2- THE 'CHEAT' SHEET FROM APEM ENGINEERS
Force-stroke curve of a horizontally positioned spring based tactile feedback mechanism Spring mechanisms
The performance of spring based mechanisms is dependent on the positioning of the spring within the device.
Vertically positioned, spring mechanisms are engaged simply by depressing the spring. It is the most versatile of the tactile feedback mechanisms in that it can accommodate a long travel distance or be used in large switches.
It bears up well in frequent use, with a maximum life of 10million cycles. The “feel” of a vertically placed spring mechanisms is always less satisfying than that of domed mechanisms or horizontally positioned spring mechanisms since there is no precise indication of actuation, no “snap”. Vertically positioned springs can suffer from inadvertent activation in high vibration applications.
Horizontally positioned, spring mechanisms have a distinct tactile feedback which is accompanied by a metallic clicking sound at the moment of actuation. These mechanisms hold up well in environments where they are subject to vibration or strong current.
Both vertically and horizontally positioned springs often require combination with a membrane to achieve an effective front seal.
Force-stroke curve of a metal dome based tactile feedback mechanism.
Metal dome mechanisms
Metal domes are formed out of stainless steel that, when compressed, collapses, actuates the switch, and then releases to its initial convex configuration. Metal domes provide a great deal of positive response, giving the user a crisp, precise activation signal. They are, however, size limited: past 16mm, the metal cannot be reliably counted on to properly return to its initial configuration. Metal domes provide superior tactile feedback and usually carry the highest price tag.
Force-stroke curve of membrane based tactile feedback mechanism
In membranes, the inside of a polyester bubble is coated with graphite. Membranes can reliably function at larger sizes than metal domes and are typically less expensive. The feel of a membrane is less precise than metal, because the collapsing dome doesn’t provide as much positive response.
This can be advantageous in high repetition uses, where transmitting too much energy back to the user can be ergonomically disadvantageous. Although still size limited, membranes can be effective in larger sizes than metal domes and usually have a longer life span.
Membranes can be configured into virtually any shape, sealing the switch without additional components.
All tactile feedback devices need to be calibrated for the intended end use. APEM QA uses a Force Stroke Machine to ensure our tactile feedback mechanisms are fine tuned.