How to choose linear scales/encoders
1, what is a linear scale?
- A linear scale is a device that contains two main parts: a sensor and a scale that encodes position. The sensor also called a transducer or reader head. The sensor reads the scale and converts the encoded position into an analog or digital signal.
- Then use a digital readout or motion controller to decode the signal into position.
- A linear scale can be an optical, a magnetic, an inductive, a capacitive, or an eddy current scale.
- Generally, used ones are optical and magnetic encoders. Optical linear scales are the most accurate of the standard style.
- Linear encoders use in metrology instruments, motion systems, high precision machining tools. They may be digital calipers or coordinate measuring machines to stages, CNC Mills, manufacturing gantry tables, and semiconductor steppers.
· 2, What is the linear scales application areas?
· • 1>, Measurement
- Measurement applications can be coordinate measuring machines (CMM), profile projector, milling machine, lathe machine, EDM, drilling machine, laser scanners, calipers, gear measurement.
- Different measuring device requests of linear scales are different. For example, CMM and profile projector request smaller body linear scales, since the installing areas are small. But milling machine lathe machine can use bigger size ones.
· • 2>, Motion control systems
- • Servo controlled motion systems use linear scales to provide accurate, high-speed movement.
- linear scales usually use on robotics, machine tools, pick-and-place PCB assembly equipment, semiconductors handling and test equipment, wire bonders, printers, or digital presses.
- A linear scale uses in the motion systems, generally requires special requests. For instance, if a linear scale connects with PLC, a 24V DC linear scale is necessary. But if used with measuring device usually use 5V DC linear scales.
· 3, How to choose the right linear scale for the right device.
· 1>, Optical linear scales
- The optical linear scale is an enclosed device,(see pic.1) , there is grading glass inside the linear scale(see as pic.2).
- The grading glass size determine the length of linear glass. The measuring range of a slim body linear scale is shorter than a standard body linear scales.
- The sensor of linear scale obtains all the technologies. the different manufacturer gets different structures. Optical linear scales use a light source and a photo-detector to determine position.
- Some manufacturers fix sensor all parts together, it is hard to repair; some manufacturers use plug and play systems, can separate all parts, and easy to repair.
- There are 5 bearings on the reader’s head, it helps the reader’s head run on the grading glass. So the grading glass shows its importance, if the bears run on the grating mark, with the time goes by it will influence the accuracy.
2>, Magnetic linear scales
Magnetic linear scales also include two main parts: sensor head and magnetic tape. The magnetic linear encoder uses active (magnetized) or passive(variable reluctance) scales and its position can sense by a sensing coil, Hall effect, or a reluctance reader. Compared with optical encoders (usually hundreds of microns to millimeters), they have a thicker scale period and a standard resolution in microns.
3, How to choose the right linear scale?
1>, First you need to consider what kind of machine will be installed linear scales.
As the application part shows there are two main areas, one is the measurement, the other is motion control systems. The different machine requires are different. You need to consider the following points.
A, measuring range
a>, The measuring range
of the optical linear scales are generally from 50mm to 3200mm, but different body size determines the measuring range. For instance, slim body linear scales are generally from 50mm to 1000mm, and per 50mm is another size. It means 50mm, next size is 100mm, and then 150mm, 200mm… And after 1000mm is per 100mm is another size, for example, 1000mm, 1100mm,1200mm,1300mm…
b>, Magnetic linear scale’s measuring range can be from 50mm to 30000mm, and can cut any length.
Different machines or devices request different resolutions for accuracy. Optical linear scales resolution can be 0.5 microns, 1 micron, and 5 microns. And magnetic linear scales can be 1 micron, 5 microns, 10 microns.
2>, You need to consider the working environment,
Scales are also classified into light, medium and heavy-duty loads that may be applied to the indicator shaft. The light encoder can only handle the 10 N radial force on the shaft. So, a heavy encoder can handle 100N radial force. As responsibilities increase, environmental ratings become more robust. Lightweight coders are usually IP40 and IP50 (dustproof); medium and heavy coders can be rated as IP65 (splash-proof).