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

由Dan Jacques，All亚博棋牌游戏egro Microsystems，LLC

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Stepper motors provide significantly more flexibility and control than traditional continuously-rotating electric motors, and they are widely used in a variety of industrial, consumer, and automotive applications. Many applications require reliable detection of reaching a mechanical end point without using an external sensor, and motor stall detection can be used instead. This article describes an innovative method of stepper motor stall detection and its application.

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;
• 他们具有良好的启动特性,格言um torque at zero speed;
• they have wide dynamic ranges and can accelerate faster than servo motors;
• 由于步进角度的狭窄，步进电机具有小的机械瞬态响应，这使得不具有复杂的控制回路的位置和速度控制。

Thus, the cost of drive solutions for stepper motors is very affordable.

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。

Figure 1: Stepper Motors

Stepper motors come in a variety of sizes and power levels, with many options for precision of stepping performance.

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.

To moderate these negative effects, an electronic integrated stall detection function can be used to ensure the load has reached its desired position, or to notify the user if the load is obstructed. When absolute positioning is not required, electronic stall detection can often times replace a costly slip-clutch or optical encoder to provide stall detection. Some common applications requiring this type of stall detection are shown in Figure 3.

图3：需要停止检测步进电机的典型应用亚博尊贵会员

如何摊位检测工作

通过测量后EMF对PWM循环数量的影响，电子失速检测。当电动机停止或缓慢移动时，几乎没有后部的EMF可以阻止相绕组中的电流。这允许电流快速上升到极限，并将PWM电流控制激活。然而，当电动机以正常工作速度旋转时，由通过相位绕组的磁极的场产生的后部EMF通过电源电压作用并减小相电流的上升时间。因此，PWM电流控制需要更长时间才能激活。假设恒定的阶梯率，这导致电动机的每个步骤的PWM周期较少。

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.

Figure 4: Typical Behavior of Stepper Motors

确定摊位的方法

Each motor winding phase has a PWM counter that accumulates 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.

Phase Limitations of Electronic Stall Detection

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。

硬（锁定）摊位场景

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).

Figure 5: Typical Hard Stall Behavior

Soft (Partial) Stall Scenario

在许多情况下，转子失速未锁定，并且随着驱动电流施加时，转子振动。在这些情况下，在看起来时，可能难以检测失速，因为它看起来仍在移动。Allegro步进电机驱动器由于所使用的差分技术而实现失速检测。当施加部分档位时，故障输出连续变化状态，表示转子进入并锁定（参见图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。