Anti-Stiction Air Cylinder Improves Membrane Switch Testing

An Air cylinder that uses a carbon /graphite piston within a glass housing produces precise and repeatable motion during membrane switch testing.

A precision low friction cylinder with a piston that slides on a cushion of air is being used to provide reciprocating motion in membrane switch test systems. An Airpel air cylinder, produced by Airpot Corp. Of Norwalk CT. generates a virtually constant level of friction at air pressures from 10 to 100 psi. As a result, the Airpel minimizes erratic movements that occur as a piston begins its travel within a cylinder.

This characteristic is vital in the testing of membrane switches, a process in which the air cylinder must generate forces that are precise and repeatable. Membrane switches are part of flat panel displays such as those used to enter orders at fast food restaurants or to enter machine control data in factories. A key is pushed on the flat panel display, resulting in the deflection of a polymer layer with silk screened ink traces. The deflecting layer then contacts another polymer layer that is also patterned with ink. When the ink patterns contact each other, the switch closes. As a result, an electrical circuit is closed and a signal is sent to a computer controller. To verify the quality of these membrane switches the testing company designs and manufactures computer controlled test systems. For quality control a keypad manufacturer pulls a few switches at random from the production line. They are electrically connected to the test system and positioned under an actuator probe that continuously monitors and logs switch resistance values.

To gather data about a switch’s performance a typical test might involve 5 million on/off cycles. The life test system measures the electrical resistance of switches and sends the data to a computer controller. In the controller, the resistance measurements are compared with the maximum allowable measurement for a switch which has already been entered by the system operator. A failed cycle occurs when the resistance reading exceeds allowable limits. To reproduce the action of activating the switch, a device is needed to automatically simulate the movement of a finger. To achieve this engineers developed silicone fingers and connected them to the piston rod of an Airpel cylinder. Depending on the design of the display panel the air cylinder is required to provide a load from 4 to 40 ounces, These loads are required with panel displays that have flat keys or dome shaped stainless steel buttons that return to their original shape after the load is removed. In the case of flat panel switches the air cylinder is required to hold the mechanical finger 1/32 inch. With the dome shaped tactile displays the Airpel typically holds the silicone finger 1/32” above the button and then deflects the button 1/16”.

A separate use of the air cylinder involves the testing of membrane switches that are part of touch screen display systems such as those used in automated bank tellers. In this example loads on the display buttons are from 2 to 4 ounces. The air cylinder does not apply direct pressure but lowers a weight on to the screen to activate the switch.

The air cylinder is microprocessor controlled to perform within test limits and tolerances during the test system’s development.  “Electrically powered linear actuators are not an option,” according to the engineer. “They cannot achieve the precise control of force that is required in this application.” Design engineers considered using conventional air cylinders with rubber seals but they realized that due to erratic piston movement these would not provide the required control over force either.

A common load requirement for membrane keypads is 10 ounces. A conventional air cylinder produces a range of forces that generate loads that vary between 7.5 and 13 ounces, According to the engineers. By contrast the range of the Airpel cylinder varies by only plus or minus 0.2 ounces from .8 to 10.2 ounces.

The primary cause of erratic piston movement in conventional air cylinders is contact between the elastomer seals and air cylinder surfaces. A gradual increase in air pressure leads to a breakaway motion by the piston as it overcomes friction. However, it is extremely difficult to consistently control the movement of the piston during this breakaway phase. Membrane switch testing requires tight control not only over the piston rod’s linear movement but also over its lateral movement. It is important that the piston rod return consistently to the same location. Moreover, the Airpel and silicone rubber fingers are required to operate in an environment chamber at temperatures that cycle between -40 and 105 °C. Two conventional cylinders of different deigns were needed to span the range of temperatures.

The Airpel has two air chambers, one behind the piston and another in front of it. It consists of a carbon graphite piston custom fitted inside a glass cylinder. The gap between the piston and the cylinder is only 0.0005” and the roundness specification for the cylinder and piston is 0.0001” When the cylinder is pressurized, the piston rides on an air cushion formed between the piston and the glass wall. The piston rod slides on a carbon graphite seal and low friction bushing.

Design engineers at Airpot selected a pivot pin as the connecting component between the piston and its rod. The pivot pin allows the piston to realign itself in the cylinder should the cylinder be subjected to lateral forces. In this case the pivot pin allows the piston to float radially to realign itself. The resulting design produces extremely low friction along the length of the piston’s travel.

To make the cylinder rugged, the engineering team at Airpot designed a steel tube to encase the cylinder and piston assembly. The air gap between the glass cylinder and the steel tube allows the cylinder to continue to operate even if it is dented. During development tests the Airpel cylinder withstood the striking impact of a 10 pound weight and continued to function as required. The air cylinder operates for more than 25 million cycles. It requires no lubrication, which eliminates the potential for oil contamination. Such a characteristic is useful in industries such as food processing and semiconductor manufacturing, where cleanliness is key. Moreover, the Airpel cylinder eliminates the potential of fires fueled by oil.

In comparison tests, piston motion in the Airpel cylinder is smoother than piston motion in a comparable conventional air cylinder. According to Airpot, at 10 psi it takes 25% of the air pressure to overcome static friction in the conventional cylinder. Only 1 psi of air pressure is required to overcome the static friction in the Airpel cylinder.

The Airpel cylinder works best with loads that are between a few ounces and 30 pounds. The air cylinder is particularly suited for use in applications that require smooth motion at extremely low pressures and for moving lightweight objects at either low or high speeds.

Airpot Corporation manufactures instrument quality, precision Air Dashpots, Snubbers, Pneumatic Actuators, Airpel Anti-Stiction Air Cylinders and -AB Air Bearing Actuators. Airpot welcomes custom requirements and works with customers to adapt products to suit their individual application needs. Value added sub-assembly services are also a specialty. Airpot Corporation is a trusted supplier to many of the world’s most prestigious original equipment manufacturers. Visit www.airpot.com for detailed specifications. Call or email our technical staff for application assistance and ordering information. service@airpot.com 203-846-2021.

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Marketing Contact:  Edward Keane                                                                          edkeane@airpot.com

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