Innovative Back EMF Based Stall Detection Simplifies Stepper Motor Designs

Innovative Back EMF Based Stall Detection Simplifies Stepper Motor Designs

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。

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;
• 它们具有良好的启动特性，零速度的最大扭矩;
• 它们具有宽的动态范围，可以比伺服电机更快地加速;
• 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。

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

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: Controller ICs

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

试验要求的要求

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。亚博尊贵会员

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

How Stall Detection Works

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.

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

Method For Determining a Stall

每个电机绕组阶段具有PWM计数器，该PWM计数器累积每个完整步骤的电流限制事件的数量，从零到全电流。计数的允许差异被编程到IC板载诊断寄存器中。当计数低于编程值时，检测到失速。

电子失速检测的相限

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.

Hard (Locked) Stall Scenario

在失速检测信号之前，旋转转子在大约2ms下停止，通过低电平表示故障。请注意，即使PWM周期的数量增加（见图5），相电流如何保持其形状。

图5：典型的硬盘行为

软（部分）摊位场景

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.: 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.