The use of video surveillance continues to grow, partly due to the development of artificial intelligence (AI) led by various "smart city" innovation programs, which include intelligent and automated monitoring of public streets, alleys, and gathering places. Security applications for video surveillance are also increasing in enclosed areas such as offices, retail stores, residential lobbies, supermarkets, museums, construction sites, industrial environments, and warehouses. This widespread use, coupled with the requirements of AI analysis, means that designers are competing to improve system efficiency and performance while also reducing costs.
With the help of compact, low-power, sensitive, and high-resolution imaging ICs, combined with intelligent and precise motion control systems, most of these improvement goals can be achieved. By utilizing the elements of this method, designers can achieve efficient remote video surveillance, which reduces the need for personnel to physically inspect areas or locations due to blurred images or events occurring outside the camera's line of sight.
However, like any growing application field, there are still various technological challenges that need to be overcome, and many of them can be solved by directly using high-energy electronic subsystems for camera pan, tilt, and zoom (PTZ).
This article explores the role of PTZ in monitoring and discusses how high energy efficiency, precision, and low-power motors and motion control electronic devices used to control PTZ functions are key to implementing video monitoring systems.
Strengthen effective monitoring through PTZ motion control
Whether used for security facilities or process monitoring, modern video surveillance systems are no longer just cameras pointing at the target area in a fixed direction. On the contrary, AI can reduce false positives and ensure optimal deployment of resources, thereby more efficiently utilizing captured images, while the use of electric PTZ allows the camera to scan left and right (pan) and move up and down (tilt), thereby redefining the monitoring area. AI and PTZ both contribute to achieving more efficient and universally 'environmentally friendly' monitoring methods. For PTZ, according to the system design, motion can be independently guided by camera components, remotely controlled by the security system, and even remotely manually operated.
This method of moving the camera through translation and tilt overcomes the tradeoff dilemma of using a wide-angle lens and a wide field of view (FOV) (that is, larger areas can be captured at the expense of scene details and curvature distortion). The PTZ function also helps to save on security system costs, as one camera can complete many static camera tasks.
The movement of the camera can be guided through different technologies. Surveillance cameras with PTZ function usually also support multiple preset positions, where users can specify the positions to be monitored, as well as the predetermined sequence and step time from one position to another. This can achieve remote monitoring of a wide area without user input.
Matching of Electronic Devices with PTZ Motors
Although motion control is the core of PTZ implementation, an important factor in efficient PTZ systems is the smooth and accurate tracking achieved through excellent motor control. Designers can consider using both brushless DC motors and more challenging but often advantageous stepper motors to achieve high accuracy, as well as utilizing ADI's Trinamic technology and IC to achieve necessary smoothness and accuracy.
Low power operation is also crucial. Many surveillance cameras equipped with precise PTZ control function are now devices that support Power over Ethernet (PoE). The latest PoE standard (IEEE 802.3bt-2018) supports up to 100 W of power supply per Ethernet cable connection.
The designers of the PTZ system have three types of motors to choose from, and the type chosen determines the control IC to be used. The choices include classic brushless DC motors, brushless DC motors (BLDC), and stepper motors.
Each motor configuration has its own advantages and disadvantages in terms of functionality, performance, and management/control requirements:
Brushed DC motors were the earliest developed DC motors and have been successfully used for over 100 years. It is designed simply, but difficult to control, and is most suitable for open and free operation, rather than precise positioning or stop and go operations. In addition, the brushes of brush motors can be subject to wear, reliability issues, and may generate unacceptable electromagnetic interference (EMI). Although this type of motor is still used in low-cost mass market applications such as toys, and even some high-end applications such as medical infusion pumps, it is usually not a feasible choice for PTZ design.
BLDC motors (also known as electronic commutation or EC motors) are suitable for closed-loop designs with position sensors, which can also be used for speed control. It has fast speed and long service life, while also encapsulating high power density.
Trinamic's TMC4671-LA multiphase servo controller/motor driver is an IC specifically designed for this task, which provides an embedded FOC algorithm for BLDC motor hard connection.
This device can also be used for other types of motors, such as permanent magnet synchronous motors (PMSM), two-phase stepper motors, DC motors, and voice coil actuators. Please note that the difference between BLDC motors and PMSM is that the former is a direct current (DC) motor, while PMSM is an alternating current (AC) motor. Therefore, a BLDC motor is an electronic commutated DC motor without a physical commutator component; On the contrary, PMSM is an AC synchronous motor that uses permanent magnets to provide the necessary magnetic field excitation.
TMC4671-LA communicates with its microcontroller using a basic SPI or UART interface. This device implements all necessary control functions and characteristics in terms of hardware, and also has error/fault condition monitoring function. It includes an integrated analog-to-digital converter (ADC), position sensor interface, position interpolator, and other necessary functions, making it a complete controller suitable for various servo applications.
In order to address the challenges of BLDC motor control, this functionality is crucial as these algorithms are very complex. Fortunately, complex details are entirely handled by the IC, so these details do not burden design engineers or system microcontrollers.
Its control loop frequency is 100 kHz, four times higher than the 20 kHz frequency of many BLDC controllers, which can bring key advantages, including faster stabilization time, faster response to torque control commands, better position stability, and lower risk of overcurrent conditions. This type of risk may damage the motor driver or motor.
Stepper motor is an alternative to BLDC motor. This type of motor is very suitable for open loop positioning or speed operation, and can provide high torque at low and medium speeds. Generally speaking, stepper motors with comparable performance are cheaper than BLDC motors, but stepper motors have operational challenges that must be addressed.
At first glance, the signal path process of a stepper motor controller seems simpler than that of a BLDC motor controller. Although this is true in some aspects, a precise and effective stepper motor controller must provide specific functions to meet the needs of the motor.
TMC5130A is a high-performance controller and driver IC with a serial communication interface, designed specifically for two-phase stepper motors. This type of IC aims to minimize or eliminate related issues to the greatest extent possible.
This device combines a flexible slope generator for automatic target positioning with a highly advanced stepper motor driver. It also includes an internal MOSFET that can directly provide up to 2 A of coil current (peak 2.5 A) and has a resolution of 256 microsteps/full steps.
However, the TMC5130A goes beyond the basic stepper motor drive function as it addresses some of the challenges designers face when deciding to use this type of motor. The two most obvious and noteworthy issues are the noise generated by the motor during stepping and the "smoothness" of motor operation. Although these may not be issues in industrial applications and other environments, they may be unsettling and even counterproductive in the use of PTZ monitoring.
For the first challenge, TMC5130A implemented StealthChop, a proprietary voltage based pulse width modulation (PWM) chopper that can modulate current based on duty cycle. This feature is optimized for medium to low speeds, which can significantly reduce noise.
For the second challenge, TMC5130A adopts SpreadCycle, a proprietary current chopping technology. This periodic current driven chopping scheme achieves slow attenuation of the driving phase, thereby reducing power loss and torque fluctuations. This technology uses a hysteresis based motor current and target current averaging method to generate a sinusoidal motor current, even at high speeds.
Other unique features of the TMC5130A include its StallGuard motor stall detection and CoolStep dynamic adaptive current drive, with the latter utilizing the former.
StallGuard provides sensorless load detection through Counter-electromotive force (EMF), and can stop the motor in a whole step to protect the motor driver and motor. Another advantage is that its sensitivity can be adjusted to meet the requirements of the application. CoolStep adjusts the motor current according to the reading of Counter-electromotive force StallGuard. Under low load conditions, the motor current can be reduced by 75%, thereby saving electricity and reducing heat generation.
When driving two two-phase stepper motors instead of one, TMC5072 provides many of the same functions as TMC5130A. This device can drive two independent coils, each with a current of up to 1.1 A (peak 1.5 A); Two drivers can also operate in parallel, providing a current of 2.2 A (peak 3 A) to a single coil.
FOC changes the situation
Another issue is the position feedback of the motor. The stepper motor does not require feedback, but feedback is often added to ensure high-precision control, while BLDC design requires feedback. Feedback is generally realized by encoder (usually based on Hall effect sensor or optical encoder), but it is limited by the update rate and resolution, as well as the additional processing burden on the system.
For BLDC motors, there is another control option. Field Oriented Control (FOC), also known as Vector Control (VC), is designed to address issues related to feedback update rate and resolution, as well as encoder cost and installation issues.
In short, FOC is a current regulation scheme for motors that utilizes magnetic field direction and motor rotor position for regulation. FOC is based on the "simple" observation that there are two components acting on the rotor of the motor. A component, called a direct component or ID, is simply pulled radially; The other component, namely the orthogonal component or IQ, applies torque through tangential pulling.
The ideal FOC provides closed-loop control of current, generating pure torque generated current (IQ) without direct current ID. Then, adjust the strength of the drive current to make the motor provide the target torque. One of the many characteristics of FOC is that it can maximize active power and minimize idle power.
FOC is a high-efficiency motor control method. This method works well under high motor dynamics and high speed conditions, and enhances its inherent safety function due to its closed-loop control characteristics. FOC uses standard resistive current sensing to measure the current intensity and phase passing through the stator coil, as well as the angle of the rotor. Then, adjust the measured rotor angle to the magnetic axis. The angle of the rotor is measured using Hall sensors or position encoders, so the direction of the rotor's magnetic field can be known.
However, it takes a long and extremely complex process from obtaining observations from FOC to forming a complete motor control scheme. FOC needs to understand some static parameters, including the number of motor pole pairs, the number of encoder pulses per revolution, the direction of the encoder relative to the rotor magnetic axis and the counting direction of the encoder, as well as some dynamic parameters such as phase current and rotor direction.
In addition, the adjustment of the proportional and integral (P and I) parameters of the two PI controllers used for phase current closed-loop control depends on the electrical parameters of the motor. These parameters include resistance, inductance, Counter-electromotive force constant of the motor (also the torque constant of the motor) and supply voltage.
The challenge faced by designers when applying FOC is that all parameters have high degrees of freedom. Although the flowchart and even source code of FOC are easy to obtain, the actual "deliverable" code required to implement FOC is complex and precise. It includes a variety of coordinate transformations, such as Clark transformation, Parker transformation, inverse Parker transformation and inverse Clark transformation (reduced to a group of Matrix multiplication), as well as intensive repeated calculations and operations. There are many FOC tutorials online, ranging from qualitative non equation/simple tutorials to complex math tutorials; The specifications of TMC4671 are in the middle and worth reading.
Attempting to implement FOC through firmware requires a significant amount of CPU computing power and resources, thus limiting the choice of processors for designers. However, with the help of TMC4671, designers can choose from a wider range of microprocessors and even low-end microcontrollers, while also avoiding coding issues such as interrupt handling and direct memory access. All that is needed is to connect to TMC4671 through its SPI (or UART) communication port, as programming and software design have been simplified to just initialize and set target parameters.
Don't forget the drive
Although some motor control ICs (such as TMC5130A and TMC5072 for stepper motors) integrate a motor gate driver function with a driving current of approximately 2 A, other ICs (such as TMC4671-LA for BLDC motors) do not. For these situations, devices such as TMC6100-LA-T half bridge gate driver ICs have increased the required capabilities. This three-layer half bridge MOSFET gate driver adopts 7 × The 7 mm QFN package provides a driving current of up to 1.5 A, suitable for driving external MOSFETs that handle coil currents of up to 100 A.
TMC6100-LA-T has the function of software control for driving current, and its settings can be optimized within the system. The device also includes programmable safety functions such as short circuit detection and overheating threshold; Paired with the SPI interface for diagnosis, it supports robust and reliable design.
In order to further accelerate product launch time, facilitate parameter optimization and fine tuning of drivers, Trinamic has provided the TMC6100-EVAL universal evaluation board. This evaluation board is convenient for hardware processing and comes with user-friendly evaluation software tools. The system consists of three parts: a backplane, a connector board TMC6100-EVAL with several Test point, and a TMC4671-EVAL FOC controller.