Stepper motors are unique in their ability to convert digital control signals—specifically electric pulse signals—into precise angular or linear displacement. This makes them a key actuator for digital-to-analog conversion. One of their major advantages is the capability to perform open-loop position control, where a pulse signal is used to achieve a specific position increment. Compared to traditional DC servo systems, this incremental control approach significantly reduces costs and eliminates the need for complex system adjustments. As a result, stepper motors are widely used in fields such as CNC machines, robotics, remote control, and aerospace. With the development of microcomputers and microelectronics, their applications have become even more widespread.
The speed of a stepper motor is determined by the pulse frequency, the number of rotor teeth, and the number of steps per revolution. Its angular velocity is directly proportional to the pulse frequency and synchronized with the pulse timing. Therefore, by controlling the pulse frequency, a desired speed can be achieved when the number of rotor teeth and steps remain constant. However, since the motor starts based on its synchronous torque, the starting frequency must not be too high to avoid losing synchronization. As power increases, the rotor's inertia also increases, leading to a significant difference between the starting frequency and the maximum operating frequency—sometimes up to ten times.
To fully utilize the motor’s fast performance, it is typically started below the starting frequency and then gradually accelerated until the desired speed is reached. The rate of acceleration is carefully chosen to ensure the motor does not lose synchronization while minimizing the start-up time. To maintain positioning accuracy, the motor must be gradually decelerated to a stop speed before stopping, which is equal to or slightly higher than the starting speed. Thus, when a stepper motor is moving a load at high speed and needs to stop accurately, it generally goes through five stages: "start-acceleration-high-speed operation (constant speed)-deceleration-stop." The speed profile is usually trapezoidal for longer distances and triangular for shorter ones, as shown in Figure 1.
The control system for a stepper motor involves assigning an initial value to the 8253 counter on the hardware circuit, allowing the system to manage the frequency changes during acceleration and deceleration. This prevents the motor from losing synchronization. For example, during point control, the motor generates enough torque to drive the load and follow the specified speed and acceleration. During deceleration, the system ensures no overshoot occurs, and the motor stops precisely at the intended position. The 8253 timer generates a square wave pulse that triggers the subdivision drive circuit, which then amplifies the signal to move the motor. Direction changes and starting/stopping are controlled by the computer through the hardware circuit.
This combination of software and hardware offers a simple and efficient control solution. The microcomputer program uses minimal memory, and the programming is not constrained by timing. As long as external interrupts are enabled, the computer can perform other tasks between each step, making it possible to control multiple stepper motors simultaneously.
To determine the initial value of the timer, the PC generates a square wave using the 8253 timer. Counter 0 operates in mode 0 to produce the pulse waveform. Counter 1 works in mode 1, counting the clock frequency provided by a 2 MHz crystal oscillator. If the initial value assigned to counter 0 is D1, the generated square wave frequency is f1 = f0 / D1, and the period is T1 = 1/f1 = D1/f0. Therefore, D1 = f0 * T1 = f0 / f1, where f1 is the starting frequency and f0 is the crystal frequency.
A mathematical model can describe the stepping motor's speed during acceleration. To prevent the motor from losing steps, the maximum operating frequency should be less than or equal to the step response frequency fs. There are two common driving modes: triangular and trapezoidal. Trapezoidal is the general case, so we focus on that. Acceleration and deceleration are managed by adjusting the timer's initial value. In the acceleration phase, the timer's initial value decreases, increasing the pulse frequency. In the deceleration phase, the initial value increases, reducing the frequency.
Assuming the number of pulses is counted from the start, the nth pulse period can be expressed as:
Tn = T1 - (n-1)Δ/f0
Where Δ is the decrement in the timer's initial value. This leads to the following expressions for pulse timing and frequency:
tn = T1 + (n-1)Δ/f0
fn = 1 / [T1 - (n-1)Δ/f0]
By simplifying these equations, a quadratic relationship emerges between the pulse frequency and time, which allows for smooth acceleration and improved positioning accuracy. Similarly, during deceleration, the pulse frequency decreases quadratically, ensuring a smooth stop without overshoot.
Overall, the pulse frequency characteristics of this design provide a reliable and efficient way to control the motor’s motion, as illustrated in Figure 3. This method has been successfully implemented in an intelligent motion control unit I developed. Using VC++ to create a user-friendly interface under Windows, I was able to easily control movement patterns, speed, acceleration, and positioning. The system also reduces the PC's workload, allowing it to handle other tasks while the motor runs. Additionally, the use of subdivision drive technology enhances the motor's precision and positioning accuracy.
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