Going Beyond User Interface Panels with Capacitive Touch: How to Differentiate Industrial Systems Without Increasing Cost

The industrial market started adopting capacitive touch-sensing technology to leverage sleeker design and greater reliability against wear and tear compared to conventional mechanical buttons. In addition, capacitive touch technology offers many features beyond mechanical button replacement (MBR), including liquid level sensing, keyless door access control, object detection, tactile feedback, gesture support in the user interface, and proximity applications. These features not only allow original equipment manufacturers to differentiate product designs, they can also reduce system design cost by integrating multiple features using system-on-chip architectures.

However, industrial systems must operate reliably in stringent work environments that include factors like dust, moisture, extreme temperatures, and high speeds, to name a few. In this two-part article, we will explore how various industrial applications can use capacitive touch-sensing technology. In addition, we cover design challenges associated with industrial system design and techniques to overcome those challenges.

Capacitive Liquid Level Sensing in Industrial Applications

Liquid level sensing is an important parameter measured in various industrial applications. Applications include measuring the level of a print cartridge, lift stations in wastewater treatment systems, liquid levels in a holding tank, and so on. There are various types of liquid level sensors, including both mechanical and electronic sensors. Today’s applications demand both resources and energy-efficient systems, which is why many designers prefer electronic and electromechanical liquid level sensors compared to mechanical-based sensors.

Among the different types of electronic liquid level sensors are resistive, capacitive, optical, and ultrasonic. Electronic sensors provide additional layers of functionality and integration like programmable analog, digital, and mixed signal capabilities. There are a great many embedded microcontrollers on the market supporting these types of electronic liquid level sensing with complete embedded resources to build a robust system design.

Figure 1: Liquid Level Sensing (Mechanical Vs Capacitive)

Capacitive liquid level sensing is a popular electronic level sensing method. It offers various advantages compared to conventional mechanical sensors including but not limited to:

No wear and tear due to moving mechanical parts
Reduced form factor with sleek, compact design
Reduced bill of materials costs by eliminating magnetic float and multiple mechanical components
High reliability and low power
Optimized resolution and accuracy to support varying price points with a single base system
Additional analog and digital resources on the same chip
Flexibility of communication interface – I2C, SPI, LIN, CAN, UART, BLE, Wi-Fi
Large portfolio of devices with wide range of sensors options

Capacitive sensors determine the liquid level by measuring changes in probe capacitance resulting from the movement of dielectric materials between the probe and the reference ground electrode such as a tank wall.

Figure 2: Capacitance and Electric Field of a Capacitive Liquid Level Sensor

Figure 3: Added Capacitance (CL) When Liquid Approaches a Capacitive Sensor

Though their sensor construction and philosophy of operation is the same, capacitive liquid level sensing differs from traditional button-touch applications. The liquid level sensor material can be a simple conductive pad (e.g. copper) embedded on a nonconductive material (e.g. PCB). The cross section view of the liquid level sensor construction is shown in Figure 2. The electromagnetic field is coupled with a conductive object like a finger or liquid, in this case water. This will result in adding a small amount of capacitance to the base system capacitance (C_P), shown as liquid capacitance C_L in Figure 3. This change in capacitance is measured by the system and reported as the presence of water to determine the liquid level.

Figure 4: Capacitive Liquid Level Sensing System

One advantage of capacitive sensing systems for liquid sensing in that there are no moving parts like floats or mechanical contacts (see Figure 4). This eliminates wear and tear, extending product life, as well as allowing for a more compact sensor design. Many MCUs integrate capacitive sensing that supports multiple sensors with a single chip. This allows the resolution of level sensing to be greatly increased without adversely impacting system cost. The sensor construction shown in Figure 4 enables liquid level sensing across the full depth of the tank. However, if the application only requires sensing when the tank is empty or full, only two sensors are needed: one at the top and one at the bottom. Since MCUs have processing capabilities, they have the intelligence and memory to accommodate firmware algorithms. Thus, the capacitive system can directly provide the volume output (amount of liquid) rather just levels. Also, designers can optionally add external non-volatile memory for data integrity during power outages.

The capacitive sensing system of MCUs enables integration with other sensors through additional peripherals, such as programmable analog and digital, on the same chip. MCUs also typically offer a flexible set of communication interfaces, such as BLE, Wi-Fi, SPI, and/or I2C, to talk to the host processor. An example of embedded controller capacitive liquid level sensing system with integration of many other peripherals is shown in Figure 5. For more specific details, Understanding Capacitive Liquid Level Sensing shows how to use capacitive sensors to measure the depth or presence of water-based liquids in nonconductive containers.

Figure 5: Liquid Level Sensing System Integration Example

Tactile Feedback and Indicators

While having a sleeker design that is more robust is important, capacitive sensing also enables industrial designers to create a better touch and feel to make systems more accessible and appealing to users. In particular, for rugged industrial operating conditions, audio, visual, and physical feedback are essential. Let us explore how capacitive touch systems can be enhanced for industrial applications through feedback.

Haptics is the term used to describe dynamic tactile or touch feedback created by vibrations that provide immediate confirmation of touch. Haptics is used with a user interface panel to create a more intuitive and engaging experience for end users. An additional benefit is increased accuracy in using the device due to the immediate feedback received. A good example is a GPS device in which the user needs feedback on button presses but cannot take his or her eyes off what the system is doing. Haptics enables a unique bi-directional communication channel that enable user interface paradigms that are either impossible or extremely difficult to produce without it.

Figure 6: Haptic Feedback System

Haptics can be integrated with any type of capacitive touch interface, be it a simple button or a sophisticated capacitive keypad or slider. There are three basic methods through which haptics can be implemented: piezoelectric actuators, linear resonant actuators (LRA), and eccentric rotating mass (ERM) actuators. Piezoelectric actuators generate vibrations upon the application of a differential voltage across both ends of it. The vibration is generated through bending or deformation for the sensor. The ability of piezoelectric actuators to produce effects with independent amplitude and frequency is their primary advantage over LRA and ERM actuators.

ERM actuators are an eccentric rotating mass, in which a small motor is driven with a voltage to spin, thus creating vibration. ERMs work perfectly for vibration alerts. However, trying to use an ERM for more common haptic applications can quickly run down a battery. LRA actuators, which are of a different mechanical construction than ERMs, consist of a spring-mounted mass that vibrates in a linear motion. The LRA must be driven at a narrow resonant frequency. It also tends to have a slightly better start-up time than the ERM.

For more information and references on tactile feedback, see Getting Started with Capacitive Sensing.

Figure 7: Haptic Ecosystem

Audio and Visual Feedback — Auto-on Display/LEDs to Indicate User Presence

In an industrial environment, users may be working in noisy and dark conditions. It is essential for them to be able to identify the location of the UI panel or acknowledgement of a touch. Capacitive touch sensing and an associated LED lighting system can address this need. As an example, a high-bright LED can be integrated under the capacitive button itself and illuminate the button whenever there is a touch. Another feedback feature can be implemented with capacitive proximity sensing. This switches on the display as soon as a user approaches the system. For example, when someone uses a machine in a dark environment, the machine can light up the screen and panel when the user moves his or her hand towards the screen. Similarly, an audio buzzer can also be easily integrated to capacitive touch button.

Smart Keyless Lock Door Entry

Certain applications require secure access to critical areas like server rooms, laboratories, and bank lockers without adding complexity to the process for the end user. Smart locks are a viable solution in which complex system design is abstracted through a simple user interface. Fundamentally, smart locks provide keyless access to a door using keypads, device, RFID, or biometric inputs. Modern MCUs with capacitive touch sensing and built-in Bluetooth Low Energy (BLE) make it possible to design such systems using a single-chip design. Capacitive keypads are used to enter the password to open the lock or BLE is used to transmit the passcode from a device like a smartphone or wearable to open the lock. Additionally, biometric data such as fingerprints can be used to access the lock.

With all these methods, smart locks essentially eliminate the need for a physical key to open a door. Enabling keyless door entry eliminates security threats like keys being stolen or duplicated. However, electronic versions of locks have their own security concerns if they are not properly designed. We will discuss security design considerations later in this section. On top of keyless door entry, smart locks can provide access control based on a password. They can also log who has accessed the door by storing the passwords used in memory. This enables users to generate reports as needed, such as the number of entries by person over a period of time, number of people entering per day, and so on.

Figure 8: Example of Capacitive door locks

Capacitive Touch Panel Smart Locks

As shown in Figure 8, capacitive touch pads provide a keypad to enter the password to open the door. This entire system can be implemented using a single MCU where the device handles the capacitive input sensing, opening/closing of the door lock through a motor, and monitoring/reporting activities. Using capacitive technology brings elegant aesthetics with a small form factor and no moving parts. The capacitive-based locks provide a streamlined and intuitive user experience like illuminating the panel when a user approaches it using proximity sensing. There are MCUs available in the market (e.g. CY8CMBR3108-LQXIT) for less than 50 cents that implement all these features at a low system cost (see Figure 8).

Figure 9: Capacitive Door Locks Block Diagram

Many MCUs provide different features along with capacitive touch sensing such as BLE and fingerprint recognition (e.g. PSoC4-BLE) for use in implementing various types of door locks with a single MCU (see Figure 10).

Figure 10: Example of MCU integrated with capacitive touch sense (CapSense) and other features

Smart Gadget-Based Digital Door Locks

Use of IoT technology enables building smart door locks using gadgets like smartphones and wearables. These devices can unlock a door whenever an authorized user’s smartphone approaches. This also enables users to remotely lock and unlock a door or to share access with others using various apps.

Figure 11: Smart Gadget based Digital Door Lock

Door Access Via Fingerprint

Industrial designs often demand strong authentication to maintain security. Hence, the traditional approach of using complex passwords alone may not be a sufficient solution from a high security standpoint. Complex passwords are cumbersome to enter and do not effectively increase security. Fingerprint-based biometric sensors can implement strong authentication for such high security needs. Fingerprint-based systems are more convenient, secure, and flexible for door locks than passwords. There are many MCU vendors offering complex software stacks helping system designers to integrate finger print solution faster and easier, like Cypress’s TrueTouch Fingerprint Reader.

Figure 12: Door access using Fingerprints

Design Considerations for Added Security

In the case of capacitive keypads, it is essential to design the mechanicals of the keypad in such a way that a user-entered password cannot be easily seen by others or a closed circuit television (CCTV) camera. For gadget-based designs relying upon BLE, a simple BLE sniffer may steal the password if the security features of BLE are not built into design. The latest updates to the BLE protocol offer several security and privacy capabilities to enable encryption, trust, data integrity, and privacy of user data. For example, the BLE link layer provides various encryption algorithms like CRC, AES, and other standards for robust and secure data exchange over BLE. There are also BLE modules that can provide additional security and privacy features to make BLE technology an even stronger wireless solution. Alternatively, adding biometry sensing such as fingerprint readers brings even greater security to the system. Fingerprint readers are convenient and offer a best-in-class secure method to access the door lock. They are the preferred choice for highly secure systems.

In part 1 of this article, we have discussed various industrial system applications using capacitive touch technology. These included capacitive liquid level sensing, tactile feedback and indicators, smart keyless lock door entry, and design considerations for added security. We have also covered how to lower the cost of these designs. In part 2, we will cover more industrial applications using capacitive touch technology like special gestures, proximity X-Y gestures over an area, detection of capacitive objects, and automation, and industrial design challenges in harsh operating environments.

References

About the Authors

Anbarasu Samiappan is a Senior Applications Manager at Cypress Semiconductor. He is managing PSoC Embedded Systems Group including Customer Technical Support and System validation functions. He is a PMI certified Project Management Professional, Gold Medalist Electronics Engineering Graduate from Anna University, and earned General Management credential from IIM, Bangalore. He has more than 19 years of industry experience. Anba can be reached at [email protected].

Jaya Kathuria works as an Applications Manager at Cypress Semiconductor Corporation where she is managing the Embedded Applications Group and Solutions Development using the PSoC platform. She has more than 11 years of experience in Semiconductor Industry. She earned executive management credential from IIM, Bangalore and holds BS in Electronics Engineering from the Kurukshetra University. Jaya can be reached at [email protected].