With public health and safety a top priority in our daily lives, automatic door locks are becoming increasingly popular, and these locks need to have sophisticated motion control. Miniature precision stepper motors are the ideal solution for this compact, sophisticated design. Automatic door locks have been around for some time, initially starting in commercial areas of hotels and offices. With the increase in the number of smartphone users and the spread of smart home technology, residential automatic door lock applications have also gained popularity. There are technical differences between commercial and residential users, such as the use of batteries versus electronic connectivity and RFID versus Bluetooth technology.
Traditional latches require the key to be inserted into the lock cylinder to lock/unlock by turning it manually, a method that has the advantage of being quite secure. One can misplace or lose the key, and the process of changing the lock/key requires tools and expertise. Electronic locks are more flexible in the sense of access control and can often be easily modified and updated through software. Many electronic locks offer both manual and electronic lock control options, providing a more robust solution.
Small diameter stepper motors for compact electronic locks are ideal for solutions with size constraints and precise positioning. Motor engineering and proprietary magnetization technologies have driven the development of stepper motors with the smallest diameter currently available (3.4mm OD). Advanced magnetic and structural analysis techniques are utilized to optimize the design and materials for the limited space available. One of the most critical decisions for miniature stepper motors is the step length of the motor, which depends on the specific resolution. The most common step lengths are 7.5 degrees and 3.6 degrees, which correspond to 48 and 100 steps per revolution, respectively, with stepper motors having a step angle of 18 degrees. With a full step (2-2 phase excitation) drive, the motor rotates 20 steps per revolution and the common pitch of the screw is 0.4 mm, so position control accuracy of 0.02 mm can be achieved.
Stepper motors can come with gear reducers, which provide a smaller step angle, and the reduction gear increases the available torque. For linear motion, stepper motors are connected to the screw via a nut (these motors are also called linear actuators). If the electronic lock is equipped with a gear reduction, the screw can be moved with precision even with a large slope.
The input part of the stepper motor power supply can take various forms, such as FPC connectors, connector terminals can be soldered directly to the PCB, and the push rod of the output part can be either a plastic slider or a metal slider, with a certain range of custom sliders depending on the travel requirements of the lock. Due to the small stepper motor and thin screws, the processed thread length is limited and the maximum travel of the lock is generally less than 50 mm. usually, the stepper motor has a thrust force of about 150 to 300 g. The thrust force varies depending on the drive voltage, motor resistance, etc.
Conclusion
With consumer interest in low margin and unobtrusive products, miniature stepper motors can accommodate this shrinking size. In addition to the compact form factor, stepper motors are easier to control, especially for precise positioning and low speed torque requirements like auto-lock. To achieve the same functionality, other motor technologies require the addition of Hall-effect sensors or complex position feedback control mechanisms. Stepper motors can be driven with simple microcontrollers, which can relieve design engineers of the concerns of overly complex solutions.
