Optical Encoders Boost Performance In Miniature Control Systems
Optical encoders have been used to measure position, speed, acceleration and motion direction for decades, but now they are finding new applications where high performance and miniaturization are mandatory design goals. New encoder technology has been developed with a focus on low power, very compact size, and low cost to make possible auto-focus and zooming mechanisms in digital still cameras and phone cameras. Consumer goods such as sewing machines, gaming equipment and surveillance cameras also benefit from compact encoders. Medical equipment and products such as insulin pumps, portable medical equipment, electronic wheel chairs and optometric equipment require low power, high resolution and full three-channel encoding capability.
Modern encoders must fit into the ever-reducing physical size of modern day applications, but high-performance applications cannot settle for a solution that offers less than full capability or that requires external circuits to generate an index channel. The codewheel design is also a factor in overall size. For example, the Avago AEDR-850x three-channel encoder does not require a separate circular track for an index channel. This allows the codewheel to have a very small diameter. In a new design with a focus on size, the encoder and codewheel requirements must be considered together.
Fig. 1. shows a closed loop motion feedback architecture. In this example, a compact optical encoder and a codewheel are mounted within the miniature motor housing. Until recently, only two-channel optical encoders were possible because adding the third or index channel increased the diameter of the codewheel beyond the available space.
New Encoder Technology
The Avago AEDR-850x reflective encoder integrates an LED light source, photo detector, interpolator circuit for higher counts per revolution (CPR), and three channel (A, B and Index) encoding capability into a single package with a 3.95 mm by 3.40 mm footprint. It is just 0.9562 mm in height. The encoder’s single epoxy dome construction helps make it small.
Using the AEDR-850x optical incremental encoder family as an example, one can develop a motor control system with an encoder and codewheel, as shown in Fig. 4 with a codewheel diameter of just 4.6 mm and code wheel optical radius (Rop) of 1.7 mm. Encoder operation requires only a 5V supply and a single 180 Ω LED biasing resistor for a three-channel encoding. The resistor, R, sets the internal LED current to approximately 15 mA.
Before the AEDR-850x, an incremental encoder with a third, or index, channel was not possible for miniature applications because the code wheel became too large with the addition of a separate index track. Patented single-track codewheel technology from Avago embeds the index information in the existing channel A and B track. This allows a compact encoder with three-channel encoding capability to be made without requiring a physically larger codewheel.
In a conventional three-channel optical encoder, typically there is a separate track for the A and B channels, and a separate optical track for the index. With the AEDR-850x, a single track with 3-channels helps an application achieve both miniaturization and performance design objectives, along with the benefit low power of LED technology. By comparison, laser-based three-channel encoders on the market are more expensive and require much higher power consumption than LED encoders. Those solutions are also much larger, as much as seven times larger than the AEDR-850x reflective encoder.
With the AEDR-850x, the index channel width is three times the angle of an opaque area width. The index is “embedded” into the existing AB track. Note that for the embedded index track design, two opaque bars are used for the index, reducing the overall physical bar count. However, this does not reduce the counts per revolution, CPR, as the count during the index transition is generated by an intelligent signal processing circuit within the AEDR-850x.
Absolute “Home Position”
A typical optical encoder has two digital output channels, channel A and channel B; the channel outputs are offset by 90 electrical degrees (°e), and this configuration defines what is called quadrature output signals. When the codewheel rotates counterclockwise, as shown in Fig. 3, channel A leads channel B. When the monitored motor shaft reverses direction, channel B leads channel A. Hardware or software easily determines rotational direction from the quadrature signals as well as relative position, speed and acceleration.
All measurements are relative in an incremental encoder system, since the two channels do not output absolute position. For absolute position information, a more complex and more expensive absolute encoder would be needed. However, by adding an index channel to an otherwise two-channel encoder, a “home’ position or zero reference is available. It is a very capable method that is universally used in lower cost systems.
The index position is an absolute position data point in an otherwise incremental system. Depending on the application requirement, the encoder’s index channel can have a gated 90 °e, gated 180 °e or gated 360 °e pulse width, as illustrated in Fig. 3.
Easy Codewheel Calculation
Once an optical encoder is selected the codewheel design begins. All that is needed is to design a codewheel that matches the encoder’s specifications for:
- lines per inch (LPI) and lines per mm, LP(mm) specification.
- CPR (counts per revolution) design goal
- ROP (codewheel optical radius)
The CPR, LPI and ROP relationships are:
CPR =LPI × 2π × ROP (inch) = LP(mm) × 2π × ROP(mm) (1)
The maximum shaft RPM for CPR and encoder frequency (f) is:
ROP calculation for a given encoder LPI/LP(mm) and desired counts per revolution, CPR, use Equation 1. With a design requirement of 828 CPR and with the AEDR-850x’s fixed 304 LPI specification, the optical radius, ROP, in mm, is:
At 828 CPR and a 1x interpolation setting, the maximum revolutions per minute (RPM) the design supports are calculated by Equation 2.
The AEDR-850x maximum count frequency, f, is 55 kHz with a 1x interpolation setting, but with the same ROP size, CPR and count frequency (f) can be increased with the AEDR-850x by selecting a different interpolation factor through two TTL compatible logic signals, SEL 4X and SEL 2x, as shown in Table 1. Having A, B and index channels that use a single codewheel track is a significant factor in miniaturization for control applications. In addition, 1x, 2x and 4x interpolation capability gives increased counts per revolution without the need for a new and larger codewheel design. This enables design reuse across different end-products, with little or no new design work.
As with all encoders, the codewheel should be kept free of dirt and contaminants for consistent performance. This is especially important for a three-channel reflective encoder. As the index track is generated by a 3 x WB opaque region, any dirt, dust, or other contaminants that block the tracks can result in the detector sensing a 3 x WB region, which will generate an erroneous index signal. Fig. 4 illustrates the potential error of dirt within a miniature three-channel codewheel.
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