Mechanical implementation of the HP Component Monitoring System

Hewlett-Packard Journal, Oct, 1991 by Karl Daumuller, Erwin Flachslander

The part count and the number of different parts are dramatically lower than for previous designs. Fewer than ten vendors are used for purchased mechanical parts.

From the mechanical perspective, the HP Component Monitoring System offered several challenges. Among the most important were the definition of the architecture of the computer module and the design of the sheet-metal and plastic parts for this component. Other mechanical highlights include the implementation of the display front assembly and the construction of the parameter modules.

Computer Module Chassis

The general design objective for the computer module was to create a flexible, compact instrument that could easily be extended and upgraded. In accordance with the modular concept of the Component Monitoring System, the computer module had to be designed so that the function cards could be handled as independent modules. All function cards were to be accessible without having to remove or disassemble major parts of the enclosure.

From the production point of view, the following general design objectives had to be met:

* Minimum part count

* Minimum number of parts with different stock numbers

* Minimum vendor count

* Use of preferred parts

* Compliance with all relevant medical safety standards

* Simple and automated assembly.

We also committed ourselves to design an enclosure that could be assembled and serviced with only one tool (all you need is a screwdriver).

The clinical environment mandates that the product be easily cleaned and have no sharp comers, sharp edges, or deep indentations. Liquid spilled over the Component Monitoring System is not allowed to create a hazardous situation for the patient or the user, nor may it leak into the unit. Last but not least, the constraints of the electronics had to be taken into consideration.

The requirements were sometimes contradictory. For example, on one hand, the chassis needs to have low RFI emissions, while on the other, it needs sufficient openings to dissipate as much heat as possible. The maximum internal temperature rise cannot exceed 15[degrees]C. Heat management is made more difficult by the fact that fans are not acceptable in the monitoring environment. This implies that natural convection is the main mechanism for dissipating heat. Extensive measurements with simulated electronic circuits and calculations in the early project phase were a good basis for the architectural design of the computer module. The knowledge gained from these studies and the demand for easy access to the function cards led to the present design.

Product Design

Two large sheet-metal parts form the enclosure of the computer module (see Fig. 1). The bottom part of the chassis is made of 1.25-mm-thick steel and has a large number of openings for ventilation. Mounting holes and pressed-in threads for the instrument's feet and locking cam are located here. The inner part of this U-shaped component has a number of indentations and cutouts. This construction allows the plastic guide for the function cards and the central plane to be snapped in place without any screws.

The second large sheet-metal part is the top cover of the chassis. Offset bends similar to those in the bottom part of the chassis make it possible to snap the plastic function card guide into the lid. Pressed-in threads are mounted on the offset flange to hold the function cards within the computer module.

Two large indentations with strong steel strips riveted to the top cover provide a quick and easy way to mount the 14-inch display on the computer module should this be desired. A combination of indentations with an undercut, feet with noses, and the cam forms a tight locking mechanism between the display and the computer module. This technique was first used by the HP Medical Products Group in 1981 for the HP 8040A cardiotocograph. This well-established mechanical interface for stacking instruments or attaching them to wall or ceiling mounts or carts was a must requirement.

After the U-shaped bottom cover and the lid of the chassis have been assembled, all function cards can be inserted by simply sliding them into the enclosure. Metal board covers mounted on the rear ends of the function cards seal the remaining openings of the computer module. Each board cover contains openings for RFI clips and the function card's external connectors, and provides a mounting hole for fixing the function card to the frame. The remaining area is perforated for ventilation, except for the space needed to silk-screen the board name and number. Mg. 2 shows the interior of the computer module with the function cards inserted.

As described earlier, the function cards are held in place by plastic guides in the top and bottom parts of the chassis. The printed circuit board guide is an injection-molded part that can be used in both locations by simply turning it over.

After the top cover is installed, the left and tight side covers can be attached to the computer module. Again, a single injection-molded part fits both sides. This part contains all the vents and openings needed for thermal management. The side covers also conceal the six screws that attach the chassis top to the bottom. The customer can easily remove the side covers for cleaning by unlatching the internal rack (see Mg. 3).