Absolute Length Gauges for Shop-Floor Metrology
The conditions surrounding production environment metrology place special demands on position measuring systems used for workpiece inspection in shop-floor metrology. The very principle of inductive systems involves disadvantages such as temperature sensitivity or nonlinear behavior. A length gauge recently developed by HEIDENHAIN, now combines the benefits of incremental optical devices and absolute measured value acquisition. Its advantages over previous position measuring systems make it an interesting alternative for shop-floor length measurement.
The requirements for quality, precision and function of products and their components exercise increasing influence on their manufacture. To be able to fulfill these requirements and nevertheless ensure economic production, an increasing amount of effort has to be invested in the monitoring of the manufacturing process to be able to precisely measure tolerances and deviations from the nominal condition and then correct them.
In automated production with high volume throughput, quality is assured with statistical methods in as much as the machine and the process capability indices allow. If this is not possible, or if it is not worthwhile under the given constraints, then the quality has to be "tested in." In any case it requires measuring, which can mean significant time expenditure depending on inspection accuracy, parts handling and number of spot samples, because the measuring process must be carefully integrated in the production process in order to keep interactions and dependencies to a minimum. In mechanical production, these tasks are usually fulfilled by displacement transducers in their various forms. They have a wide range of tasks: in the simplest cases, capacitive or inductive proximity sensors test the presence of components during or after assembly. At the upper end, inspection, measuring and testing equipment possess the capability to remeasure dimensional, form and position tolerances of a workpiece with precision in the micrometer range. This makes constant or known ambient conditions necessary and places specific requirements on the measuring device.
Production-integrated linear measurement
In the large market for displacement transducers, inductive sensors for path measurement in the range of a few millimeters are employed in millions of metrological applications. Half of these are proximity sensors. The other half are proportional transducers that convert a measured value to a corresponding output signal. In almost all cases, the standard device here is a linear variable differential transformer (LVDT) in full-bridge or half-bridge versions. In this measuring device the complete electronics are relocated in the subsequent electronics, which greatly reduces the size of the measuring device. Further advantages are its low price and the wide variety of available versions. However, the very principle of this type of inductive measured value generation brings with it certain disadvantages.
An LVDT plunger coil system has only a relative small linear range around the center of the soft-iron core so that the utilizable measuring stroke (depending on the mechanical design) covers only a few millimeters. For this reason, LVDTs are normally used only for measuring tolerances that stay within the limits of their linear stroke. The nominal dimension is fixed by the mounting position of the stylus, which reduces the flexibility and handling of a measuring device if other dimensions are to be measured. Another critical disadvantage of the LDCT's measuring principle in typically harsh shop-floor environments is its sensitivity to thermal influences. Even small fluctuations in temperature can lead to drift of the measured value to such a large degree that a recalibration can become necessary. In spite of these constraints, which are familiar enough to the user, the benefits of LVDTs have sufficed to establish it in the market.
For measurements within micrometer accuracy in automation technology, various nontactile principles are used in addition to tactile linear encoders. These are usually optical devices based on the light-barrier principle (with either transmitted or reflected light). Other devices measure the distance between a measured object by the dynamic pressure of escaping air. Camera systems are sometimes used that can often be integrated well in a manufacturing process.
Optical incremental length gauges, are increasingly being used as an alternative to LVDTs. As the devices become more sturdily designed and provide better protection against contamination, they are beginning to move many demanding measuring tasks from the lab to the manufacturing process. The user profits from several decisive advantages of a glass measuring standard compared with other measuring principles:
- High measuring accuracy and linearity over the entire measuring path (up to 100 mm)
- Temperature stability and defined thermal behavior
- High repeatability at high resolution
- Low error within the signal period (= short-range error)
The Specto series of optical length gauges from HEIDENHAIN, have been on the market for many years now.
Fig. 1. The length gauge combines the benefits of incremental optical devices and absolute measured value acquisition.
Absolute measured value acquisition
Absolute encoders feature an immediate connection between position and position information. Even after traverse at high velocities, a reliable measured value is always available. Criteria such as cutoff frequencies and electronically limited maximum traverse velocities therefore play no role.
A measuring device has to be referenced when put into operation, which means that the position of the measured value must be defined within the external coordinate system. Usually, a physical standard (held in a defined position) is used to set this datum: the length gauge contacts the surface of the measured object and then the surface of the known standard. This is necessary if, after it is switched on, the measuring device cannot define its position, as for example on incremental encoders without evaluation of a reference mark. On an absolute encoder, the position is known immediately upon switch-on. This advantage alone significantly facilitates handling and commissioning of the testing equipment.
With the new Acanto length gauge, the manufacturer supplements the advantages of incremental optical devices with absolute measured value acquisition (Figure 1). These devices now provide the measuring means to profit from immediate knowledge of the absolute position. If the ambient conditions are also included in the evaluation, in the ideal case it is possible to do without recalibration during the test procedure.
Fig. 2. A typical application: Measuring the diameter and length of a shaft automatically.
The Acanto is a length gauge with a compact design and an IP65 degree of protection. It is available in measuring lengths of 12 mm and 30 mm (Figure 2). This enables it to be used in a large variety of applications. To simplify installation and handling, versions are available with either an axial or radial, pluggable cable connection. The plunger is moved pneumatically and is available in a spring-loaded version. A ceramic-polymer sliding guide for the plunger ensures low-friction, low-wear operation and a long service life (≥ 5 million cycles). The length gauge is mounted by clamping the shank (8 mm diameter). The positions are physically encoded on the glass scale with a resolution of 23 nm. The system accuracy, which is the linear error over the entire measuring length, is ±2 µm. The short-range error (interpolation error) is 0.7 µm. However, this plays a secondary role for repeated measurements in the range of only a few micrometers. The temperature drift is typically 1 µm/K.
The position information is transmitted over the EnDat 2.2 serial interface. The EnDat master is responsible for communication with EnDat encoders from the manufacturer. This allows simple transmission of position data and additional information to the higher-level application.
The EnDat master can be integrated by means of a micro controller (µC) or a Field Programmable Gate Array (FPGA) or ASIC. Integration in an FPGA or ASIC is chosen for high transmission frequencies with pure serial data transfer. Various versions are available through MAZeT (www.mazet.de) in Jena, Germany. Besides the absolute position information, the EnDat interface also provides diagnostics for the encoder. Valuation numbers provide information on the current quality of the measured-value generation. Data saved in the device memory can be interrogated, and warnings and alarms are supported. This makes it easy to monitor the length gauge's operating condition, which significantly increases the availability of a testing system, particularly in applications with many axes.
FROM: Dr. Johannes Heidenhain GmbH