基于创新的返回EMF的失速检测简化了步进电机设计

基于创新的返回EMF的失速检测简化了步进电机设计

By Dan Jacques, Allegro MicroSystems, LLC

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步进电机提供比传统的恒旋转电动机更具灵活性和控制，它们广泛应用于各种工业，消费和汽车应用。亚博尊贵会员许多应用需亚博尊贵会员要可靠地检测在不使用外部传感器的情况下达到机械端点，并且可以代替电动机失速检测。本文介绍了步进电机失速检测的创新方法及其应用。

Introduction to Stepper Motors

A general definition for a stepper motor is an electromechanical machine that moves a rotor shaft in small precise increments without feedback to govern motor speed. Stepper motors offer several advantages:

• 当负载波动时，它们表现出良好的速度稳定性，因为步进可以保持恒定的扭矩;
• 它们具有良好的启动特性，零速度的最大扭矩;
• they have wide dynamic ranges and can accelerate faster than servo motors;
• due to the narrowness of the step angle, stepper motors have small mechanical transient responses, which makes position and speed control possible without a complicated control loop.

因此，步进电机的驱动解决方案的成本非常实惠。

避压器发生了某些缺点;虽然，电子产品的进步已经有助于最大限度地减少它们的效果。开环操作无法提供有关绝对位置的信息或电机是否响应输入命令。如果电动机速度或绕组电流不正确控制，则共振会导致振动。如果速度太高，电机可能会失去步骤。

图1: Stepper Motors

步进电机有各种尺寸和功率水平，有许多选择踩踏性能的精度。

图2：控制器IC

具有全面的高度集成的控制器IC，可用于行业标准，低调，表面贴装封装，具有热耗散垫。

Requirement for Stall Detection

In practical systems, a means of stall detection is required to detect when the rotor is stationary, which can be caused by any of several conditions. Rotation of the electrical field generated by the driver may lose synchronicity with mechanical rotation of the stator, or the mechanical load may exceed the design limits of the motor. Any obstruction of the load path, including a fixed mechanical stop, also can cause the motor to stop rotating, but in these instances, without information on absolute position, the motor will attempt to drive through the obstruction in order to ensure that the load reaches the end point. This can cause wear, audible noise, heating, and mechanical failures. In addition, driving a stepper into a fixed stop by design inherently reduces the efficiency of the system, which is critical in battery-operated applications.

为了中等这些负面影响，可以使用电子集成失速检测功能来确保负载已达到其所需位置，或者如果负载受阻，则通知用户。当不需要绝对定位时，电子失速检测通常可以更换昂贵的滑动离合器或光学编码器以提供失速检测。需要这种类型的失速检测的亚博尊贵会员一些常见应用如图3所示。

图3: Typical Applications Requiring Stall Detection for Stepper Motors

如何摊位检测工作

Electronic stall detection works by measuring the effect of back EMF on the number of PWM cycles. When a motor is stopped or moving slowly, there is little back EMF to impede the current in the phase windings. This allows the current to rise to the limit quickly and the PWM current control to activate. However, when a motor is rotating at normal operating speeds, the back EMF generated by the fields of the magnetic poles passing over the phase windings acts against the supply voltage and reduces the rise time of the phase current. Therefore, the PWM current control takes longer to activate. Assuming a constant step rate, this results in fewer PWM cycles for each step of the motor.

可以在图4中可以看到这种效果。显示和偏移的两个相位，使得每个步骤覆盖。B相被延迟90电力。这允许直接比较绕组电流。当相B电流升高时，电机仍在运行正常，反电动势率采用以限制当前的上升时间。

The stall is applied at time t = –4 ms. A visual comparison shows that phase A current rises slightly faster causing the device to apply more PWM cycles to control the current. These additional cycles provide the count difference necessary to detect a stall condition. The stall is detected at time t = 0.

图4.: Typical Behavior of Stepper Motors

Method For Determining a Stall

每个电机绕组阶段有一个PWM ac的计数器cumulates the number of current limit events at each full step, from zero to full current. The allowable difference in counts is programmed into the IC onboard diagnostic register. A stall is detected when the count falls below the programmed value.

电子失速检测的相限

电子摊位检测有一些条件需要正常工作。在档位之前，电机必须足够快地踩到后部的EMF以降低相电流转换速率。另外，电机不能处于完整的步骤模式，相位电流方案必须符合在步骤0,16,32和48的0％和100％电流，并且两个阶段必须具有相同的轮廓。

Stall Detection Scenarios

有许多因素可以贡献摊位，因此使用高级IC可以正确评估失速信号，例如如图4所示的那些。在下面的附图中，呈现了两个替代方案，以及性能证明Allegro IC检测方法。

Hard (Locked) Stall Scenario

A spinning rotor was stopped approximately 2 ms before the stall detect signal indicated a fault by going low. Note how the phase current maintains its shape even though the quantity of PWM cycles has increased (see Figure 5).

图5.: Typical Hard Stall Behavior

软(部分)失速的场景

In many cases the rotor stall is not locked, and the rotor vibrates as the drive currents are applied. In these cases it can be difficult to detect a stall as it appears the motor is still moving. The Allegro stepper motor drivers achieve stall detection due to a differential technique used. When a partial stall is applied, the fault output continuously changes state, indicating that the rotor is going in and out of lock (refer to Figure 6).

图6：典型的软摊位行为

摊位检测产品

下表总结了具有失速检测的一些高级Allegro设备的功能。有关这些设备的其他信息可在Allegro网站www.allegromicro上提供。COM。