High-Speed Displacement Detection Solution for Full-Screen Smartphone Pop-up Cameras
By Dr. Chiming Zhao, Senior Product and Marketing Manager, iSentek
The year 2018 kicked off the era of full-screen smartphones. vivo first launched the vivo NEX with an elevating front camera, followed closely by OPPO's release of the Find X motorized slider phone with a dual-rail periscope design. Tech giants like Huawei, Xiaomi, Lenovo, and Samsung also launched or planned pop-up camera phones. Full-screen phones with high screen-to-body ratios optimize aesthetics and the user experience, breaking free from complex lens designs like the "notch," "waterdrop," and "cyclops." However, they also introduce engineering challenges such as drop stability, abnormal feedback, and pop-up displacement control. iSentek's IST8801 linear Hall sensor provides a high-precision and fast-response displacement detection solution for pop-up cameras.
Implementation Methods of Pop-up Cameras
There are two common implementation methods for smartphone pop-up cameras: mechanical and motor-driven. Mechanical types, like the Xiaomi MIX 3, require users to manually slide the cover to pop up the camera. Motor-driven types, such as the vivo NEX and OPPO Find X, automatically control the motor for elevation and are typically equipped in higher-end models.
The motor-driven control in mainstream models uses a displacement sensing module—composed of a linear Hall sensor and a magnet—to assist the system in controlling the camera's elevation speed. Its main functions include: providing feedback when stalling or motor step-loss occurs, feeding back moving position to control speed, and detecting manual push-back resets. Since the detection working distance is very short, mostly within 1 centimeter, high precision and high-speed feedback capabilities are required.
Simple motor control relies on a set number of rotations for a stepper motor or an elevation time limit to achieve open-loop control of the camera's rise and fall. The drawback of this method is that if a stall or motor step-loss occurs, the camera cannot reach its designated position or will malfunction. In view of this, newer motor-driven cameras use closed-loop control to solve these problems: adding a displacement sensor and a magnet enables the motor's action to cooperate with the sensor, combining with algorithms and APPs to achieve a precisely operated user experience. The displacement status detected by the sensor informs the system to process the action through an interrupt function.
The detailed configuration of current pop-up camera displacement sensing mechanisms includes the following two types:
One magnet paired with one magnetic sensor: The advantages are low cost and a small footprint. However, the magnetic field created by the magnet will rapidly decay as the distance from the source increases; therefore, a single-sensor solution has limitations on the camera's travel distance. The sensor loses its sensing capability when it exceeds a certain distance from the magnet. Taking Figure 1 as an example, it can be seen that the sensed magnetic field strength drops significantly as the magnet's displacement distance increases. The situation depicted in the figure is only suitable for stroke detection under 4mm. A single-sensor configuration is also susceptible to external magnetic interference. With a magnetic interferer of sufficient strength, the displacement judgment will inevitably be inaccurate. Increasing the magnet's strength to counter this would increase the cost.
One magnet paired with two sensors: This solution can effectively solve the travel distance limitation and external interference problems. The linear movement range of the magnet is set between the two sensors. When the magnet moves away from one sensor (the sensor senses a weakening magnetic field), it is simultaneously approaching the other sensor (which senses an increasing magnetic field). The two sensors can relay this information to complete the magnetic field detection work within the range.
At the same time, the magnetic field strength value received by the system is calculated through the detection values of the two sensors. Subtracting the two detection values (Δ) yields the corresponding relationship between the magnetic field and distance, simultaneously eliminating external uniform magnetic field interference (and reducing interference for non-uniform magnetic fields). By adding the two detection values and then subtracting the absolute difference (Σ−abs(Δ)), the system can determine whether there is external magnetic interference. Taking Figure 2 as an example, if Σ−abs(Δ) is greater than 20G, it indicates there may be external magnetic field interference.
Regarding the selection of magnets in the above displacement sensing solutions (including magnet material, size, temperature stability, etc.), magnet N/S pole orientation, and the relative positioning of the sensor and magnet, iSentek provides comprehensive solution simulation, evaluation, and matching to meet the needs of different groups and product types. In addition, the algorithm services provided by iSentek can issue interrupt signals based on the needs of various scenarios, making judgments and handling situations such as stalling, idling, external force impacts, and magnetic interference, which improves motor safety and enhances the user experience.
Figure 1: One magnet and one sensor solution, detected magnetic field strength variation. Figure 2: One magnet and two sensors solution, detected magnetic field strength variation.
Performance Requirements for Linear Hall Sensors
The IST8801 linear Hall sensor provided by iSentek differs from traditional analog outputs by being a digital output chip. The primary difference is that the voltage signal output value of an analog output is voltage, which is easily subjected to interference; a digital output is less susceptible to interference, and the output data is relatively easier to process. iSentek's optimized circuit design can also compensate for linearity errors that may occur in the digital output during the data conversion process, providing a stable linear output.
Other important performance requirements for sensors include:
High Output Rate: The advantage of a high output rate is that it can feed back more data points to the system during the camera's elevation time, allowing for more precise control of the elevation speed and the camera's position. The maximum output rate of the IST8801 is 500Hz. Assuming the camera's elevation speed is 10mm/s, the detection accuracy can reach within 0.02mm. If the output rate is not high enough, the camera position registered by the system will not be precise, which may lead to incorrect actions, causing the camera to fall short of its designated position or causing the motor to continue running after the position is reached.
Interrupt Function: The IST8801 features a built-in interrupt function. Several thresholds can be set within the sensor's detection range. Once it is detected that the camera position has been reached or the value range has been hit a certain number of times (to avoid misjudgments caused by noise interference), the sensor will send an interrupt signal to the system. The system uses the feedback signal from the sensor to determine whether the scenario encountered by the motor is a stall, magnetic interference, or other anomalies, thereby deciding whether forward rotation, reverse rotation, stopping, or notifying other components to work is needed. The interrupt function also greatly helps in saving the power consumption of the entire system.
High Sensitivity and Low Noise: During the displacement of the pop-up camera, the signal change is weak in the section where the magnet is farther away from the sensor. Sufficiently high sensitivity is required to sense minute signal changes. Moreover, minute signals are easily susceptible to noise interference, causing misjudgments. When the noise level is close to or greater than the signal change size, the system cannot determine whether the signal change is caused by camera displacement or noise influence. At this point, low noise characteristics become critical. Under a 40mT dynamic range, the IST8801 has a sensitivity of 0.2 LSB/µT (16 times that of competing products). The optimized chip design reduces noise to only about 3µT, making it clearly distinguishable from signal changes.
Miniature Chip Size: For smartphones that strive to be "light and thin," every inch of internal mechanical design is precious. The continuously increasing functional components make the mechanical layout extremely challenging. Taking the vivo NEX as an example, the motor device of the pop-up camera takes up one-third to one-half of the circuit board area. Integrating the electronic components originally required by the phone into the remaining space is a daunting challenge for mechanical engineers. iSentek can provide highly efficient, small-size chips (1.28 x 0.88 x 0.53 mm³). When paired with the placement configuration, magnet orientation, and magnet selection design services provided by the overall solution, it compresses the module volume, increases the flexibility of the overall system's mechanical layout, and effectively reduces the design burden on mechanical engineers.
Pop-up Phones Still Have Room for Growth in the Future
In the battle of full-screen smartphone pop-up cameras, domestic smartphone brands have reversed the decline, pivoting to lead international trends and guiding major foreign brands to be the first to realize new concepts. In the future, driven by the demands for a concise appearance and improved user experience in consumer electronics, hiding complex accessories within the sleek exterior of mobile phones through technological means has become an inevitable trend. iSentek is deeply rooted in magnetic sensor technology, possessing proprietary chip design, manufacturing management, proprietary IP, and system algorithm service capabilities, allowing us to provide customized solutions for different applications and assist in the realization of pop-up functions across various fields.