce the cost and size of HMI designs.

Image of LK162A LCD module from Matrix Orbital

Figure 3: The LK162A LCD module from Matrix Orbital can be combined with a capacitive HMI controller.

A PSoC device includes configurable analog and digital blocks, and programmable interconnect that allows a designer to create customized peripheral configurations for each application. A CPU core, flash program memory, SRAM data memory and configurable I/O, provide the base processing for HMI applications.

This is coupled with CapSense technology that handles capacitive sensing and scanning without requiring external components. This can be used as the sensor under sealed buttons to provide the user interface in an industrial design. An LCD display can show the button function, which can be reconfigured according to the data it receives.

The architecture has three main areas as shown in Figure 4, consisting of the PSoC CORE, the CapSense System and the SYSTEM RESOURCES. A common, versatile bus allows connection between the I/O and the analog system.

Diagram of PSoC devices from Cypress Semiconductor

Figure 4: The PSoC devices from Cypress Semiconductor combine a CapSense controller with a CPU and a flexible analog multiplexer.

Each CY8C20336A device includes a dedicated CapSense block that provides sensing and scanning control circuitry for capacitive sensing applications. The analog system contains the capacitive sensing hardware with an internal 1 V or 1.2 V analog reference, which together with the PSoC core supports capacitive sensing of up to 28 inputs.

One of the algorithms supported by the chip is SmartSense, which removes the need for manual tuning of the CapSense applications. This establishes, monitors, and maintains all required tuning parameters. It also provides a robust noise immunity, allowing designers to go from prototyping to mass production without re-tuning for manufacturing variations in PCB and/or overlay material properties.

Alongside a 24 MHz M8C CPU core, the PSoC core combines SRAM for data storage, an interrupt controller, and sleep and watchdog timers. The CPU core is a 4 MIPS, 8-bit Harvard architecture microprocessor that handles the control algorithms and the conversion of the voltage readings on the pins to a capacitance value.

The third element of the design is a flexible analog multiplexer that connects to every GPIO pin. The pins are connected to the bus individually or in any combination.  The bus also connects to the analog system for analysis with the CapSense block comparator.

The multiplexer can also be used for more complex capacitive sensing interfaces such as sliders and touchpads, as well as allowing an analog input from any I/O pin. It can also be set up to provide a crosspoint connection between any I/O pin combinations. This avoids the device being locked into a particular pin configuration for more flexible board development.  

To provide feedback that a button has been pushed, the CY8C20336A includes a haptic controller with up to 14 different effects. These effects can be used with three different, selectable eccentric rotating motor (ERM) modules to provide the feedback.

Conclusion

There are many ways to build equipment interfaces for the Internet of Things. Integration allows multiple controllers to be combined for capacitive buttons, as well as the addition of haptic motors to provide feedback to the user. Higher performance ADCs provide more sensitivity and responsiveness for traditional resistive touchscreens. Designers can build their own interfaces, combine buttons and LCDs, and implement discrete touchscreens or integrated modules to display IoT data.

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European Editors

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Digi-Key's European Editors