Mechanical design of the HP 54600 Series oscilloscopes - Technical

Hewlett-Packard Journal, Feb, 1992 by Robin P. Yergenson, Timothy A. Figge

[NOTE: SOME FORMULA'S HAVE BEEN OMITTED]

Simplicity of manufacture and a minimum of parts were the approaches taken to achieve high quality and reliability. Robotic assembly wasn't a consideration, so rotating motions were often chosen to mate components in final assembly.

The mechanical design team for the HP 54600 Series oscilloscopes shared the overall design objective of achieving the highest quality and reliability of any oscilloscope available. Quality and reliability must be designed in from the start. The mechanical design team felt that the most effective way to ensure that the instrument attained these goals was to make it simple to manufacture with as few parts as possible to go wrong in the field.

With this in mind, fasteners have been eliminated where possible. Snap fits and sliding and rocking component lock mechanisms are employed. As a result, the HP 54600A and 54601A require a total of only 22 threaded fasteners for final assembly. Fig. I shows the components that go into the final assembly.

It was decided early on that robotic assembly would not be cost-effective in any portion of the assembly operation. This allowed us to free our thinking a bit and forget the straight-line-motion assembly techniques often used to evaluate manufacturability. As a result, over half of the assembly operations consist of a simple rotational attachment technique. Typically, one mating end has fingers and slots that interlock so that when one component is rotated into place, a slight interference at the joint results in a retention load on the two components. The other end is aligned by component details and held together by a screw or snap feature. Fig. 2 shows examples of this concept. This method of assembly can reduce threaded fasteners by a factor of two. The rotary motions of the parts, which would be difficult and time-consuming for a robot, are simple operations for manual assemblers.

The use of keyhole standoffs also helps reduce fasteners. For example, the power supply mounts with a 9-mm sliding motion to engage the keyhole standoffs and one screw to eliminate sliding. Also, one side of the power supply printed circuit board mounts to a card guide formed in the side of the sheet-metal deck with the same motion required by the keyhole standoffs.

The knobs on the front panel of the HP 54600 use an internal interference protrusion with a reverse draft angle to ensure that they stay fixed on the encoder shafts. This eliminates the installation time and complexity of set screws. The feature works amazingly well, resulting in knobs that push on quite easily, but require a substantial increase in force to remove.

The electronics of the HP 54600 family consists of four assemblies: power supply, display, keyboard, and system board. Each of these components arrives at the assembly line fully tested and ready for installation. To keep interconnection problems to a minimum, only two ribbon cables are used inside the HP 54600. These features result in a total assembly time of less than 12 minutes, one-third that of previous designs.

Even with an estimated mean time between failures of 50,000 hours it still seemed appropriate to include serviceability as an objective. Removing two screws (see Fig. 3) from the cabinet allows the remaining chassis assembly to be slid out of the cabinet to permit removal of the front panel, system board, display module, power supply, or fan. Although caution must be used when the cabinet is removed, the instrument remains fully operational to allow easy access for troubleshooting. With the cabinet removed, access holes in the front panel allow calibration for flatness and overshoot (see Mg. 4).

To satisfy the instrument's cooling requirements the enclosure is pressurized with an 80-mm dc brushless fan. The air flows across the power supply and display modules and then wraps around the sides to flow over the system board and out the ventilation holes, which are along the bottom of the cabinet. The narrow ducting area along the bottom of the cabinet results in a venturi effect which provides a relatively high airflow velocity across the system board (see Fig. 5). The 20 cubic feet per minute of air flowing through the box results in an average air temperature rise of only about 8'C above ambient. To satisfy environmental conditions requiring additional cooling, a thermal sensor inside the instrument ramps the fan voltage from 8.5V to 14V depending on incoming air temperature. This way the instrument is extremely quiet at standard room temperatures without compromising component life when used in harsh conditions.

Portability

General-purpose oscilloscope users want to be able to move an oscilloscope easily, so portability ranked high on the list of mechanical design objectives. The size evolved out of an earlier decision to put the whole oscilloscope on one board and to minimize raw board costs by having four boards per panel. This gave us a tentative board size of about 8.5 by 11 inches. The project's first manager suggested putting the oscilloscope board opposite the power supply and display modules with the sheet metal deck separating them. This configuration helps isolate sensitive electronics from the magnetic fields associated with the power supply and display modules and to a large degree defines the footprint of the instrument. A 3/4-width cabinet (about 12.5 inches wide) meets HP industrial design standards and works well with the 11-inch board dimension. The height is based on clearances required for the display module. Although the final configuration diverges from the deep aspect ratio of some other oscilloscopes, the resulting display area is nearly twice that of these other products.