by史蒂夫Meyer., Contributing Editor
改进的位置传感器可以降低运动系统成本并提高性能。
Motion control is, first and foremost, about control. The old engineering adage applies; you cannot control what you cannot measure. Rotary and linear position sensors play the crucial role of providing the mechanism to measure motion. Understanding how feedback works as part of the motion system is important in applying the right technology at the right price and getting the best results.
Like most devices based on electronics, feedback devices for the motion control industry have improved accuracy and reduced cost in the last few years. Despite the importance of feedback to motion control systems, feedback technology is sometimes mis-applied.
A brief tutorial on the optical encoder
光学编码器简单地是打破光源的旋转轮。闪光灯由光电晶体管拾取并转换为方波脉冲，通常为0到5 VDC。当编码器附接到移动负载的电动机时，每个方波与特定量的旋转运动成比例。
到目前为止，光学编码器是用于测量运动的最常见传感器。当电动机附接到引导螺丝或带驱动时，可以根据相同信号的函数测量有效载荷的线性运动。两个通道或正交方法的一个好处是运动方向直接包含在两个通道之间的相位关系中。Z通道通常是每转的1脉冲“标记”脉冲，提供额外的信息，用于校正累积误差。
每种技术都有它的我们aknesses, and optical encoders are no exception. Optical encoders can be sensitive to vibration because the disks are low in mass. If the disk vibrates sufficiently it can cause read errors. Mechanical assembly can be complex since the disk must be centered in between the light source and sensors. On balance, the low cost and reliability have made the optical encoder incredibly popular and the simplicity of quadrature output makes it easy to integrate in control systems.
Lots of options
由于反馈的重要性，制造商多年来开发了一系列反馈选择。近年来伺服电机的旋转反馈从每次革命的13位或8192个脉冲到20位，每次革命超过100万脉冲，并且在一些产品上更高。
Too much of a good thing?
There are lots of high-resolution options available to the user, but too much feedback can create some problems that are difficult to diagnose; for a position loop, start with the mechanical accuracy required. Digital control requires a minimum of two samples to measure a real value, so two times the mechanical resolution should be the minimum feedback required. Five to 10 samples may be required depending on the profile of the motion and the degree of control required. Much more than that and the control system will start to “hunt” in between mechanical errors in the rest of the equipment
旋转与线性
Since positioning is a property of something mechanical, it is always preferable to measure directly. Linear measurement systems have been around for a long time, but there has been a significant gap between sub-micron linear position sensors and general-purpose position sensors like limit switches or proximity devices. High accuracy sensors found in CNCs and semiconductor equipment tend to be expensive. For expensive capital equipment, this is not an issue, but for the mainstream motion control user, something else is needed.
输入磁反馈
霍尔效应晶体管已经存在一段时间，作为磁场传感器。通过在霍尔效应晶体管旁边的永久磁化的条带，产生正弦波。使用两个平行行的交错磁性“磁极”并将模拟波形数字化导致正交输出就像光学编码器一样。磁性编码器具有许多有趣的属性，使其成为许多运动控制应用的理想选择。
由于磁性编码器不使用光源，因此它们可以在受污染的环境中运行。灰尘，污垢和油不会影响精度。包封化合物或环氧树脂不会改变其传感能力，因此整个印刷电路可以密封用于清洗环境，其中光学编码器是不切实际的。磁感测方法对气隙的微小差异不敏感，这也使得磁铁环的位置非常容易。
Timken.linear magnetic feedback technology
Linear actuators generally try to regulate position indirectly from a rotary sensor on a motor. This approach has inherent limitations due to torsional compliance and backlash in most drivetrains. Direct measurement of linear position offers the simplicity of controlling position without consideration for the mechanical error in the drivetrain. Position loop stability is much easier to control since the position is being controlled by direct measurement. In addition, “book keeping” to manage error is eliminated and the control system tuning is simplified since the load is being controlled directly from the feedback source.
The Timken Hall Effect sensing array chip works equally well with linear magnet strips. In the same way small magnetic poles are magnetized on a ring of material, poles are magnetized on linear strips of magnetic material. Currently available magnetic linear position sensors can provide position feedback of 0.0004 in. per pulse at price points ranging between $150 and $200 per system.
This makes it economical to add feedback to many applications that were not previously suitable. At these price levels, it becomes economical to use feedback with lower cost mechanical components and reduce the overall cost of actuators generally. Since the system is based on Hall effect sensors, it is immune to all forms of contamination except ferrous particles. This environmental immunity makes it possible to package actuators for harsh atmospheres with relative ease.
完美匹配
德克萨斯州的启动公司目前在开环和闭环版本中开发一系列低成本，高性能执行器的一系列低成本，高性能的执行器。指导原则是策略性地使用常用的组件和材料，以便制造廉价的执行器，该执行器能够更昂贵的系统工作。较低的成本执行器导致成本更低，这允许引入以前不经济的新产品。该公司最终开发了几种大型笛卡尔系统，包括4 x 4英尺的系统，目前正在被测试，可用于任何过程，添加剂或减法。
Moving forward
According to Robert Stiffler, business development manager at Timken, the company continues to extend the performance of its magnetic sensing technology. Available magnet strips from Timken are limited to sizes ranging from of 18 to 24 in. They can be assembled “heel and toe” to create larger scales, but longer scales would be more convenient for larger equipment. Timken is currently evaluating a number of strategies to increase scale length. Accuracy is also expected to improve to ± 0.0002 in. in upcoming versions of the product.
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Timken.
timken.com.
GEC Solutions
Gecsol.com
Filed Under:机电提示,Encoders • optical
