New HMI... When Do I Start the Backlighting Design?
Backlighting design can sometimes be like fitting a square peg in a round hole. If the backlighting is an afterthought of the HMI, meeting the design criteria can be difficult and costly. But it can be an easy process and often keep the solution cost low while still meeting or exceeding the design criteria.
Designers, engineers, and project managers sometimes put off the backlighting design thinking it will be easy to do later, or just not knowing how and when to start. When this happens, we can see some drawbacks ranging from lower capacitive touch responsiveness, to overall higher solution cost… sometimes costing hundreds of thousands of dollars in extra tooling and extra dollars per part.
Typical results of delayed backlighting design are:
- LEDs placed in wrong position
- Lower capacitive touch responsiveness
- Less lighting functionality
- Lower backlighting uniformity or brightness
- Higher total solution cost
Industrial designers and OEMs are responsible for the design criteria of the HMI. This includes both the look and the functionality. Once this design criteria are understood, the backlighting design should begin. The backlighting design is crucial to the visual aspects of the HMI and the user experience. The structure of the backlighting and placement of the LEDs can make a significant impact on the functionality and interactions, through capacitive touch, or tactile buttons.
Performance and Cost Drivers
With the understanding of which buttons, texts or icons are interactive, and which ones are isolated for individual illumination; the backlighting design can customize the layout to accommodate the look and functionality required. By placing LEDs away from capacitive touch buttons and steering the light into the right zone, the HMI will maintain a clean capacitive touch response and clean isolation from one illuminated zone to another.
Often small modifications to the backlighting layout and design can make significant performance and cost impact. This is magnified as the product goes to production. Moving an LED position or an alignment feature are usually not significant cost drivers to the PCB and facia. However, these features can be costly on the light panel if placed incorrectly. A boss or alignment pin in an optical path will result in a compromise to the lighting performance, or require additional LEDs to compensate.
Manufacturing costs and throughput are driven by the assembly aspects of the HMI. This can be greatly affected by adhesive layers, heat stake positions, and ease of alignment during assembly. With inclusion in the design process the light guide can be designed for lowest cost assembly, and help keep the overall solution costs low.
The backlighting design should begin as soon as the HMI design criteria are defined. Starting the design at this time can provide the lowest solution cost with the best performance. By placing LEDs and mechanical features in the best position, the backlighting will yield the best performance without compromise or additional cost. Designing the adhesive layers or alignment features will yield fast and easy assembly, again translating to lower solution cost with best performance.
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