Abstract : Taking a fitness device with a power of 3-5 kW as the object, this paper draws on the technology of related foreign products and conducts market research to ensure the market advantage of the control unit's performance-price ratio. This control unit fully utilizes the resources of a microcontroller and an AC variable frequency speed control device, and realizes the development of a speed control control unit for the fitness device by controlling the frequency converter through the microcontroller. This system has the advantages of simple control circuit, high speed control accuracy, and low cost. Keywords : Fitness device; Frequency converter; Microcontroller; Communication protocol 0 Introduction With the continuous improvement of living standards, people have higher requirements for health. Fitness devices such as electric treadmills are widely purchased due to their simple form and suitability for indoor exercise in homes and gyms. Their drive motors include DC motors and AC motors, and the speed control unit is the core to ensure the good performance of the fitness device. Currently, AC variable frequency speed control systems are favored in industrial production due to their excellent speed control and braking performance, high efficiency, high power factor, and low motor cost. This paper uses a FR-E520 low-power Mitsubishi inverter and an AT89C51 microcontroller as the human-machine interface. Based on the frequent and pulsating load changes of the moving device, a nonlinear variable load control method is adopted to achieve digital AC variable frequency speed closed-loop control. This allows the moving device to automatically and smoothly accelerate and decelerate, achieving stepless speed regulation with an accuracy of 0.5% to 1%. This speed control unit, with its advantages of high speed regulation accuracy, simple operation, and personalized settings and monitoring, realizes the commercialization of variable frequency speed regulation technology. 1 System Design This system uses the AT89C51 as the human-machine interface control component. Through an RS-485 communication interface, it sets the FR-E520 Mitsubishi inverter, forming a speed closed-loop control, which is a typical DDC (Direct Digital Control) system. This control system can realize various motion modes, speed settings and displays, and can accurately control the rotational speed. The system hardware block diagram is shown in Figure 1. To improve the anti-interference capability of the microcontroller and ensure operational safety, the communication circuit between the microcontroller and the frequency converter adopts opto-isolation. The system hardware mainly consists of three parts. Read the full article: Development of a Speed ​​Control Unit for Fitness Exercise Devices