TRA-BOND 2143D is the recommended adhesive to mount a scale on a hub. This two-part adhesive is easily mixed and cures at room temperature. A UV cured alternative with similar shear strength is AA 352. For vacuum applications, D.E.R 331 resin combined with Versamid 140 curing agent is recommended.
No. After the scale is removed the adhesive will no longer be effective. The act of removing the scale may also compromise the performance of the encoder system.
Isopropyl alcohol is recommended. Do not use acetone. Note that as many particles as possible should be removed before cleaning using clean, deionized compressed air.
Visit the ‘Cleaning Encoder Optics’ Technical Paper for more information.
The output clock frequency determines the maximum frequency of A/B channel transitions. It is expressed in megastates/sec or counts/sec.
The frequency should be chosen to be equal or less than equal to the maximum input frequency of the receving electronics. Note that if the speed is such that the selected frequency is exceeded, the encoder will buffer the excess counts.
Should motion suddenly stop the encoder will operate in burst mode emptying the buffer. This situation should be avoided when the encoder is part of a servo loop.
A Megastate represents the quadrature state of the A and B channels of an incremental encoder. For example A high, B low. There are four megastates per cycle of the A or B channel.
Megastates/secs can be considered as counts/sec – an industry standard term to specify the combined rate of edge transitions on the A or B channel.
Drives and controllers are typically specified by maximum encoder frequency – counts/sec or in the case of Celera Motion encoders, megastates/sec.
“Calibration” for the Optira really means training the sensor to recognize where the edges of the Index Window are, and centerting the LSB in the center of the index window. An LSB index option for an analog Optira is not a valid configuration. An analog Optira does not have an LSB, as it outputs analog sin and cosine signals.
Resolution defines the smallest measurement that can be made, and thus defines the smallest position increment possible. Accuracy is how the system moves relative to perfect motion. If we commanded a 100 mm move, did the system actually move 100mm ? The error is Accuracy. Repeatability is how well we can repeat a move. Using the 100 mm example, if we did 1000, 100 mm moves, and every time were within 100.5 +/- 0.1, the repeatability would be 0.1 and the Accuracy is 0.5 mm. It is also important to note that accuracy vs. position variation causes velocity variations of the moving object.
MMA is “Micro Motion Absolute”. This is a method where multiple distance coded index marks are applied to the scale. With a small amount of movement, the encoder can calculate it’s starting position and be treated as an absolute encoder. This method requires a simple algorithm to be programmed into the customer’s drive or controller. The MMA Tech Note describes the function and algorithm.
For a typical servo application the customer needs ~10X more resolution than accuracy to have good servo performance. In most cases, when the customer does not know what they need, it means they need low resolution. For linear, this might mean micron per count resolution, or for rotary thousands of counts per rev. High resolution would be 5-100 nm per count or for rotary >500,000 counts per rev. For rotary, it is very much dependent on radius. For very large diameters (>200 mm), the above numbers might not be considered high resolution. For low speed application, higher resolution is critical for smooth velocity profiles.
The two biggest differences between these encoder types are the environment in which they can operate in, accuracy and resolution. Magnetic and inductive encoders can survive in very harsh environments, whereas optical encoders cannot. Optical encoders, like the MicroE product line offerings, have ultra-high resolution, whereas inductive and magnetic max out near 21 bits (2 million counts/rev). Magnetic encoders generally have lower repeatability and lower accuracy, and are sensitive to nearby electrical and magnetic fields.