Innovative Back EMF Based Stall Detection Simplifies Stepper Motor Designs

Innovative Back EMF Based Stall Detection Simplifies Stepper Motor Designs

由Dan Jacques，All雷竞技竞猜下载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.

步进电机简介

步进电机的一般定义是一种机电机，可以小于精确增量的转子轴，无需反馈以控制电机速度。步进电机提供了几个优点：

• they exhibit good speed stability when the load fluctuates, because the stepper can maintain constant torque;
• they have good startup characteristics, with maximum torque at zero speed;
• 它们具有宽的动态范围，可以比伺服电机更快地加速;
• 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.

Certain disadvantages occur with steppers; although, advancements in electronics have helped to minimize their effects. Open loop operation fails to provide information about absolute position or whether the motor is responding to input commands. Resonance can cause vibration if motor speed or winding current is not controlled properly. Motors can lose step if the speed is too high.

图1：步进电机

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

图2.: Controller ICs

Full-featured, highly integrated controller ICs are available in industry-standard, low-profile, surface-mount packages with thermal dissipation pads.

Requirement for Stall Detection

在实际系统中，需要一种失速检测装置来检测转子何时静止，这可能是由几种条件中的任何一个引起的。由驾驶员产生的电场的旋转可能丢失具有定子的机械旋转的同步性，或者机械负载可能超过电动机的设计限制。负载路径的任何阻塞，包括固定机械停止，也可以使电动机停止旋转，但在这些情况下，没有关于绝对位置的信息，电机将试图通过阻塞驾驶，以确保负荷达到终点。这可能导致磨损，可听噪声，加热和机械故障。此外，通过设计将步进进入固定的停止固有地降低了系统的效率，这在电池操作的应用中是至关重要的。雷竞技最新网址

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雷竞技最新网址

How Stall Detection Works

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。

This effect can be seen in Figure 4. Two phases of the winding current are shown and offset so that each step is shown overlaid. Phase B is delayed 90 electrical degrees. This allows direct comparison of the winding current. When phase B current is rising, the motor is still running normally and back EMF acts to limit the current rise time.

在时间t = -4 ms施加失速。视觉比较表明，电流稍微升高，使设备施加更多PWM周期以控制电流。这些附加循环提供了检测失速条件所需的计数差异。在时间t = 0检测失速。

图4：步进电机的典型行为

确定摊位的方法

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.

Phase Limitations of Electronic Stall Detection

There are a few conditions required for electronic stall detection to work properly. Before the stall, the motor must have been stepping fast enough for the back EMF to reduce the phase current slew rate. In addition, the motor cannot be in full step mode, the phase current scheme must conform to 0% and 100% currents at steps 0, 16, 32, and 48, and both phases must have same profile.

失速检测方案

There are many factors which can contribute to a stall, so it is important to use an advanced IC that properly evaluates stall signals, such as those shown in Figure 4. In the following figures, two alternative scenarios are presented, and the performance of the Allegro IC detection method is demonstrated.

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).

图5：典型的硬盘行为

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）。

Figure 6: Typical Soft Stall Behavior

Stall Detection Products

The following table summarizes the features of some advanced Allegro devices with stall detection. Additional information about these devices is available on the Allegro website, www.allegromicro. com.