A New Linear
Optical Encoder
The small size, ruggedness, and high-speed capabilities of a new open, optical encoder allow easy integration with a wide variety of positioning equipment including high-speed x-y stages, measuring machines, and long machine tool axes.
Howard Salt,
Renishaw Inc.
Photo 1. RG scales are shown in use on both axes of a flying probe that tests specific points on PCBs for continuity, circuit breaks, etc. The long axis going from the front has a buried readhead, though the scale is visible. The other scale and readhead are visible in the vertical plane.
As machines have evolved from manual operation to full automation, the instruments that measure their movements have evolved in similar fashion. Today's manufacturers demand greater accuracy (often to the submicron level), increased reliability, and higher resolution not only from their machines, but also from feedback devices. To measure machine motion, many manufacturers depend on optical linear encoders.
A typical optical encoder consists of two parts: a scanning unit and a scale. The scanning unit contains a light source, a condenser lens, a reticle with grated windows, and photovoltaic cells. Most manufacturers use line-grated glass or metal scales that mount on a machine base, while the unit is connected to the moving slide of the machine.
When the scanning unit moves, a parallel beam of light passes through the condenser lens, then through the windows on the scanning reticle, and onto the reflective grated scale. Reflected light passes back through the scanning windows and strikes the photodetectors. These sensors convert the fluctuation in light intensity into analog sinusoidal electrical signals that are phase shifted by 90°. These outputs are sent to a digital readout or numerical controller for interpolation and subsequent decoding to give an up/down count showing the position of the moving slide.
A New Direction
The RG2, a new open, optical encoder (see Photo 1) made by U.K.-based Renishaw, uses a self-adhesive steel-tape scale and patented optoelectronics for high-speed, noncontact position feedback (see Figure 1). The readheads can be as small as 10.7 mm high and weigh only 22 g (<1.0 oz.), yet handle speeds up to >5 m/s. Small size, high-speed capabilities, and ruggedness allow the system to be easily integrated with linear motors, actuators, high-speed x-y stages, measuring machines, microscope stages (see Photo 2), and long machine tool axes.
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| Figure 1. In the RG encoder, reflected light scatters off the scale onto the readhead optics. Spatial filtering ensures pure sinusoidal fringes at the detector plane, despite contamination or minor damage on the scale. Industry standard analog or interpolated digital signals are output directly from the readhead. |
Principle of Operation
The RG2's noncontact readheads, which can be interchanged for better resolution at any time (0.1–5 microns), house an IR LED light source, index grating, patented photodetectors, and other electronics including monitoring circuit and integral interpolation (for square wave output readhead).
The RG2 readhead projects light onto more than 100 scale facets, which scatter it back onto a transparent phase grating in the readhead. The combined effect of the scale's periodic pattern and that of the grating produces sinusoidal interference fringes on photodetectors in the readhead. The optical scheme can be likened to an electrical noise filter that passes only pure signals to the detector array. The result is cyclical errors typically <0.1 micron, even in the event of scale damage or head misalignment. Electronics in the head convert these signals into two true, pure waveforms of equal amplitude, 90° phase shifted. Patented optics ensure negligible deviation from the 90° phase shift, providing superior quadrature control and allowing fast, accurate operation with a wide range of standard counters.
In an analog output readhead, the sine waves become the 20 micron sine and cosine incremental signals, either in voltage or microcurrent form. The RG2 analog voltage readheads produce a 2-channel, 1 Vp–p differential, sinusoidal signal for remote interpolation and are capable of speeds
>5 m/s. Microcurrent readheads generate a two-channel, sinusoid signal in quadrature and are capable of 1 m/s speeds.
Where appropriate, the signals are fed to internal or external interpolation electronics. The interpolator subdivides the 20 mm signal and generates two digital RS-422 square waves in quadrature of resolutions 0.1, 0.5, 1, or 5 micron, with speeds up to 5 m/s. These are output in differential form for good RFI immunity.
The Optical Scale
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| Photo 2. High-accuracy translation stages are typical applications in which the RG encoder enhances performance. Compact dimensions allow installation close to the guideway of this microscope stage, minimizing guideway error. |
The universal component of the RG2 system, and key to its accuracy, is the optical scale. This consists of a faceted, gold-plated steel strip with 20 micron pitch, specially coated to protect it against routine handling and contamination. When the 6 mm wide, 0.2 mm thick strip is applied directly to a machine and clamped with epoxy-applied fasteners, the ends will shift substantially <1 micron, even over a 0°C–70°C temperature cycle and whether on aluminum or Invar, an iron-nickel alloy containing 40%–50% nickel and having an extremely low coefficient of thermal expansion. The scale's adhesive backing, formulated for exceptional shear strength and resistance to oils and many solvents, bonds to all common materials including granite and ceramic.
The scale is factory calibrated to ensure linearity and accuracy of ±3 micron/m, but actual linear accuracy typically falls within ±1.5 micron/m. The scale is available cut to length (as short as 10 mm) or on 50 m reels for the user to cut. Axes as long as 70 m are easily handled.
The Optoelectronics
A novel feature of the scale and optics is that no single scale facet is crucial to signal integrity. This system averages the reflection from 80+ facets and effectively filters out signals that do not match the scale period before the photodetectors convert them to electric currents. This ensures signal accuracy and stability, even when the scale has significant contamination or damage. The internal optics are, in fact, capable of speeds of $7 m/s, and require appropriate counter/controller clock speeds to handle the incoming signals. The optical arrangement also allows a relatively large separation between the scale and readhead (0.8 mm, ±0.11 mm).
Accuracy
Ordinary two-point linear error compensation is all that's needed to achieve scale accuracy of ±3 microns/m. That is, the readout for an installed length of scale must be compared with a known measurement standard and the resulting compensation value applied, via the control, to future measurements. Accuracy can be further enhanced by using more compensation points and reducing the distance between them. The maximum slope compensation for any uncorrected, 1 m length of scale is <±40 microns, typically around ±20 microns (see Figure 2).
Installation
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| Figure 2. Any axis in the RG encoder system has a measuring error with slope and nonlinearity components. Because of the variations of customer installation, the slope is not prespecified but is easily compensated with a single coefficient. Nonlinearities cause the measuring error to deviate from the end point slope, lying within an error band whose half height is the linearity error. The magnitude of the error depends on scale length. Nonlinearity specs are <3 mm/m or <1 mm/60 mm. A small cyclical error, the subdivisional error (SDE) is related to scale pitch, and is typically around 0.3 mm, even with a contaminated or misaligned scale. |
A built-in signal condition monitoring circuit, unique to the RG system, drives an integral, two-color LED on the head to aid setup and also provides an alarm signal for remote monitoring at the user interface. This circuit continuously monitors the amplitude of the sine and cosine signals before interpolation and drives the LED setup indicator, which shows green to indicate optimum signal, orange for acceptable, and red for a warning condition. The LED is used during initial readhead setup to achieve optimum alignment, eliminating the need for a specialized setup and test kit. During installation, the readhead is simply swept up and down the scale, and ideally should show green throughout this process. Although red does not mean failure, it does indicate a strong possibility of a miscount. The color of the LED can be affected by speed, misalignment of the readhead with the scale, or excessive contamination on the scale. For alignment purposes, the readhead-to-scale air gap and parallelism are the most important adjustments.
The scale applicator ensures that the scale is laid parallel to the axis guideway. Because the applicator has the same mounting configuration as a readhead, it can simply be substituted in the readhead mounting bracket during scale lay-down. The only requirement is a clean, dry surface. End clamping of the scale is highly recommended to ensure zero differential movement between the scale and machine under all thermal conditions.
RG2 Advantages
- Noncontact operation eliminates lip seals, wear, friction, and hysteresis.
- Compact, low-mass design makes it well suited to linear motor integration. All units in the RG2 series have very low profiles. The ultra-low RGH25, for example, measures only 36 mm long by 14 mm wide by 10.7 mm high and weighs 22 g (<1 oz.).
- When installed to the manufacturer's specifications (a bracket for independent adjustment is recommended), RG2 standoff is 0.8 mm, with a tolerance of ±0.1 mm. Yaw is 0 ±0.5°, roll is 0° ±1°, and pitch is
0° ±1°. - Speeds of 3 m/s and 5 m/s at resolutions of 1 micron and 5 microns, respectively, are attainable. The 20 micron pitch scale with ±3 micron linearity ensures high accuracy when interpolating to fine resolutions.
- Magnetic fields from linear motors cannot disrupt operation. A custom-designed, double-shielded, and highly flexible robotic cable provides outstanding EMI/RFI noise immunity and reliability. The cable has a flex life of >10 3 106 at a 50 mm bend radius.
- IR light-scattering technology and averaging mean that fingerprints, dust, and smears on the scale will not disrupt signal integrity. Scale surface contamination may reduce signal strength, but should not affect measuring accuracy. However, the system should not be used in the presence of coolants, oils, or extreme humidity. The scale surface can be wiped clean with a dry cloth or IPA alcohol.
- A choice of two signal formats with reference marking is available: analog 1 Vp–p voltage, microcurrent, and digital square wave (RS-422) with resolutions of 0.1, 0.2, 0.5, 1, and 5 micron.
- The scale anchors to the machine, eliminating the need to compensate for differential thermal expansion and, thus, reducing workload on controls.
- One scale functions with any readhead, allowing quick field upgrades or changes in system configuration. The user cuts the scale to length.
Additional system components include:
- Unidirectional reference marks that are synchronized with the incremental channels and repeatable to the unit of resolution for a given readhead.
- Epoxy-mounted scale end-clamps are recommended for thermal mastering. They may be used at the designer's discretion, depending on the length of the scale, accuracy requirements, and the thermal management of the machine structure. The ultra-low-profile clamps (0.2 mm) fit beneath the readhead so that overtravel or removal does not disturb the scale.
- The scale applicator slips into the readhead mounting bracket during installation and, while stripping off the backing tape, automatically lays the scale parallel to the axis.
Howard Salt is Business Manager, Calibration and Encoder Systems, Renishaw Inc., 623 Cooper Ct., Schaumburg, IL 60173; 847-843-3666, fax 847-843-1744, www.renishaw.com
