Selecting the most relevant product for a project is sometimes difficult. The first step in getting the right feedback is asking the right questions:
- How much space do I have for the switch?
- Will it be constantly or infrequently actuated?
- How distinct do I need the actuation signal to be?
- How strong?
- Will it need to be environmentally sealed?
- How long a life must it have?
- How great a consideration is cost?
- Will the device be subject to vibration?
1. PRINCIPLES
Tactile feedback can be achieved via a spring mechanism, a metal dome or a polyester membrane. With all three materials, deforming the initial and preferred state of the device enables contact with a switch actuator.
But that’s where the similarity ends, and choosing between the options is a lot easier when you know the strong and weak points of each type. Tactile feedback mechanisms vary greatly as to cost, reliability and feel.
But perhaps most importantly, they require varying degrees of force and travel (pressure and distance) to engage the switch. These last two variables impact the life expectancy of the device and the ease of use for the end user. The tactile feedback of a computer keyboard switch, constantly in use and requiring little force, should not resemble that of a forklift safety switch, where gloved, infrequent use calls for significant resistance and a highly charged feedback.
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.
APEM products with this mechanism: IP series
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.
APEM products with this mechanism: IM series
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
APEM products with this mechanism: IA series ; MEC switches
Membrane mechanisms
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.
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