Motor Locks Up Reliability

DN Staff

October 26, 2004

3 Min Read
Motor Locks Up Reliability

When solenoids failed to meet the durability needs of an automated lockbox for real estate agents, designers found the solution in a new DC micro motor. It's the same kind of motor also used in pill-dispensing machines, security cameras, and miniature robots, among other applications.

Measuring just 12 mm in diameter and 29 mm in length, Sanyo's new 12 GN series DC gear motors fit the compact design of SentriLock's REALTOR(TM) lockbox system, introduced in thousands of homes this past summer. Agents use a smart card to release the lock's shackle and then punch in a PIN to open the door to the compartment holding the homeowner's key.

"In locks, solenoids provide good power at the start but they will often lose power across the full stroke, causing the device to stick." notes mechanical engineer Shane Snipe, manager of Sanyo's DC Micro Motor Div. ( "What this application needed was reliable, constant torque."

Designed for applications that require intermittent actuation, the new motor weighs only 8.7 grams yet provides a starting torque of 300 mNm and a rated load of 20 mNm. It is available in four gear ratios: 1/75.7, 1/134.5, 1/196.6, and 1/297. No-load speed ranges from 62 to 246 rpm, depending on the model. Among other key features:

* No-load current of 120 mA.
* End play of 0.02 to 0.35 mm in the drive shaft.
* Lateral play of less than 0.04 mm at the tip of the drive shaft.
* Vibration of less than 40 m/s 2.
* Noise level below 55 dB at 5 volts.

In addition, the motor performed well in the REALTOR lockbox application during environmental tests by an independent lab, including temperature extremes from -40 F to 185 F, as well as salt air and moisture conditions.

In the lockbox system, the motor's gearhead attaches to a spur gear that drives a linear actuator in one direction to release the shackle-and in the opposite direction to open the door to the box containing the house key. Five patents are pending on the SentriLock system, involving both the mechanical operation of the system and its smart card technology. Visitors can view a demo of the product on

Engineer Scott Fisher, president of SentriLock, says he selected the Sanyo motor because it met the size, torque and operating voltage requirements of the application-5 to 6 volts. The motor connects to an off-the-shelf, dual-cylinder lithium battery (type 2CR5). By the fall of 2005, Fisher expects that more than 100,000 of the new lockboxes will be installed around the country, with a total market potential of over 2 million units. The system may also be adapted to other applications.

While other manufacturers, such as MicroMo Electronics and Danaher's Portescap unit, produce micro gear motors for precision medical devices and other high-end applications, Sanyo's Shane notes that the 12 GN Series design meets the market need for reliable, low-cost motors for high-volume applications. At just $3.00 a motor in quantities of 100,000 or more, the 12GN series are no more expensive than solenoids, adds Shane.

COMPACT MUSCLE: Sanyo's new DC micro motors, weighing just 8.7 grams, provide needed torque in applications where solenoids fall short on durability.

Besides the lock box application, engineer customers are selecting the new 12 GN series of micro motors for a wide range of applications, including: hand-held insulin dispensers, power tools for applying adhesives, miniature robots for hobbyists, security cameras, and automated pill-dispensing machines in remote areas. Another prime target: hotel door locks that need motors that can survive 500,000 cycles. "About one third of the hits we're getting on our website involve these new micro motors," says Shane.

Late this year, Sanyo will introduce even smaller 8 and 10 mm gear motors, aimed at such applications as lens movement in digital cameras and linear actuators for industry.

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