Compliant MicroElectroMechanical Systems (MEMS)

Dee Anderson
(801) 422-6266
dee_anderson@byu.edu

Compliant Mechanisms are mechanical devices that gain movement from parts that flex, bend, or have “springiness." Such mechanisms, with built-in flexible segments, are simpler and replace multiple rigid parts, pin joints, and add-on springs. Hence, they can often save space and reduce costs of parts, materials, and assembly labor.

Other possible benefits of designing compliance into devices may be reductions in weight, friction, noise, wear, backlash, and, importantly, maintenance. Compliant devices can range from giant tools bigger than a person, to hand-held items, or to micro-mechanisms – so small that hundreds fit on a silicon microchip. They can facilitate tasks that have never been done before, such as provide constant force in compression. See the list of compliant mechanisms below for examples of specific applications.

General Information

Compliant mechanisms comprise a novel class of devices: one or more of their elements bend or deflect to allow motion and transform energy or transfer force. This is in contrast to rigid-link mechanisms that only move as their moveable joints allow. Within the flexible (or compliant) element, energy is stored in the form of strain energy, later to be released and perform some useful task.

Compliant mechanisms may be reliably designed and analyzed using well-understood “pseudo-rigid-body” models, which model compliant elements as one or more rigid-body joints. BYU Professor Larry L. Howell wrote the first, published textbook on the subject, “Compliant Mechanisms” (published by John Wiley & Sons, Inc., © 2001). One can imagine a variety of devices that typically have complex springs and mountings or joints, but could be made cheaper or easier with flexible parts – things like grippers/pliers, clips, latches, hinges, triggers, switches, connectors, robot arms, etc. Some of the marketable compliant mechanism are as follows:

• Bistable (2-position) Mechanisms move between two stable conditions (On and Off positions) and are useful as switches, circuit breakers, clamps, snap hinges, closures, positioning devices, etc. Though they require external force to move from position 1 to 2, no holding energy is required to remain in either position. Other advantages may include cheap and easy fabrication, no required assembly, adaptable, durable, and scalable for any force or motion required. Plastic prototypes have exceeded a million cycles in durability tests.
• Ortho-Planar flat Spring is a thin flat spring in a circular or polygon pattern whose small central platform moves up or down perpendicular or orthogonal to the plane of the flat material of which the spring is made. The platform doesn’t rotate so there is no rubbing nor wear. It can be actuated physically or by an electromagnet and used in keyboards, pneumatic valves, electrical contacts, positioners, damping devices, touch probes, speakers, precision antennae, circuit boards, etc.
• Centrifugal Clutches made as compliant mechanisms eliminate numerous segments, springs, pins, rivets, etc. Flexible segments are designed into the single moving part so that when the hub (driven by a motor) spins the clutch up to speed, centrifugal force causes the heavy segments to engage the drum and drive the machinery. Special qualities include smoother engagement, higher torque, ease of adaptability and replacement, and potential for reduced cost. Small and medium horsepower applications include go-karts, mini-bikes, trimmers, tillers, chain saws, chippers, amusement rides, agriculture and industrial machine couplings. Think higher margins!
• Over-Running Pawl Clutches, with or without centrifugal throw out, provide torque in one direction but freewheel in the other. They are used for one- and two-way rotation, as in pull-starts for small engines, bicycle and “Big Wheel®” free wheels, fishing reels, gear drives, winches, conveyors, elevators, counters, collators, feed mechanisms, and many other machines. As with other compliant clutches, advantages include reduced part count, low cost, low wear at high speeds, efficient load transfer, and reduced backlash.
• Near-Constant-Force Compression Mechanism use compliant technology to achieve near-constant pressure with a deviation of only 2% in the compression forces. Several configurations have been designed to work over a range of travel patterns. No near-constant- force compression mechanism is known on the market yet, so the opportunities are great. Uses of near-constant-force compression mechanisms might include fitness products, robot end effectors, tool holder, motor brush holder, wear test apparatus, and safety devices.
• Near-Constant-Force Electrical Connectors use compliant technology to maintain constant connection between electrical connections over long periods of time. License has been granted for communication electronics (i.e. cell phones and Palm Pilots&tm;) but license is available for other applications. Advantages are reduced wear, low maintenance, ease of manufacture, reduced part count and cost. The majority of computer hardware and automotive electrical problems arise from faulty electrical contact integrity. The NCF electrical connector improves connections and reliability.
• Bicycle Brakes of compliant design provide absolute parallel motion, have visual appeal, and are preferred by experts for their strength, superior control, even wear, and reduced noise.
• Compliant Bicycle Derailleur equals the performance of a regular bicycle derailleur with decreased weight. Flexible links replace parts such as springs and other materials.
• Compliant Grippers and pliers can be produced from an injection-mold process and constructed of one piece, significantly reducing assembly requirements. Compliant mechanism grippers do not use pins or springs yet function exactly like traditional grippers. The grippers can be used in a variety of applications: in photo labs, in chemistry labs, with electrical equipment, etc.
• Microelectromechanical Systems (MEMS) are small, compliant devices for mechanical and electrical applications. MEMS are fabricated using techniques developed for the production of computer chips. Most MEMS devices are barely visible to the human eye with many features 1/50 the diameter of a human hair.

The Market

The applications of compliant mechanisms are varied and practically unlimited. Any device that uses springs, joints, pins, or latches can be a potential benefactor of compliant mechanisms.

One advantage of compliant mechanisms is reduction in parts required to accomplish a specific task. Some mechanisms may be manufactured from an injection-moldable material and constructed of one piece. Part reduction can dramatically reduce manufacturing and assembly cost.

Compliant mechanisms have a smaller number of movable joints, such as turning and sliding joints. The result is reduced wear and reduced need for lubrication. Backlash reduction due to a decreased number of joints increases mechanism precision.

Compliant mechanisms weigh less than their traditional counterparts. Weight reduction has the advantage of reduced shipping costs.

The Product

Compliant Mechanisms Research Group Web page at BYU
http://research.et.byu.edu/llhwww