Another form of interlock is the dead mans switchin numerous���safety settings, especially for the operators of trains, lawnmowers, chainsaws, and many recreational vehicles. In Britain,these are called the drivers safety device.Many require that the���operator hold down a spring-loaded switch to enable operation ofthe equipment, so that if the operator dies (or loses control), theswitch will be released, stopping the equipment. Because some operatorsbypassed the feature by tying down the control (or placinga heavy weight on foot-operated ones), various schemes have been
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developed to determine that the person is really alive and alert.Some require a midlevel of pressure; some, repeated depressionsand releases. Some require responses to queries. But in all cases,four: Knowing What to Do: Constraints, Discoverability, and Feedback 143they are examples of safety-related interlocks to prevent operationwhen the operator is incapacitated.LOCK-INSA lock-in keeps an operation active, preventing someone from prematurelystopping it. Standard lock-ins exist on many computerapplications, where any attempt to exit the application withoutsaving work is prevented by a message prompt asking whetherthat is what is really wanted (Figure 4. 6). These are so effective thatI use them deliberately as my standard way of exiting. Rather thansaving a file and then exiting the program, I simply exit, knowingthat I will be given a simple way to save my work. What was oncecreated as an error message has become an efficient shortcut.Lock-ins can be quite literal, as in jail cells or playpens for babies,preventing a person from leaving the area.Some companies try to lock in customers by making all theirproducts work harmoniously with one another but be incompatiblewith the products of their competition. Thus music, videos, orelectronic books purchased from one company may be played orread on music and video players and e-book readers made by thatcompany, but will fail with similar devices from other manufacturers.The goal is to use design as a business strategy: the consistencywithin a given manufacturer means once people learn the system,they will stay with it and hesitate to change. The confusion whenusing a different companys system further prevents customers from�FIGURE 4.6 A Lock-In Forcing Function. This lock-in makes it difficultto exit a program without either saving the work or consciously sayingnot to. Notice that it is politely configured so that the desired operationcan be taken right from the message.144 The Design of Everyday Thingschanging systems. In the end, the people who must use multiplesystems lose. Actually, everyone loses, except for the one manufacturerwhose products dominate.
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