Ergonomic guidelines for designing machines

May 6, 2004
A few design principles keep machines safe and easy to use.

Stanley Caplan
Human Factors Consultant
Usability Associates
Rochester, N.Y.

Design requirements often carry statements saying something to the effect that "Operation of the machine shall be user friendly." And every engineer wants to design products that are easy and safe to use, but as most engineers know, this is easier said than done. Every switch, handle, knob, and control is a potential source of misuse and frustration for users that can lead to machine damage or injury to users.

This was driven home to a copy-machine manufacturer when it installed beta equipment in a sheriff's department. A deputy could not figure out how to open a locked paper drawer, so he used his pistol to solve the problem. The result: A service call to replace damaged parts and remove the bullet.

The challenge for engineers is to make sure controls and other areas on machines users might touch — also called touch points in ergonomic parlance — are functionally obvious, don't expose users to risk of injury, minimize mistakes, and make it easy to recover if users do make a mistake. This is a tough task because every machine is different. But over time, certain principles become evident. Here are some guidelines for designing touch points on consumer, business, industrial, and medical products. They can be used during development or transformed into design specifications. The important ones become specifications and concern frequently used touch points and those that can cause damage to machinery or users.

Ensure that users with heights between the population's 5th and 95th percentiles can see all necessary touch points.

Distinguish between operational and maintenance touch points.

Keep required amount of force consistent with purpose for force. For example,-the force to manually adjust the paper-guides in your printer should be small enough to let anyone slide them apart or together, but the guides must stay in place during use.

Follow local conventions for up and down, rotation, color, and other stereotypical cues.

For global products, make touch points flexible enough that local conventions can be used.

Anticipate misuse. Design in means to prevent misuse or make the machine tough enough to withstand it.

Prevent doors from closing completely if an internal component is improperly seated.

Make door locations obvious and the method for opening them consistent with direction of opening, i.e., don't have users push in to have a door open outward.

Make touch point appearance consistent with expected use.

Falling covers are hazardous to fingersand heads. Ensure covers and doors stay up when opened. If held up by a mechanism, make sure it stays open despite wear and tear

Doors should close and latch regardless of where users push on them. This is especially important for interlocked, hinged doors requiring a switch or sensor at the top or bottom.

Avoid connections or situations where removing a component takes significant force. A sudden reduction in this force often causes the component to "eject" itself and fly out of the user's control or the user's hand to recoil onto a potentially dangerous surface.

Inserting a component should not require exact positioning. If it does, make sure engagement points are clearly visible and obvious. And it should only be possible to insert the component one way, the correct way.

Add an audible "click" or stop to signal a component is in its final position. And don't design in false cues.

Ensure there's enough room for people with hands at least as large as those in the 95th percentile to grab handles and fit between components.

Consider clothing users might wear. Ties, jewelry, and billowy blouses can get caught in rollers, pinch points, and other tight spots.

Add an aid for removing a component if there is no way to grab it. AAA battery compartments, for example, often have small fabric straps under the batteries. Pulling on it lifts the batteries out of their spring-loaded compartment.

Anticipate unintentional contact with hot or sharp surfaces and protect users from them. For example, think about where a user might put his left hand if the right hand is removing a component. If it takes enough force, he might put his left hand on an adjacent surface to provide leverage.

Include illustrations that show components in the same orientation as they would be seen by the user.

Place labels next to areas they apply to.

Use font size and style that is readable from where user will be when looking at labels.

Assume there will be poor lighting and make sure there is high contrast between text, markings, and background. Unpainted, embossed lettering and markings are especially hard to read under such conditions.

Put legends above the switches and buttons they identify. Putting legends below the controls mean they will often be obscured by the user's hand.

Don't put legends directly on frequently used buttons. They will wear off.

Following these guidelines sometime requires technology such as position sensors, but they can often be met with creative, no-tech solutions. For example, in one business product, the company wanted to include a foldout shelf to add work surface without adding to the machine's footprint. But knowing someone might want to use the shelf for a step to reach a burned out light or string party decorations, they used support brackets that looked too flimsy to support a person's weight. In another case, a thin, metal door was molded with ribs on the inside. The ribs served two purposes. They added rigidity to the door so it would fully close when pushed and they were placed so that the door would not close if internal components weren't fully seated.

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