The 3 Dimensions of Resolution

Resolution is defined as the smallest measurable target change that can be detected at the sensor output. When 3D sensors are implemented in real world process control applications, resolution is a very important parameter. Proper process control relies on being able to detect small changes in the object, to be able to reliably track trends, and make a response to these changes at an appropriate threshold of change.

For a single point displacement sensor, resolution is the smallest target movement in the Z axis that the sensor can detect. For 3D sensors, resolution has 3 values, one for each of the X, Y and Z axes of measurement.

Z-Axis Resolution

For all 3D sensors, Z-axis resolution, like displacement sensors, is the smallest target movement in Z that the sensor can detect. Verifying resolution of a sensor can be done by placing a target object on a moveable device equipped with a position measurement device such as a micrometer, LVDT or position readout, and displacing the object by small amounts to determine resolution. The device must have ability to move in precise small steps, much smaller than the resolution expected from the sensor.

Z-Axis Resolution

Z-axis resolution is smallest when the target is at the near end of the sensor measurement range (MR) and becomes a bit larger at the far end of the MR.

X-Axis Resolution

For all 3D sensors, X-axis resolution (also known as profile data interval) is determined by the number of pixels on the sensor imager in the X direction (eg., across the laser line or field of view). For a specific field of view (FOV), using a sensor with more imager pixels will improve X resolution. Also, since the sensor FOV is trapezoidal in shape, X-axis resolution is best when the target is at the near end of the sensor measurement range.

X-Axis Resolution

Y-Axis Resolution

Y-axis resolution is different for profile (laser line) and snapshot (structured light) sensors.

Snapshot Sensors

Snapshot sensors capture a full 3D point cloud from a stationary target. Both X and Y axis resolution are determined by the number of pixels on the imager and the sensor FOV, as described in X-Axis Resolution.

Profile Sensors

For profile sensors applied to 3D point cloud measurement of moving objects, Y-axis resolution is primarily determined by both the profile scan rate and the target motion speed, which determines the Y-axis spacing between sequential profile “slices” as targets pass the sensor. For a given target motion speed, increasing the profile scan rate will improve the Y-axis resolution.

If part motion speed is constant, a time-based trigger can control sensor profile scan rate. However, almost all transport mechanisms will vary in speed over time.  Therefore, use of an encoder on the conveyor/transport mechanism to trigger the sensor at specific motion intervals is highly recommended to ensure constant Y-axis resolution.

Today’s all-in-one 3D smart sensors have a direct encoder input to simplify integration.

Specifying Resolution

Because 3D sensor resolution varies over the measurement range, expert sensor manufacturers will specify resolution at both ends of the MR to aid in sensor selection.