How to Choose the Right Mark Sensor? What to Consider When Selecting a Mark Sensor?

Open any mark sensor selection manual and you will see technical parameters such as RGB vs single-color light source types, response time, switching frequency, jitter, gray scale, single-point or multi-point teaching modes, NPN/PNP, push-pull outputs or analog outputs.

So, faced with such complex specifications, how do you choose the right mark sensor product?

Sensor selection always starts from basic application needs. For mark sensors, this naturally begins with mark detection performance, including:

Mark recognition capability

Positioning and measurement performance

Signal compatibility

1. Mark recognition capability

When choosing a mark sensor, first determine the appearance of the target surface, including background patterns and mark color. Different colors reflect light differently, so pick the detection light source type based on the background and mark colors.

In practice, because surface patterns vary, RGB three-color sources are often a good choice for easier setup and maintenance while providing robust mark detection. For complex multicolor backgrounds, choose sensors with color modes, though this may increase cost.

Recognition capability is also affected by detection distance. Sensor datasheets usually list sensing distance (e.g. 13 mm) and tolerance (e.g. ±3 mm). These indicate the effective sensing range and allowable variance. The sensitivity characteristic curve shows detection performance at different distances.

Mark sensors measure surface gray levels; an important accuracy metric is gray scale resolution. Gray levels are typically expressed as percentages from 0% (white) to 100% (black). Higher gray scale resolution means better ability to detect low-contrast marks. For example, the KTM series may offer 20 gray levels, meaning 2^20 gray steps.

However, even strong recognition can be affected by field conditions: faded prints reduce contrast, dust or oil contamination weaken reflected light, etc. To address this, choose sensors with diagnostic outputs (such as IO-Link). Diagnostics monitor the gray contrast in real time and can signal the controller when contrast falls below a threshold, prompting maintenance actions like material replacement or cleaning.

2. Positioning and measurement performance

Mark detection is ultimately used for positioning, so pay attention to response characteristics. One key metric is switching frequency in Hz, the number of on/off cycles per second. This should be much higher than the mark passing frequency to avoid missed triggers. Many sensors exceed 10 kHz, and advanced models can reach 50–70 kHz.

Another metric is response time, typically in microseconds. This delay is used to timestamp the switching event for precise position calculation.

3. Signal compatibility

Ensure the sensor output matches the control system input. Consider output voltage levels and type (NPN, PNP or push-pull). If analog gray values are required, select sensors with analog outputs. Also determine whether you need dark-on or light-on output polarity depending on background and mark colors.