Active latch locks motion with tiny power

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Tom Shelley reports on a low cost clutch mechanism that locks large forces or torques with tiny amounts of power

Tom Shelley reports on a low cost clutch mechanism that locks large forces or torques with tiny amounts of power By using a piezoelectric actuator to rotate a locking blade, it is possible to devise a clutch mechanism that can lock more than 13.5kN of force or 3kN-m of torque using only about 1mW. The base mechanism has been developed for locks and security applications, but is already been developed to lock up large swing doors, and has the potential to lock up any kind of moving device, whether the movement is linear or rotary. The key mechanism in the "Active latch" is Simon Powell's hairpin piezo electric actuator featured in Eureka's March 2002 cover feature story, and used in the Diamond H gas control valves described in last month's feature story. In the present development, the same actuator sits inside a lock plunger. It acts on a tooth at one end of a locking blade in such a way that when energised, the locking blade can engage in a cavity in the side of the plunger, locking it in the extended position. The plunger in turn engages in a slot in a ring or bar. When un energised, the plunger can be depressed, allowing the ring to turn or the bar to pass. In its present commercial form, model AL1, the device is 31mm long, 21mm wide, and 12mm thick. It weighs 30g, runs off 6VDC, and can block 20Nm of torque on a ring with 6mW of power per operation. Lifetime is more than one million cycles at 2.4Nm. A key override slot is fitted as standard. Makers Servocell offer a freely web downloadable design guide with more than a dozen possible ways of using the latch in different devices. The simplest is to use the mechanism to lock a handle to a hasp, in order to open a door. The latch is connected to the drive shaft of the lock. In the un-energised state, the ring, attached to the handle, moves past the plunger, failing to engage it and the lock mechanism. Alternatively, the latch can be used to lock up a door handle, preventing it from rotating and opening a door. In another concept, the latch is fitted into the pocket in a tumbler, which is sprung to hold a lock tooth out of the path of a linear slider with a slot in it. Another concept has the latch plunger engage in a reciprocator. The possible variations are endless. Because the basic mechanism uses very little power, it can be run from batteries. If used to engage an unlocking mechanism, as in the first example, power is only consumed when the unlocking action takes place, so a small battery can power the mechanism for years. If used to lock under power, power consumption will be greater, but allows doors to be opened in the event of power failure, which might be desirable in a fire situation. But the operating principle is quite general, and far from restricted to locks, since the mechanism can be make large or small, and can be used as the basis of a clutch using far less electric power than the more conventional electromagnetic kind. Servocell Pointers D/T/A-Text: Latch mechanism prevents plunger from being depressed using a very tiny amount of electric power D/T/A-Text: Key mechanism is a piezoelectric actuator rotating a locking blade D/T/A-Text: Latch can be used to lock rotary or linear mechanisms