By Dr. Chiming Zhao, Senior Product and Marketing Manager, iSentek
Under the wave of "Internet +," door locks, as the first line of defense for home security, are evolving towards becoming smarter. Studies indicate that in the first half of 2018, orders for smart door locks reached 8.3 million units, and it is estimated that they will exceed 30 million units in 2019. Current automatic deadbolt solutions mostly use optical sensing, magnet sensing, or inclined tongue modification. These solutions either have poor precision and easily damage the lock tongue, or they require modifying the lock body or door frame structure, making installation inconvenient. iSentek's smart deadbolt solution can solve both major problems: its measurement error is within 1 degree, and it can complete measurements simply by being installed on the circuit board, eliminating the inconveniences of damaging the lock tongue or modifying the lock body and door frame.
Implementing Smart Door Lock Deadbolt Solutions on the Circuit Board
iSentek's solution is composed of a magnetic sensor paired with an accelerometer for dual verification, plus a calibration program specially developed for lock applications. The primary 3-axis magnetic sensor measures the earth's magnetic field strength, outputting a 3-axis magnetic signal. The system uses this data and the data converted into an angle to determine the door's status. After obtaining the data, the system can implement the following applications:
Automatic Deadbolt: After the door lock is installed, the system uses the detected magnetic field data to determine that the door is completely closed, immediately driving the motor to automatically throw the deadbolt.
Alarm Prompt: When the door lock is not fully closed, and the system determines that the door panel angle has deviated for too long, it will actively trigger an alarm prompt: "Door is not closed properly!"
Provide Door Panel Status Remotely: The system monitors the sensor's feedback data and uses a mobile APP to return the status in real-time—whether the door is closed, whether it is locked, etc. In addition, if the door is slightly opened during the period when the user sets it to be closed, the system will immediately send a text message to notify the homeowner.
Key Points for Applying the Circuit Board Smart Door Lock Deadbolt Solution
Implementing the deadbolt solution on the circuit board has the advantages of a simple design, convenient construction, and high precision that is not easy to damage the lock tongue. However, designers also need to pay attention to the following points:
Magnetic Sensor Precision: Precision refers to the relationship between the actual value and the measured value. For the lock tongue to successfully enter the lock hole, the error must be within 1 degree. Therefore, the precision provided by the magnetic sensor must also be within 1 degree. iSentek's solution can control the error between the lock hole and the lock tongue to within 1 degree, which can significantly reduce the risk of lock tongue damage. A high-priced smart door lock that is easily damaged due to inaccurate sensors not only requires the manufacturer to pay for new locks, but also leaves the user constantly worried about thieves breaking in during the door lock failure period. The user experience is undoubtedly poor.
Door and Lock Material Interference: Some door panels and lock materials contain magnetic materials such as iron, cobalt, and nickel (or the door lock has magnetic components). These materials will cause soft iron and hard iron effects. Both effects will affect the measurement of the magnetic sensor. iSentek has measured and developed a special algorithm through actual measurements. A simple initialization calibration process can easily complete calibration under various environments. Just one simple switch of the door 90 degrees twice can easily complete the calibration even by the user, avoiding misjudgments caused by soft iron and hard iron effects.
External Interference: Sensors with high sensitivity are easily disturbed by external factors. When a home appliance or a magnet gets close to the door lock, there is a chance it will cause the magnetic sensor to misjudge. Aiming at this problem, iSentek's solution pairs the magnetic sensor with an accelerometer for dual verification, preventing the door lock from misjudging. For example, when a magnet gets close to the door lock, the magnetic sensor data changes but the accelerometer data does not, indicating this is external interference and not movement of the door panel.
In addition, magnets approaching will cause hysteresis effects (after the magnet leaves, residual magnetism will remain, called hysteresis). Magnetic sensors from some manufacturers, due to technical or material issues, are prone to sensor failure due to hysteresis effects and need to be re-calibrated. We cannot predict how often each door lock will be subjected to magnetic interference. If it frequently needs re-calibration, the convenience the smart door lock strives for will be completely lost. iSentek products use Anisotropic Magnetoresistive (AMR) technology and feature built-in degaussing circuits. Even when encountering common magnetic field interference in daily life, the sensor output will return to near its original state after the interfering magnetic field leaves. Without needing to consider the hysteresis effect to constantly calibrate, only then can a smart door lock truly be called "smart."
iSentek's magnetic sensors have already gained recognition and mass production from top drone manufacturers for compass applications where precision requirements are extremely high. Regarding smart door lock applications, we promote the IST8310 series of products and solutions. Different from solutions that require modifying the lock body and door frame, which generate extra costs, an increasing number of domestic and foreign lock manufacturers are turning to completing the design on the system circuit board.
Magnetic Sensor Calibration for Smart Door Locks
To cope with different lock structure and material designs, as well as the diverse environments of buildings; and also the influence of door panel and lock body materials on magnetic sensors, the iSentek algorithm team has developed an exclusive calibration algorithm. After the door lock is installed, the installer is requested to switch the door 90 degrees twice to collect data, and the system will automatically perform calibration. Then, completely close the door to let the system record the 3-axis magnetic data returned by the sensor, defining the "door closed" data, to complete the magnetic sensing system exclusive to that user's door panel.
Manufacturers can open up the calibration process for users to use. If there are system abnormalities later (such as sticking a magnet near the door), the user can calibrate it themselves.
Attached Figure 1 is a comparison of algorithm errors. The red and green lines show the errors after calibration using general manufacturer algorithms (5-8 degrees); the purple line shows the error after calibration using iSentek's algorithm (within 1 degree). The precision levels are on completely different scales.
Figure 1: Angle Error After Algorithm Calibration: The red and green lines represent the calibration results using a general manufacturer's algorithm (5-8 degrees); the purple line represents the calibration result using iSentek's algorithm (under 1 degree).
Conclusion
iSentek's smart door lock deadbolt solution can be implemented on a circuit board, without problems of optical sensing, magnetic sensing, or inclined tongue sensing solutions' lock tongue colliding with the door frame, causing damage or misjudging the status. This solution combines a high-precision, low magnetic hysteresis 3-axis magnetic sensor, dual verification by an accelerometer, and an easy-to-use calibration algorithm, meeting lock customers' market demands for precise and easy-to-install new generation smart door locks.
