Views: 0 Author: Site Editor Publish Time: 2024-11-26 Origin: Site
As a vital actuator in mechatronics, the stepper motor holds a prominent position among key products and finds extensive application in numerous automation control systems. With advancements in microelectronics and computer technology, the demand for stepper motors has been escalating daily, spanning across various sectors of the national economy.
A stepper motor is an electromechanical device capable of directly converting electrical pulses into mechanical movements. By manipulating the sequence, frequency, and count of the electrical pulses supplied to its coil, one can control the direction, speed, and rotation angle of the stepper motor. Notably, precise position and speed control can be achieved without resorting to a complex, closed-loop feedback control system involving position sensing. Instead, a simple and cost-effective open-loop control system, comprising the stepper motor and its corresponding driver, suffices.
Based on the external control pulses and direction signals, the stepper motor driver precisely controls the stepper motor's windings to be energized in a specific timing sequence in forward or reverse direction through its built-in logic circuit, so that the motor can realize forward/reverse rotation or locked state.
Take a 1.8 degree two-phase stepper motor as an example for illustration: when both windings are energized and excited at the same time, the output axis of the motor will remain stationary and locked at the current position. The maximum torque to keep the motor locked at the rated current is the holding torque. If the direction of current in one of the windings is changed, the motor will rotate by a step angle (1.8 degrees) in a specific direction.
Similarly, if the direction of the current in the other winding also changes, the motor will rotate by one step angle (1.8 degrees) in the direction opposite to the previous direction. When the currents in the coil windings are excited by sequential changes in direction, the motor will rotate in successive steps in the given direction with great accuracy. For a 1.8 degree two-phase stepper motor, 200 step angles are required to complete one week of rotation.
Two-phase stepper motors are categorized into two winding types: bipolar and unipolar. Bipolar motors have only one winding coil per phase, to realize the continuous rotation of the motor, the need for the same coil of current in the order of variable excitation, the design of the drive circuit needs to be equipped with eight electronic switches in order to achieve the order of switching. Unipolar motors have two winding coils of opposite polarity in each phase, and by alternately exciting the two winding coils on the same phase, the motor can realize continuous rotation. The design of the drive circuit requires only four electronic switches. In the bipolar drive mode, the motor's output torque is increased by approximately 40% compared to the unipolar drive mode because the winding coils on each phase are excited by 100%.
Moment load (Tf)
Tf = G * r
G: Load weight
r: radius
Inertia load (TJ)
TJ = J * dw/dt
J = M * (R12+R22) / 2 (Kg * cm)
M: Load mass
R1: Radius of outer ring
R2: Radius of the inner ring
dω/dt: Angular acceleration
The speed-torque relationship curve serves as a crucial indicator of the output characteristics of stepper motors.
The speed value of the stepper motor motor at a certain point.
n = q * Hz / (360 * D)
n: rev/sec
Hz: Frequency value
D: Drive circuit interpolation value
q: stepper motor step angle
For example, a stepper motor with a pitch angle of 1.8°, with a 1/2 interpolation drive (i.e., 0.9° per step), has a speed of 1.25 r/s at an operating frequency of 500 Hz.
In this zone, the stepper motor cannot be directly started or stopped. Instead, it must first traverse the self-starting zone and then accelerate to reach the operational zone. Similarly, braking in this zone cannot be done directly to avoid causing the stepper motor to lose synchronization. It must first decelerate into the self-starting zone before braking.
The maximum pulse frequency at which the stepper motor can start and operate without losing synchronization, when the motor is in a no-load state.
The highest pulse frequency that the motor can be excited to run at without losing a step, under no-load conditions.
The maximum load torque that the stepper motor can handle to start and initiate movement at a specific pulse frequency, without losing synchronization.
The maximum load torque that allows the stepper motor to operate stably at a specific pulse frequency, without experiencing any loss of synchronization.
When the stepper motor's operating frequency is within the continuous operation region of the speed-torque curve, it is crucial to minimize the acceleration and deceleration times during startup and shutdown. This ensures that the motor operates for a longer duration in its optimal speed range, thereby enhancing its effective running time.
As illustrated in the diagram below, the stepper motor's dynamic torque characteristic curve remains as a horizontal straight line at lower speeds. However, at higher speeds, the curve decreases exponentially due to the impact of inductance.
We know that the stepper motor load is TL, suppose we want to accelerate from F0 to F1 in the shortest time (t r), how to calculate the shortest time t r ?
(1) Normally, TJ = 70% Tm
(2) tr = 1.8 * 10 -5 * J * q * (F1-F0)/(TJ -TL)
(3) F (t) = (F1-F0) * t/tr + F0, 0<t<tr
Exponential acceleration in high speed condition
(1) Normally
TJ0 = 70%Tm0
TJ1 = 70%Tm1
TL = 60%Tm1
(2)
tr = F4 * In [(TJ 0-TL)/(TJ 1-TL)]
(3)
F (t) = F2 * [1 - e^(-t/F4)] + F0, 0<t<tr
F2 = (TL-TJ 0) * (F1-F0)/TJ 1-TJ 0)
F4 = 1.8 * 10-5 * J * q * F2/(TJ 0-TL)
Notes.
J indicates the rotational inertia of the motor rotor under load.
q is the rotation angle of each step, which is the step angle of the stepper motor in the case of the whole drive.
In the deceleration operation, just reverse the above acceleration pulse frequency can be calculated.
In general, when a stepper motor operates under no-load conditions and its operating frequency approaches or equals the rotor's inherent frequency, resonance may occur, leading to severe out-of-step phenomena.
Ensure that the motor's operating frequency does not fall within the resonance range to prevent resonance from occurring.
Utilize a micro-step driving mode to reduce vibrations by dividing the original single step into multiple smaller steps, thereby enhancing the resolution of each motor step. This can be achieved by adjusting the phase-to-current ratio of the motor. It's worth noting that microstepping does not improve the step angle accuracy but enables smoother motor operation with reduced noise. Typically, the torque is approximately 15% lower during half-step operation compared to full-step operation, and it decreases by 30% when using sine wave current control.
