A feedback system is necessary for accurate earth tide measurements with a LaCoste and Romberg gravity meter. Feedback systems apply and adjust a force to balance the constantly changing gravity tide, thereby keeping the gravity meter beam at the reading line. The gravity tide signal is then proportional to the feedback force applied.
Feedback systems are also useful for point-to-point gravity surveying. If the gravity meter can be read without turning the measuring screw, circular error and backlash concerns are eliminated. This requires that the beam position be sensed electronically, and a force then generated to push the beam to the reading line.
The MVR (Maximum Voltage Retroaction) Feedback
System was designed in 1989 by Dr. Michel Van Ruymbeke of the
Royal Observatory of Belgium (Van Ruymbeke, M., 1989).
The MVR Feedback System is an optional feature available for all LaCoste and Romberg Model "G" and "D" Land Gravity Meters. Note that installation of the MVR system requires disconnection of any CPI system already installed.
The surface mount electronics and small size of the card provide a very compact system.
FEATURES OF THE MVR FEEDBACK SYSTEM OPTION |
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Starting about 1989, the MVR Feedback Card has been installed on many LaCoste and Romberg Model G and D gravity meters by Dr. Van Ruymbeke at the Royal Observatory of Belgium. We refer to this as a "Field Installation" procedure since it does not require the gravity meter to be returned to the factory. The card is installed between the black lid and the top of the meter heater box. Any existing CPI card is first disconnected, and the MVR Feedback card is connected to the gravity meter capacitor plates through the standard connector and black cable. A 9.0 volt regulator is installed to power the Feedback card. The Feedback output is connected to both the galvanometer and Analog Output Connector on the side of the meter. The Reading Line adjustment is made by removing the black lid to adjust the potentiometer located on the Feedback card.
The LaCoste and Romberg Factory Installation procedure has a few differences from the Field Installation. Installing the card under the black lid is adequate for the special case of tide monitoring in a temperature-stable room. However, factory testing has shown that this location does not provide adequate temperature stability for the card when the meter is exposed to changing ambient temperature conditions.
For outdoor work, we must install the MVR card deep inside the white box on the side of the heater box. We use the same location normally used for the CPI card, where a small aluminum box attached to the heater box forms a small "oven" surrounding most of the electronic card.
Also, the reading line adjustment potentiometer ("MVR ZERO") is removed from the MVR card and re-located to the upper left corner of the white box. This permits the operator to make the adjustment through the access hole in the black lid without removing the black lid.
