Defining The Life Cycle Of A Membrane Switch

How many cycles will a membrane switch survive?

When the Membrane Switch Subcommittee of ASTM International began its study of switch characteristics in 1992, members unanimously agreed that the term “life” was too general and vague. To put the committee’s dilemma in perspective, consider how many times you’ve seen switch designers provide drawings that identify “Life = 1 million.”

The simple statement breeds a long list of questions about switch performance:

• Will the switch really see one million cycles? Or is there a factor of safety built into this?
• What electrical current will the switch see at each closure.
• What force is assumed applied for the one million depressions? Is it a force that is just enough to make contact or 100% harder?
• Is it OK if the graphic layer has worn before reaching one million? How much wear it acceptable?
• What change in actuation force is acceptable before one million cycles?
• If the switch resistance is less than 100 ohms initially, what happens if it increases to 200 ohms before it’s one millionth cycle?
• What change in tactile ratio is expected.
• If the contact bounce time is less than 10ms when the switch is new what if it increases to 25ms before reaching one million cycles?
• What size, shape, and hardness probe should be used when testing this switch for one million cycles to best simulate the real conditions?
As you can see it is impossible to make a single statement, Life = 1 million, without better clarifying the conditions and parameters for switch performance.

For this reason the very first project tackled by the Engineering Task Group of ASTM 1992 was defining a practice that did nothing else but depress and release a membrane switch to a predetermined number of cycles. Contact Closure Cycling, ASTM 1578, explains how to set up and expose a membrane switch to repeated closures. It does not have a measured result. In order to see the effects of repeated cycling you must measure a switch characteristic before & after the cycling using a different ASTM test method. i.e. Circuit Resistance ASTM 1680.

It is important that the switch under test be pressed with a force hard enough to achieve switch contact closure on each cycle. ASTM document F1578 was created to address these issues: Standard Practice for Contact Closure Cycling of a Membrane Switch It is very important to understand that ASTM 1578 is a procedure for cycling a membrane switch but does not, by itself, have a measurable result. In order to cycle a switch using F1578 the following must be known:

• Probe Force
• Duty Cycle
• Test Rate
• Voltage across or Current through the switch (optional)
• # of Cycles
• Probe type

Probe Force:

The test probe must depress the switch during each cycle to reach contact closure. It should be set up to hit the switch with a force at least 10% higher than the actuation force. Therefore, the actuation force of the switch to be tested must be known prior to setting up the cycle test.

Actuation force is defined by ASTM as “the maximum force measured prior to or including the point at which a specified resistance is achieved”, i.e. contact closure. ASTM test F1597 is used to measure the Actuation Force (Fa) of the switch under test. Switch Closure and Actuation Force (Fa) for typical Tactile & Non-Tactile switches are shown in figure above. Once the actuation force is know the test probe is set up to hit with a force at least 10% higher than the actuation force:

Probe Force (min) = Fa x 1.10

Duty Cycle:

This is the ratio of switch closure to total cycle time. If the probe is held down longer than it is up the each cycle would look like the above graph of Force vs. Travel. If the total cycle time is .2 seconds and the probe down time is .13 the duty cycle would be:

Duty Cycle = .13 / .20 = .65 or 65%

Test Rate:

This is the actual number of cycles per second. In the above example one cycle was .2 seconds therefore the test rate:

Test Rate = 1 / Cycle Time = 1 / .2 = 5 cycles per second

Voltage across or Current through switch:

If it is required, a voltage source or current source must be used to electrically load the switch. Obviously electrically loading the switch during Contact Closure Cycle testing better simulates actual conditions as long as the power consumed is not under or over actual operating conditions.

  • of Cycles:
  • This is the number of times the probe will hit the switch and return before the test is stopped.

    • Probe Type:

    ASTM defines two standard probes similar to the one shown in the figure above(see F1578 for more options). Obviously these probes will not fit every application and if a different size, shape or durometer is used it has to be reported.

    While ASTM 1578 outlines cycling procedures, it does not, by itself, determine whether or not the switch fails. In order to see the effects of repeated cycling you must measure a switch characteristic before & after the cycling using a different ASTM test method. i.e. Circuit Resistance ASTM 1680.

    Rather than a vague statement of “Life = 1 million,” switch performance criteria should be reported as follows:

    Contact Closure Cycles per ASTM 1578

    • Probe Force = 10% higher than measured Actuation Force
    • Duty Cycle = X %
    • Test Rate = X cycles / second
    • Current applied = X ma
    • # of Cycles = 1 Million
    • Probe type: Option Fig. 1

    Actuation Force per ASTM 1597

    • X ounces +/- Y ounces
    • Z % allowable change after 1 million Contact Closure Cycles.
    • Probe type: non-elastic, X diameter
    • Specified Resistance: X Ohms

    Tactile Ratio per ASTM 1570

    • X +/- Y
    • Z% allowable change after 1 million Contact Closure Cycles
    • Probe type: non-elastic, X diameter

    Contact Bounce Time per ASTM 1661

    • X ms max
    • Z% allowable change after 1 million Contact Closure Cycles
    • Contact Bounce Measuring Instrument Method
    • UTV : X volts
    • UTV : Y volts
    • Probe type : Option Fig. 3

    Circuit Resistance per ASTM 1680

    • X ohms
    • Z% allowable increase after 1 million Contact Closure Cycles
    • Probe type : Option Fig. 1
    • Force on probe: X ounces
    • Termination Points: pins x & pin y on connector

    As you can see proper Contact Closure Cycling of a membrane switch requires the insurance that the probe force is high enough on every hit to reach the specified resistance.
    It is nice to be able to test quickly but as the test rate increases (cycle per second) there has to be a way of measuring the force being applied dynamically, i.e. as the probe is cycling at the desired test rate. Flat load cells are available that can do this. A dedicated testing lab such as Lab-Tech has the equipment to not only measure the actuation force and perform Contact Closure Cycling, but can also dynamically measure the probe force. This is especially important when using a pneumatic cycle tester since on each cycle the air pressure has to build to a pre-determined value which is directly related to the probe force.

    Originally posted at Membraneswitchnews.com Membraneswitchnews.com by Alan Burk
                                                                  Back to CAM Graphics Resource Page