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The Parfocal Calibration and Fast Compensation of High Zoom Ratio Infrared Continuous Zoom Lens -Hardware Circuit Design

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The Parfocal Calibration and Fast Compensation of High Zoom Ratio Infrared Continuous Zoom Lens -Hardware Circuit Design

Issue Time:2021-10-08
one of the parfocality methods-hardware circuit design

Infrared continuous zoom lenses generally use a spatial cam method for zooming, integrating a continuous gradual field of view, which is convenient for rapid conversion between the three functions of search, monitoring, and tracking. Compared with the use of multiple lens methods, continuous zoom lenses are not only small in size and light in weight, which is conducive to system integration but also can save a lot of costs.

However, due to mechanical processing and assembly errors, it is difficult to guarantee the parfocality of the infrared continuous zoom lens (clear imaging at any position of the continuous zoom). Even if the parfocal adjustment reaches the standard at room temperature (25°C), due to the influence of assembly stress and thermal stress, and with the change of the ambient temperature, the parfocality of the continuous zoom lens will change greatly.

High zoom ratio infrared continuous zoom lens is more sensitive to temperature. The greater the difference between the ambient temperature and the normal temperature, the more obvious the deterioration of parfocality. Therefore, a continuous zoom infrared lens with a large zoom ratio cannot guarantee parfocality.

In order to ensure the parfocality of a continuous infrared zoom lens with a large zoom ratio, it is necessary to achieve rapid compensation during the zooming process to eliminate the non-parfocality of different zoom positions at different temperatures.

There are two commonly used compensation methods, one is to use autofocus, and the other is to use temperature compensation. The auto-focus method must be performed after the end of the zoom. Based on the monotonicity of the image gray gradient, the auto-focus reliability is low for moving targets, but this method does not require calibration, and the installation cost is lower.

The temperature compensation method needs to be quickly compensated according to the previously calibrated reference value, with fast compensation speed, high reliability, and easy tracking. However, when fitting the position points, it is necessary to calibrate at different ambient temperatures, which is complicated to debug, and for targets less than the minimum imaging distance, the calibration value cannot be clearly imaged, and further focusing is required.

In order to ensure the parfocality of the continuous zoom infrared lens with a large zoom ratio, this article will specifically introduce one of the parfocal methods-hardware circuit design, so that the continuous zoom infrared lens with a large zoom ratio can be used in different environments, which can effectively align the target.

The control circuit of the infrared continuous zoom lens mainly includes the following parts:

Power management

Power management provides stable power for the entire circuit, mainly using switching power supply to provide power for FPGA, MCU, motor, and auxiliary circuits; while the sampling circuit part (including the analog circuit, reference source, and temperature sensor in GD32F450I, GD32E103T8U6) uses LDO Linear power supply. In this way, the power supply efficiency can be improved, and the sampling accuracy can be guaranteed.

Zoom control and compensation control

Since the compensation circuit needs to be controlled in real-time during the zooming process, in order to improve the control accuracy, two MCUs are used to control the compensation circuit separately.

The main control adopts CORTEX M4 chip GD32F450I, the main frequency is 200 M, and it can be overclocked to 400 M. The ARM simplified instruction set control, with 512K of SRAM and 1792K of FLASH, can satisfy the storage of the calibration position.

Using incremental encoder, with higher resolution, better accuracy can be obtained. A sampling rate of 20 M is used to ensure that the compensation accuracy meets the requirements during high-speed compensation. The auxiliary control adopts the CORTEXM4 chip GD32E103T. The speed of the zoom motor is slower than that of the compensation motor. The zoom position sampling can use either an encoder or a potentiometer.

The potentiometer is convenient to control, but the sampling time is longer, and the encoder needs to be initialized, but the calculation is simple, the sampling time is shorter, and the accuracy is higher. The zoom encoder uses a sampling rate of 6M.

In the process of zooming, search the compensation table in advance to find out the design point of the out-of-tolerance compensation position (it can be set according to different zoom segments, the larger value is set for short focal length, and the smaller value is set for long focal length).

The current zoom value needs to be transmitted from the auxiliary control to the main control in advance, and the main control can quickly pre-compensate according to the current position and accurately compensate at the stopping point or the final position.

Pre-compensation is mainly to complete the parfocal adjustment of the infrared continuous zoom lens with a large zoom ratio to ensure that the image of the continuous zoom lens is clearer during the continuous zooming process.

FPGA circuit

FPGA circuit is mainly used to calculate the gray gradient of the current image, and transfer the gray gradient value to GD32F450I according to the frame rate
In the main control unit, the data is transmitted in DMA mode. This frame rate clock is used for GD32F450 synchronous sampling and reading the current encoder position value.

This position can be used to verify whether the compensation movement is normal. If the gray gradient is not input in the hill-climbing mode, the GD32F450I main control unit will determine that the current target is not at the infinity position, or the current target is not at the set value of the farthest focal length, and the GD32F450I main control the unit will change the motion control algorithm and switch to autofocus mode control.

The autofocus algorithm is mainly used to verify the compensation control motion algorithm. If the image gray gradient sorting is normal, the temperature compensation control algorithm will complete the parfocality control. If the current lens is in tracking mode, the continuous zoom lens can only use the compensation control algorithm for parfocality compensation.

Temperature check

The temperature measurement circuit is mainly used to measure the temperature of different positions. The large zoom lens is more sensitive to temperature than the continuous zoom lens, so needs to measure multiple location points, the use of the multi-point average temperature method can better reflect the current ambient temperature. In actual use, the three temperature measurement points are also used for redundant design. When a certain temperature point is abnormal, a 2 to 1 method is used to determine the current temperature.

Serial control

The serial port control circuit is used to communicate with the system, receive system commands, and return to the current state.

Through the above design of the hardware circuit, it is possible to efficiently meet the goal of parfocality of the infrared continuous zoom lens with a large zoom ratio. The infrared continuous zoom lens designed and manufactured by Quanhom can effectively focus on the target even in harsh environments. And we also take into account the lightweight and cost of the product, which is very suitable for remote monitoring and homeland security, and the product supports the SXGA (1280x1024 12μm) format.

If you want to know more about infrared continuous zoom lenses after reading the above, please contact Quanhom for professional advice.

As a leading manufacturer of Opto-electromechanical components, we have a professional and experienced professional team that continuously develops first-class new infrared technology, and has decades of experience in designing complex infrared optical-related products. Our high-quality thermal infrared lenses (LWIR, MWIR, and SWIR) are well received by users, and our thoughtful one-stop service has also won praise and trust from many customers. If you want to buy our infrared continuous zoom lens, please contact us immediately!