Two of the Nivkal system are installed at two locations in the Geodätisches Institut (Institute for Surveying) at the Technische Universität München, Germany. It consits of two parts: one to measure the locations of the markings on the rods in the position in that they are used in the field (vertical) and another one in a climate chamber to measure the temperature dependencies. Although mainly a research project, today most of the rods that are calibrated there are for commercial use. The manufacturers of these rods send them in to get the calibration protocol. This is then shipped with the rod to the final customer.

Because of the type of measurements for which these rods are needed any inaccuracy will add up during a measurement campaign. Therefor it is vital to know the exact location of the marks on the rods. Actually these marks are not printed onto the rod itself, but onto a metal stripe (made from Invar, a material with very low thermal expansion) that is embedded in the rod. It can move freely in the axial direction and is suspended at the steel foot of the rod. On the upper side a constant force is applied.

Picking up the marks

The markings are detected by a special microscope (designed and manufactured (by hand!) at the University in Karlsruhe, Germany). Whenever a settable threshold is crossed, it sends a trigger impulse to the main electronics and the laser transducer. This is the main electronics box of the Hewlett Packard laser interferometer. This interferometer constantly measures the distance travelled since the start of the measurement cycle. When a trigger occurs it "freezes" the current value and provides it on the IEC bus. From there the central control logic fetches it, preprocesses it and stores it in a buffer (together with other data, see below) for later transfer to a HP workstation.

The laser beam is emitted by the so-called "laser head", split into a reference beam and a measurement beam and these beams are deflected by prisms to go to the base of the microscope (the reference beam) and the base of the moving carriage (the measurement beam) where they are reflected by mirrors. They then travel back to the interometer and finally the interference fringes end up in the laser head again where they are converted into an electrical signal and sent to the transucer box.

The Main Rack

The main rack holds virtually all the electronics of the system. At the bottom of the rack there is the HP workstation, that runs the GUI and the surveyor specific software (written at the Institute). Above it an inkjet printer is mounted. But the most important part is located in the upper part, where the electronics of the interferometer and other parts are located.

In the back of the upper half there is the main controller for the measurement. It runs a full measurement cycle, as commanded by the user program, and transfers the data to the workstation where it is further operated on, stored and the certificates are printed. The main controller is a dual processor system where one processor is responsible for the control of the motor and the reading of the laser interferometer on a trigger of the microscope. It checks the data from the laser for plausibility and sends it to the other controller. This stores it in a buffer and appends some meteorologic data according to a set of rules. Both controllers run FORTH.

Meteologic data is needed to compensate the change of the speed of light in the atmosphere. Although the effect is in the order of 10-4 it is too big to be neglected here. The full travel distance is about 3m and the measurements must be accurate to a micrometer. The workstation later corrects the raw readings with the aid of the meteorologic data. (In the first version of the system all the work was done by one CBM 3032, so the current version is some overkill - but then there was no GUI and it was impossible to preview the data on the screen.)

Our Contribution

We developed and supplied the main controller and we were responsible for the correct integration of the electronic parts. This included the interface specifications between the program on the workstation and our controller. The guys at the university did the mechanical work and the design of the user software. We work together on this project since the early 80's - a new or upgraded version every couple of years, just as the requirements change.

One slightly different version of this apparatus was shipped to the Landesvermessungsamt in Bad Godesberg (near Bonn). There the main controller used is a RTX2000, running FORTH.

The latest version (for the Fachhochschule Neubrandenburg) is a complete redesign of the electronic parts. The transducer of the laser is now a card in the PC (Agilent does not manufacture the old tansducer anymore, but only this card instead). The microscope was substituted by a high resolution digital camera and the dual-controller interface is gone altogether. Its task is now done by a daemon under Linux, running on the PC. Consequently the new rack is considerably smaller than the old one (half the height), as the HP workstation became superfluous.

If you are interested in using the Agilent 10887P calibrator board, as the new "transducer" is called now, under Linux, habe a look at our download section. There you find a driver for this board.

© Paul Elektronik, 1998