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LinuxCNC

Linear scales

Pinouts

Because there are different standards...

Pin numbers >	1	2	3	4	5	6	7	8	9
Sino RS-422 Ditron-compatible	A'	GND	B'	Shield	R'/Z'	A	+5V	B	R/Z
Vevor	A'	GND	B'		Z'	A	+5V	B	Z
ToAuto, Jing, and others.	+5V	GND	A	B
Yuheng Optics Co RS-422		GND black			Z'	A	+5V red	B brown	Z

Yuheng 9D color code:

1 - A' Blue

2 GND - black

3 B' brown

4 shield

5 Z' yellow

6 A White

7 +5 red

8 B green

9 Z grey

1 2 3 4 5
6 7 8 9

Stepper Motor(needs rewrite)

The stepper motors traditionally were permanent magnet stepper motors with a permanent magnet making up the internal rotor. The train of input pulses (typically square wave pulses) cause a precisely defined increment in the shaft position. Each change in pulse input moves the shaft through a fixed angle. For a 4 pole motor the wave train would look something like below.
drive wave forms

A 4 pole stepper has 16 positions of 22.5 deg. Various tricks - micro-stepping (duty cycle trick) gives more resolution, but steppers have some rather nasty drawbacks:

Advantages - cookie cutter cheep, no need of an encoder, simplicity, no brushes to wear out, high low speed torque.
Disadvantages - Low efficiency(thus high heat), torque drops with speed, resonances, dropped steps, high current DC power supply required, micro-stepping usually required. High speed means producing high speed wave forms - causing RF interference and possible dropped steps.

To overcome these limitations - many - most steppers are not really what they say they are - they are actually servos in drag. The input looks like a stepper, but the signal is read by a microprocessor that converts it to a position and tells a servo motor to go to that location with feedback from a rotary encoder.

The biggest problem is while the interface is simple - it is not an efficient way to communicate information - speed - and location. A computer can't deal with normal interrupts as it puts out the wave form.

Servo Motors

These are typically brushed DC motors that are driven with a PWM H-bridge. There is some sort of an encoder.
Advantages - high power and efficiency(cooler) for size and weight, encoder determines accuracy and resolution, fast acceleration, reserve torque and power for short time, torque is flatter, quieter, smoother.
Disadvantage - system requires 'tuning' to get maximum performance, motors can run-away if loop signals are severed, brushes wear out (2-3khours of operation), complex buyer choices,

There are now AC servos - that don't require the big expensive power supply and no brushes to maintain.

Which type is best?

Depends on the application - 3D filament printers work fine with cheap steppers, but running a milling machine Servos will shine - hard spots in metal can cause dropped steps and destroy position with a stepper.
A key though - using the Stepper with internal servo might seem like a good idea, but you will be limited by the manufactures ability to adjust the loop - mind numbing software interfaces to make small changes - if the adjustment you are after is possible at all. Better to just use a real servo and put the loop control where it belongs - inside LinuxCNC

Encoders

Both optical and magnetic (similar to magnetic tape) encoders produce a quadrature wave form and and index pulse that lets the receiving electronics keep track of the position of a rotating shaft or linear position (linear position most often has an index pulse every so often and may have a home position output as well. An improved version uses gray-coded encoding (see below).

A Rotary encoder's quadrature waveform output

A schematic of a Rotary encoder's quadrature encoding disc

A Rotary encoder's quadrature encoding disc

Gray coded encoders

Gray codes instead of having a two bit output as in a quadrature encoder, have instead a binary word encoded with 3, 8, 16 or even more bits printed on the optical plate. Instead of using normal binary counting, the code increments with a system so that only one bit changes at a time when moving over each position. With normal binary counting several bits can change at once (i.e. 01111 + 1 = 10000), but in the real world there one bit will always lag the others creating possible transient erroneous codes. Gray coded encoders still repeat (there is often an index bit at the full cycle) but the possibility of over running a count, is eliminated. Gray coded encoders are not often used due to their higher costs and they have tweaked quadrature scales to the point that they just work.

A gray code encoding disc

8 bit gray encoder

Resolver

Today the term Resolver sometimes is used when people actually mean an optical or magnetic rotary encoder, below is about the original and correct meaning.

A system that uses a transformer with three windings - A reference winding that rotates (rotor) and sine and cosine stator windings that are 90° from each other on the circumference of the housing. A sine-wave is fed in to the rotating reference winding and the amplitude and polarity of the outputs reveals the quadrant and position with in the quadrant. The sub quadrant position is based on the ratio of the sine vs cosine voltages. SIN θ / COS θ = TAN θ, where θ = shaft angle

In the schematic above, a sine-wave is transmitted via a rotary transformer to the rotating reference winding, the 'rotor'. If the rotor is perpendicular to one of the 'stators' none of the signal is received by that stator. If the rotor is parallel a stator, that stator will receive the full signal. Thus the amplitude and polarity of the outputs reveals the quadrant and position within the quadrant. The sub quadrant position is based on the ratio of the sine vs cosine voltages. SIN θ / COS θ = TAN θ, where θ = shaft angle.

Detecting the outputs

There is more than a little hand-waving about how these signals are detected. Comparing the amplitude of the reference vs stator signals could be accomplished with peak detection, but such a system would have poor noise immunity and thus would have poor accuracy. Instead a four quadrant multiplier is used as peak detection throws out a lot of signal information.

Using a multiplier, the reference is compared to the signals throughout the whole cycle.

It is also explained that the 'phase information' provides position information - this is true, but misleading - it is better said that the polarity of the signals indicate the quadrant the rotor is in at anyone time.

Synchros

Similar to the resolver (see above) but with three stators 120° apart.

LVDT Linear Variable Differential Transformer

Originally a massive electric motor (US pat 808944 from 1906 Contactless AC-motor reverser) - these were used to control motor position. For details find a used copy of Herceg, Edward E. (1976). Handbook of Measurement and Control. Schaevitz Engineering. LCCN 76-24971

LVDTs are the standard of linear contact measurement - frictionless - repeatability 0.01um. The core moves linearly inside a transformer consisting of a center primary coil and two outer secondary coils wound on a cylindrical form. The primary winding is excited with an ac voltage source (typically several kHz), inducing secondary voltages that vary with the position of the magnetic core within the assembly. The core is usually threaded in order to facilitate attachment to a non-ferromagnetic rod which, in turn, is attached to the object whose movement or displacement is being measured.

One does not have to spend $1k to get the electronics for a LVDT head - just get an AD698 which can also be used to build a simple system that works with a half-bridge and full-bridge heads.

Key patent diagrams:

LVDT-1906-808944.png
LVDT-1934-2050629.png
LVDT-1940-2196809.png
LVDT-1947-2427866.png

Paper - need copy:

The linear variable differential transformer
H Schaevitz - Proc Soc Stress Anal, 1947

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