Raster Encoder Accuracy and Printing Accuracy in Inkjet R&D (Worth Saving)

Raster Encoder Accuracy and Printing Accuracy in Inkjet R&D (Worth Saving)

Table of content.

Definition of a Linear Encoder (Grating Scale)

A linear encoder (grating scale) is a type of encoder commonly used to measure linear displacement. It consists of two main parts:

  • the scale grating, and

  • the reading head.

Typically, the scale grating is fixed, while displacement is detected through the movement of the reading head. The reading head includes a light source, lenses, an index grating, and photosensitive elements.

 

 

Grating and Moiré Fringes

The scale grating is the most critical component, as it directly determines the measurement accuracy of the encoder. Linear encoders operate based on the Moiré fringe phenomenon.

A grating is created by engraving a large number of parallel, equally spaced lines onto a glass substrate. The dark lines are opaque, while the gaps between them are transparent. High-precision gratings may contain thousands or even tens of thousands of lines within a single centimeter.

Moiré fringes are visual interference patterns formed when two sets of lines or objects overlap at a constant angle and frequency.

 

Opaque and Transparent Sections of the Grating

Each time the grating moves by one pitch, a Moiré fringe shifts by one fringe spacing. The photosensitive elements in the reading head undergo one full signal cycle. In this way, periodic optical signals are converted into sinusoidal or square-wave electrical signals.

★ Tips

Encoder resolution refers to the number of optical signal cycles per unit length.
 For example, if there are 100 signal cycles within one inch, the encoder resolution is 100 DPI.

By counting signal transitions, the system determines how many transparent grid units the reading head has passed, and thus calculates displacement:

Example:

  • Encoder resolution: 100 DPI

  • Detected signal transitions: 2000

  • Travel distance = 2000 ÷ 100 = 20 inches

  • 20 × 25.4 = 508 mm

 

Bidirectional Movement and Signal Interpolation

Four photosensitive elements are installed in the reading head: A+, B+, A−, and B−, with phase differences of one-quarter cycle.

  • A+ − A− produces signal A

  • B+ − B− produces signal B

If signal A leads signal B, the movement is forward; if A lags behind B, the movement is reverse.

Signals A and B also provide strong noise immunity. If the system detects an A rising edge without a corresponding B rising edge, the count is ignored. Similarly, false triggers caused by noise are filtered out by cross-checking signals A and B.

 

Signal Multiplication (Interpolation)

 

Within one complete signal cycle:

  • Signal A produces one rising and one falling edge

  • Signal B produces one rising and one falling edge

  • 1× interpolation: count only the rising edge of A

  • 2× interpolation: count rising and falling edges of A

  • 4× interpolation: count rising and falling edges of both A and B

This allows the encoder to measure displacement with much higher resolution.

★ Tips

Common encoder base resolutions are 150 DPI and 180 DPI.
 With interpolation, achievable resolutions include:

  • 150 / 300 / 600 DPI

  • 180 / 360 / 720 DPI

 

 

Encoder Accuracy vs. Encoder Resolution

Accuracy refers to manufacturing error, for example:
 ±0.275 μm per 10 mm

This means that over a 10 mm travel, the possible error is ±0.275 μm.
 Note that accuracy does not scale linearly—±0.275 μm per 10 mm does not imply ±2.75 μm per 100 mm.

Resolution, on the other hand, is often achieved through signal subdivision, either by physical means or electronic interpolation.

For example:

  • Grating pitch = 25 μm

  • One signal period per 25 μm movement

  • With 50× electronic subdivision, the system outputs 50 counts per 25 μm

  • Effective resolution = 0.5 μm

If movement is less than one grating pitch (25 μm), no signal is generated.

 

Printing Accuracy Concepts

Printhead Resolution

Defined as the number of printed dots per inch along the printhead length.

Example (Ricoh MH5340):

  • Single color: 600 DPI

  • Dual color: 300 DPI

  • Four colors: 150 DPI

 

★ Tips: DPI and PPI

DPI (Dots Per Inch)
 A unit used to measure print resolution, indicating how many dots are printed per inch. Higher DPI generally means higher print accuracy.

  • Web images: typically 72 DPI

  • Photo printing: 300 DPI or higher

PPI (Pixels Per Inch)
 Measures pixel density on display devices such as monitors, TVs, and mobile screens. Higher PPI results in sharper and more realistic images.

 

 

Horizontal and Vertical Printing Resolution

Example: 600 × 1200 DPI

  • 600 dots per inch horizontally

  • 1200 dots per inch vertically

PASS Count

PASS refers to the number of scanning passes the printhead makes over the substrate.

Single-pass resolution refers to the vertical resolution achieved in one scan, which depends on the number of ink colors used.

Example (Ricoh MH5340):

  • 1 color: 600 DPI per pass

  • 2 colors: 300 DPI per color per pass

  • 4 colors: 150 DPI per color per pass

 

 

Relationship Between Encoder Resolution and PASS Count

Horizontal resolution is determined by encoder resolution.

Example using a 150 DPI encoder and Ricoh MH5340 with two colors:

Available print modes:

  • 150 × 300 DPI

  • 300 × 300 DPI

  • 600 × 300 DPI

To achieve 600 × 1200 DPI:

  • Using 600 × 300 DPI mode:
     PASS = (600 × 1200) ÷ (600 × 300) = 4 passes

  • Using 300 × 300 DPI mode:
     PASS = (600 × 1200) ÷ (300 × 300) = 8 passes

 

 

Minimum Print Resolution of a Printhead

This refers to the smallest printable resolution per unit area when all available channels are used.

Example:
 Kyocera KJ4B-QA

  • Minimum print resolution: 600 × 600 DPI

  • Means 600 dots per inch horizontally and vertically within a 1-inch area

Back to blog