Voice, Touch, and More: Design Considerations for HMI

Partnering with the right design team and EMS provider can help ensure HMI design success and high yield production.

24 August 2021

Many of today’s modern electronic products have an advanced human machine interface (HMI) that creates a meaningful, engaging user experience. The HMI in many products can be designed using hardware built into the device, using a smartphone or web application, or a combination of these. Hardware-based HMI elements have gone far beyond simple elements like switches and knobs; today’s electronics contain more advanced elements like touchscreen displays, sensors, voice recognition, facial recognition, and even gesture recognition. These elements come together to create an engaging user experience while also providing desired functionality.

Integrating all these elements into a new system while ensuring compact form factor, heat dissipation, and reasonable assembly costs is challenging from a mechanical design perspective. There are also challenges related to EMI and EMC to consider when designing an HMI system. Finally, DFM/DFA are important aspects of HMI design as the system needs to be manufacturable at scale without overly complex assembly and costs. Here are some of the major HMI design challenges design teams need to overcome to create a scalable product.

HMI Design Challenges

Mixed-Signal Design with Voice and Touch Sensors

The main PCBA that supports an HMI system must support digital interfaces from the main system controller (MCU, FPGA, etc.) and analog signals from voice and touch sensors in the HMI design. This is an area of electronics design that is sometimes misunderstood, leading to bad grounding and routing practices that lead to EMI. As a result, designers often find their product will not comply with EMC regulations in their target markets, as well as industry standards (e.g., IEC 61000-4-6). Some designers may even end up resorting to measures like board-level shielding or additional enclosure shielding to suppress EMI.

While this may be fine for a functional prototype, it can be unscalable in production as these shielding elements add weight, materials costs, and assembly costs. The form factor may also need to change to accommodate these additional shielding measures. Although mixed-signal design in an HMI system can be challenging, an experienced design team can propose creative solutions that follow best electrical practices without compromising on form factor and costs. Innovative EMS companies like PCI will use the latest processor and peripherals for each product while considering the Product life cycle of the product at the design stage. This will prevent lost time during the manufacturing stage, helping customers get to market quickly.

Noise-Immune Voice Recognition

Intuitive HMI systems with voice recognition capabilities need the ability to differentiate human speech from background noise in different environments. The first challenge in voice recognition design is in selection of a microphone for sensing position and distance, as well as its placement in the overall assembly. The second challenge is in choosing the appropriate algorithm for differentiating speech from background noise, as well as the host controller to run these DSP algorithms efficiently. In addition, these elements need to be free of electrical noise when integrated into the PCB used to control the HMI.

Designing With Touchscreens

Touchscreens can be challenging to work with from an EMI perspective and from a board design perspective. HMI design engineers have had to become more strategic about combating EMI in HMI design as touchscreens have become much more common in everyday products. Touchscreens in HMI designs are primarily capacitive, providing multiple low impedance routes for capacitive coupling from other components in the system. Even when best mixed-signal design practices are followed in an HMI system, capacitively coupled conducted EMI is known to cause touchscreens to malfunction. However, with correct component selection, a system grounding strategy, proper flex cable design, and filtering strategies, it is possible to greatly reduce EMI in touchscreens and other analog sensors in an HMI system.

Touchscreens are often the largest component in an HMI assembly, and they can dominate the form factor of the system. If a flat form factor is needed, such as in a tablet, component selection can become a major challenge, especially considering limitations on component stocks in today’s volatile supply chains. Your manufacturer can assist with a procurement and design strategy that ensures you can source critical components to support touchscreen displays without sacrificing form factor.

Mechanical, Thermal, and ESD Challenges

Three important aspects of reliability in HMI systems are thermal reliability, mechanical reliability, and ESD immunity, and some industries carry strict performance standards in these areas. For example, HMI systems in industrial, automotive, and specialty areas like fitness equipment must comply with a range of performance criteria with regards to vibration, electrically fast transients, mechanical shock, thermal shock, and thermal cycling. HMI design teams need to consider these reliability requirements in all aspects of the design, ranging from component selection to rugged PCB design and material selection for enclosures. An ideal EMS partner can determine the important performance criteria for your product and perform testing required to prove compliance.

Ensure Successful HMI Manufacturing with a Proven EMS Provider

Balancing aspects like form factor, user experience, and signal integrity in a new product can be challenging, especially when advanced capabilities like voice and touch are needed as part of HMI design. While commercial off-the-shelf HMI components can be woven together to create a workable user interface, companies that want to be truly competitive should partner with an advanced EMS provider that can supply all elements of a new product. This includes the PCBA, displays, integration of advanced sensors for touch and voice, and an enclosure with a sleek form factor. In addition to HMI design, electromagnetic radiation from the main PCBA can interfere with sensors, screens, or other elements in an HMI system. An EMS provider that also specializes in EMI testing and EMC can diagnose compliance issues and propose creative solutions.

Partner with PCI

OEMs that want to ensure high quality and yield should partner with an experienced EMS provider that excels in HMI design and manufacturing. At PCI, we have more than 30 years of EMS experience focused in industrial, commercial, consumer, and telematics market segments. Our Lean Six Sigma manufacturing expertise enables us to customize our manufacturing line to meet our partners’ requirements.

If needed, we provide our customers the flexibility needed to quickly scale production as needs arise. We provide our partners with high-quality products at lower manufacturing costs thanks to our shorter change-over time and leaner material control. Contact PCI today to learn more about our capabilities.