Take an exact-sized modular design approach for medical device enclosures

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Even low volumes can benefit from modular instrument cases for medical devices.

The initial design is essentially done. It could be a device for fluid flow/control, a diagnostic instrument, or other medical device using an aluminum electronic enclosure. Now the design engineer needs to house the boards and electronics into an enclosure, and it needs to get done quickly and cost-effectively. There are four main areas that the enclosure needs to balance: be the right size and fit, fast assembly time, allow quick time to market for the design, and help keep costs low. A modular enclosure design can help achieve these goals.

Modular design
Space is a premium in medical electronics, and the drive to make devices smaller is ever-present. For various types of instrument cases, it can be difficult to find the appropriate size and configuration for an application. Often, the engineer will have to incorporate a fully customized design or settle on a size that’s not quite ideal. This can be expensive and time-consuming, and can result in an inferior design solution. An “exact-sized” modular design approach for enclosures can help solve these problems.

An exact-sized modular design starts with the aluminum extrusion, which are cut to any specified length. A highly functional extrusion can be designed to accept tapped strips, sealing cords for electro-magnetic compatibility (EMC), grounding terminals, optional direct mounting of a panel or pc board, or screws. See Figure 1 for an example.

1. A versatile extrusion can be used to allow the side panels, pc board, screws, grounding terminals, etc., latch together in an easy fashion. This can reduce assembly time and the number of components.

This flexibility saves a few manufacturing processes, keeping costs low. These cost advantages far exceed the extra cost of an advanced extrusion. The benefit to the designer is an exact-sized case that is customized with a low cost (even in small volumes). Without a modular design, this kind of customization would have only been cost-effective in high volumes.

The extrusions do more than just allow a modular enclosure frame to be assembled. An extrusion solution can be designed symmetrically to ensure that parts can’t be mounted incorrectly. The sidewalls, mounting panels, or even pc board can be mounted directly into the extrusions, greatly reducing labor. Even nuts/bolts, grounding terminals, etc., can fit right into a carefully crafted extrusion design.

Assembly time
Assembly time is a hidden cost that can add up and a designer should incorporate the amount of assembly required when considering an enclosure design. With an exact-size modular enclosure, the extrusion shape can allow the extrusion and panels to be directly mated together. This prevents drilling and tapping the extrusion, saving costs. It’s possible to incorporate a T-shaped groove in the extrusion to allow versatile mounting. Various components can be mounted without customization and with less assembly time. Further, the T-shaped grooves can allow grounding terminals to be placed. This provides a quick and convenient way to have a secure connection to ground.

Design for EMC
EMC is an important consideration in enclosure design, particularly in medical electronics. Depending on the device’s use, electronics equipment must meet compliance standards like FCC and CE. The goal is to create ground continuity over the outer skin of the enclosure and to block given wavelengths from passing through any openings. To ensure compliance, a wide range of issues must be considered. These include material type and thickness, conductive plating, gasketing, power filtering, vent-hole size, seam length, access panels, and ac cable routing. To provide a solid basis-point, the extrusions for the instrument case can be designed with 90-degree angles to cover any open seams. Further, the shape of the extrusion can allow the insertion of EMC sealing cords.

For both aesthetics and EMC considerations, the top and bottom covers can be folded over in the sheet metal stamping process, as shown in Figure 2. This helps provide a tighter seal and boosts the unit’s EMC protection. The enclosure’s rounded look makes the unit more attractive. There are other ways to easily and cost-effectively improve the aesthetics, including design techniques to minimize unsightly parts of the enclosure. For example, screws can be placed in hard-to-see locations, or the enclosure can be indented where the screws are placed to maintain a flat surface. Also, cover panels and bezels can hide these areas as well as add to the appearance of the chassis in their own right.

2. The top and bottom covers can be folded over in the sheet metal stamping process, which aids in both aesthetics and EMC considerations.

Aesthetics can also be enhanced with a digitally-printed front panel. Digital printing on the aluminum panel can use the same brilliant colors and creative designs that come from a standard digital printer. The designs can help an OEM’s unit stand out, allow sharper and precise logo use, and more. One possibility is to create a design that will help make a diagnostic device less intimidating for children. With more playful colors or even cartoon characters (the possibilities are endless), the panel can act as more than a chunk of aluminum or an EMI-barrier, and actually help enhance the soothing environment for the patient.

Walter Schindler is a mechanical engineering manager at Elma Electronic Inc.

Walter Schindler, Elma Electronic Inc.
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