Figure 22 :: Setting Up The Servo's Scanning Characteristics; Figure 23 :: The Effect Of Servo Lag With No Correction And After Applying A 2.5 Degree Correction - Lightware LW20 Product Manual

LW20 / SF20 LiDAR sensor
Product manual
Configuring the scanning motion is done by selecting the "Sweep" button under the "Setup" tab. Note that the settings entered here
are limited by the physical and electrical characteristics of the servo. Only the first return signal is shown on the graphic.

Figure 22 :: Setting up the servo's scanning characteristics

The speed of the scan is determined by how fast the LW20 updates and the number of steps that the servo moves with each reading.
The "Steps per reading" box increases or decreases this speed as does the update rate setting in the "Laser" section. Running the
servo faster increases its power consumption but provides a faster response to changes in measurements. If the servo moves too fast
its range of motion may be decreased.
Each direction of the scan is indicated in a different color on the graphic. This is to emphasize the effect of "servo lag" that most
servos exhibit when moving at a constant speed. Servo lag comes about because servos are designed to aim in a fixed direction and
their control systems are optimized for this type of action. When the servo moves continuously, the control loop isn't able to catch
up with the aiming direction and always lags behind.
The number of degrees of lag is different for different servos, at different speeds and at different power supply voltages. To correct
for this lag enter a value in the "Lag offset" box. This is typically between 0.5 degrees and 10 degrees depending upon the torque,
speed and quality of the servo. By making small adjustments to the lag offset the images from each direction of scan can be made to
coincide, as seen in the picture below.

Figure 23 :: The effect of servo lag with no correction and after applying a 2.5 degree correction

Once the servo lag has been corrected, the mechanical motion of the servo will remain consistent. From time to time the settings
should be checked in case wear on the gears or changes to the power supply have affected the servo.
In most applications, the field of view of the scanning pattern will need to be reduced. This could be because of fixed objects inside
the scanning area or to remove errors at the edges of the scan caused by the servo changing direction.
| | |
LW20 / SF20 LiDAR - Product manual - Revision 2 17 of 25 © LightWare Optoelectronics (Pty) Ltd, 2017 www.lightware.co.za