Introduction to “Real-time Control” and Why It’s Important

Every day, consumers interact with a variety of systems that perform actions based on external conditions. Take a car, for example. When you step on the accelerator, the car accelerates almost instantly, that is, the moment you press the pedal, the car accelerates, with no noticeable delay.

To introduce the article topic from the car example, let's assume that the car is a system. When the external condition (driver) steps on the accelerator to increase the speed of the car, the system achieves the so-called "real-time control". Real-time control is the ability of a closed-loop system to collect data, process data, and update the system within a defined time window. If the system misses the defined time window, its stability, accuracy, and efficiency will be reduced. The reduction in control ability may affect the performance of the system; for example, the required speed cannot be achieved or even overheating. This article will introduce the functional blocks of real-time control systems and illustrate them with a robotic application as an example.

Although communication between system components does not have to be involved in system control, it should also play a role in the main control loop. The main functional blocks involved in real-time control are sensing (collecting data), control (interpreting and using data), and actuation (updating the system) (see Figure 1).

Figure 1: Main functional blocks of a real-time control loop

These sections are described in detail below.

• Sensing refers to measuring external factors such as voltage, current, motor speed or temperature. These critical parameters need to be measured accurately and precisely to provide the system with reliable data at a specific point in time.

• The central processing unit applies control techniques to the input data in order to calculate the next output command. Microcontrollers (MCUs) or controllers, such as C2000™ real-time MCUs, Sitara™ Arm®-based MCUs, integrated brushless DC motor drives, and DC/DC controllers, provide outstanding processing power to help ensure that the system meets the ultra-short time windows that are typically microseconds to milliseconds.

• Drive applies the calculated output command to the system, controlling the output. Changing the duty cycle of a pulse width modulator (PWM) unit that drives a power electronics system is an example of drive. TI products that help enhance drive performance include analog drivers, isolated gate drivers and gallium nitride (GaN) field effect transistors with integrated gate drivers.

• Finally, deterministic high-speed communication interfaces (such as fast serial interfaces or Ethernet) enable timely communication between the system and external devices or internal components.

Taking the robot as an example, real-time control can precisely control the position and speed of the motor, and the positioning accuracy of the robot arm is less than 100µm . This level of accuracy is achieved by continuously measuring the motor current and voltage as well as the motor position. The processing unit compares the measured values ​​with the calculated values, as shown in Figure 2, and then adjusts the PWM signal sent to the motor based on the comparison result. In addition, to meet the accuracy and time requirements of the system, the entire process needs to be completed within a few microseconds.

Figure 2: Fast current loop diagram

Real-time control is also fundamental to achieving efficient and reliable power systems. For example, real-time control helps charging stations maintain a stable output power and regulates the current flowing into the car battery to protect battery life and avoid overheating. Combining real-time control with new technologies such as MCUs and GaN can improve power density and efficiency, helping to minimize power losses in applications.

As the performance of modern motor drive systems continues to improve, so too do the demands placed on real-time control. For example, high-precision, high-speed computer numerically controlled machinery (the machinery that controls complex machine tools such as grinders and lathes) can achieve accuracies of less than 5µm at speeds exceeding 20,000RPM. This is only possible with very fast control loops, which means that the latency between signal measurement and system adjustment is typically less than 1µs.

Given the highly time-sensitive computational requirements, many designers have adopted a combination of field-programmable gate arrays, external fast analog-to-digital converters, and multiple MCUs. However, TI's C2000 MCUs and Sitara processors offer increased analog integration to execute current loops in less than 1µs, known as fast current loops. By taking advantage of fast current loops in modern control topologies, designers can develop smaller, higher-performance systems at lower costs.

Costs can be further reduced by using fully integrated solutions, such as TI's MCF8316 motor driver. These devices come with pre-programmed brushless DC motor control algorithms that require only fine-tuning, which can be implemented during the system design phase by configuring the integrated electrically erasable programmable read-only memory through the MCU's simple I2C interface. They also provide hardware configuration that allows system designers to tune the motor without an MCU. The MCF8316 integrates six metal oxide semiconductor field effect transistors that provide current to the motor, enabling a complete real-time motor control solution in a 7mm x 5mm package.

Real-time control is an important part of applications such as grid infrastructure, appliances, electric and hybrid electric vehicles, power transmission, motor drives and robotics. To further improve the response speed, all of the above applications need to continuously shorten the time window for performing actions. TI's full range of sensing, processing, control and communication technologies can provide high power efficiency and performance as well as low latency response time, which helps to achieve smaller and more reliable real-time control systems.

Additional Resources

• Refer to the application note “Basic Development Guidelines using C2000 Real-Time Microcontrollers”.

• Read the white paper “Time-Sensitive Networking for Industrial Automation.”

• View the Real-Time Control Reference Guide.

• Read the article “How MCUs can unlock the full potential of electrified designs.”

• Check out the dual-axis motor drive reference design using fast current loop (FCL) and SFRA on a single MCU.