Absolute and incremental measurement
Measurement solutions come in two flavors: Absolute and incremental. With an absolute measurement system, the system will generate an absolute signal, e.g. the position. An incremental system counts the number of steps between two positions. The clock is an absolute measurement system, it will tell you a point in time. A stop watch is an incremental system, it will tell you how many seconds (increments) have gone by since the start of the measurement.
For incremental systems, the design is simple. There are either pure incremental systems, that will provide the number of steps between the start of the system and now. Increments can be counted in any direction.
Incremental scale with reference
The second type of incremental scales has a reference, where a reference position is aligned with on increment of the incremental scale.
Typically, motion control systems are based on incremental systems. They provide a relative movement scheme as input. Incremental systems will require a home run to find the reference position and then count from there.
Incremental scales are rather simple and easy to produce.
There are many possible solutions for absolute measurement. BOGEN can produce scales for any absolute pattern and provides several different types of sensing heads for some of the technologies.
Multitrack binary pattern
Using a binary patterns, multiple tracks can be written in parallel. Through the binary system using the zero as a north pole and the one as south pole, a magnetic pattern can be written, where each combination of the different tracks provides one position.
In this example a four track pattern is used. The combination of the four tracks provide 16 different positions. The scale is said to have a resolution of 4 bit, since the scale can discern 2 to the power of 4 different positions.
To write the scale, the 4 patterns need to be aligned as much as possible. To read the scale, typically one sensor is required for each track.
Multitrack Gray pattern
The Gray pattern is a variation of the binary pattern. The key design rule for a Gray scale is, that only in one track the information changes between the different types.
The scale basically contains all the same combinations as the Gray scale, but ensures that the sensor will not have a position inaccuracy of one increment if positioned at any border. Based on the number of tracks, the Gray scale has a resolution equal to the number of tracks.
Pseudo random code
The pseudo random code is an extension of the binary code for linear applications. Instead of using several tracks in parallel, the scale is constructed by having a binary combination that are different at any increment of the scale. Typically the scale is produced to the same number of bits as the number of sensors in the sensing head.
There are different algorithms that can create such a pattern, as it looks like random it has received the misleading name.
Pseudo random code with additional incremental track
This principles extends the pseudo random code with an additional incremental track. While the pseudo random code identifies each position, the incremental track is used to interpolate between the increments.
This pattern provides a higher resolution than the number of sensors in the pseudo random track, since the interpolation of the incremental tracks is multiplied with this number for the overall resolution.
The nonius track uses two tracks with different pole counts, typically with a difference of one.
The nonius pattern is analyzed with two parallel sensor elements, each on one track. With the phase shift between the two tracks, the absolute position can be calculated.
BOGEN’s patented always absolute pattern is a high resolution absolute pattern.
The position is determined by the angle of the pole border. The always absolute pattern can be narrower than any two or more tracks patterns and provides more resolution than a one track pseudo random code.