Optical Tip of the Month - FineLock for Trimble S Series Instruments
On Trimble® S Series instruments, a great advantage of Autolock® technology for searching, locking, and tracking a target is the large field of view of the tracker receiver. However, when measuring to prisms that are close together, this is no longer an advantage. For example, when measuring deformation in a tunnel during construction, there will often be multiple prisms installed at regular intervals. If prisms are too close together with an Autolock measurement, the instrument cannot lock onto the center of one single prism as the tracker receiver is getting multiple reflections.
Using Trimble FineLock technology, only the inner fine detector of the tracker receiver is used which has a very narrow field of view, removing the interfering reflections from other prisms, and allowing the instrument to center on a single prism.
Trimble FineLock is available on Trimble S7, S9 and S9HP instruments.
FineLock technology has a field of view of 2.5 MRAD (0.16 gon; 0.14°). To calculate the minimum offset targets need to be from each other, requires using a larger angle than just using the field of view of the sensor.
Minimum spacing between prisms
As a start for simplicity we will assume both prisms are at the same or very similar distance. The reflected beam of a prism at 10 m distance is projected on the FineLock receiver diode and with the attached FineLock aperture it generates a laser spot of 40 μm diameter. This is about 20% of the fine detector chip size. With a safety margin of 10%, the angle between 2 nearby prisms should be at least 1.5 MRAD. As shown in the figure, the prism outside the field of view does not cause any interference:
However, this is a simplistic case, where the primary target is locked and in the center of the tracker, for example in Monitoring installations where the instrument is turning to a pre-programmed point with a known target. If the instrument is not turning to a specific point, a minimum target separation of 1.5 mrad can result in the situation shown below where both laser spots are projected on the receiver diodes.
So typically, the 1.5 mrad target distance is still not sufficient. A sufficient target separation is reached with an angle difference of 2.75 mrad which is slightly larger than the size of the FineLock receiver diode. However, this is still assuming that the two targets are at a very similar distance, which is often not going to be the case.
If the interfering prism is located at a shorter distance than the one to be measured, the situation is much more complex. A target at a shorter distance always generates a larger and more diffuse spot than a target at a longer distance, which in turn requires the angle difference to be larger in general.
Taking all of this into account, an angle difference of 4 MRAD (0.23°/0.25 gon) is recommended which results in the specified minimum spacings shown below:
Long Range FineLock
When you need FineLock functionality beyond the 700m range of standard FineLock there is the Long Range FineLock solution. This is a transmitter module that sits under the telescope on certain Trimble S9 instruments, where you would otherwise have a Tracklight or Vision module.
The range for the LongRange FineLock is 250-2500m, though standard FineLock is available on LongRange FineLock instruments for measurements in the 20-700m range. When using LongRange FineLock you need to do 2 face measurements to get accurate height readings given that the transmitter module is offset below the telescope.
For more details on FineLock and Long Range FineLock, as well as the FineLock lens Aperture and field software controls, see the full bulletin here:
Lean More on the S Series click here: