Impulse Gravimeter Circuit #2005
( 3-25-1990 )
Introduction:
Impulse gravimeter Circuit #2005 is fabricated in a plastic box 1.5" x 2.5" x 5" in size but with an aluminum cover plate. All operating controls are brought out on this panel, as well as an output jack and a small (1.5" square) zero-center calibrated meter, used to give a display of the output voltage of the unit. A dual 1.5 volt battery supply is self-contained in the unit. The unit is primarily designed to demonstrate the gravitational ‘impulses’ generated by the numerous Newtonian-type gradients developed in this universe.
The Circuit:
The circuit used in this unit (Figure 1) is a typical electronic-type gravimeter, but has had the parameters adjusted for this particular application. The unit was designed around a quad-type C-MOS IC op-amp which performs very well with a 1.5 volt power supply. This ICL7641 device can operate over the full range of the supply voltage, i.e., over the rail-to-rail range of 1.5 volts. However, the operating parameters for this unit were adjusted to that the unit operates in the sensitive range of =/- 0.5 volts in order to agree with the scale calibration on the display meter used.
The detector section (IC1) is typical, but the sensing capacitor, Cin, and the feedback resistance, Rf, have been adjusted to this particular device for optimum performance at the +/- 1.5 volt supply operation with an output in the +/- 0.5 volt range and also to keep the input circuit resonances very much below 1 Hz, so as to avoid the many ELF ‘resonances’ seen in this universe (which would complicate the desired responses). Some measure of sensitivity control was provided by the gain control in the feedback circuit. The output of the detector section is buffered by IC2 before being fed into the amplifier stage.
The amplifier stage, IC3, is an inverting-type having a gain of only about 5x to limit the response of the unit to the desired output ranges. An off-set control (Set) is included here to enable the adjustment of the averaged output signals of the unit to about center-scale on either side of the zero-center meter circuit. This enables operation of the unit with a choice of pulse polarity, e.g., the negative mode of operation would allow an increase in the detected value of the earth’s g-field to read up-scale on the meter circuit.
The output of the gain stage is also buffered by IC4 before it is fed to the internal low-pass filter and meter sections of the unit. In this particular unit, the filter and meter sections were combined for use as a portable calibrated gravimeter unit. However, an output jack was provided for use with an external high impedance meter or a strip chart recorder unit. The filter section was designed for a cut-off frequency of about 10 Hz for a good response to the gravitational signals from the Local Supercluster of Galaxies, as well as the Milky Way Galaxy. However, this internal filter can be cut-out (in the filter of position) if one desires to follow the very fast responses of the very remote events in the universe.
Operation:
Operation of the unit is very simple: Turn on the power supply and allow the unit a minute or two to stabilize. Set the off-set and gain controls to about their mid-positions, and the filter to the off position. The gravity signals should now be quite apparent as fairly rapid excursions on the meter. The gain control can now be adjusted for the desired level of excursions --- higher gain means greater amounts of excursion. The off-set control should now be adjusted to read the impulses on either side of the zero center scale, depending on the desired pulse polarity. Place the filter switch to the on position to limit the unit response to the nearby galaxies (normal mode of operation). The off-set control should be used to properly position the response on the built-in meter or an external meter or strip chart recorder unit. The output polarity is adjustable with the off-set control or just the reversing of the output leads. The experimenter should become quite adept at using this unit in very short order.
Conclusions:
While this unit was primarily intended for response to the fairly rapid ‘universe’ gravitational response, it may also be used as a g-factor gravimeter by loading the output with a capacitor in the range of 21,000 uF to 100,000 uF (observing proper polarity), to increase the output time constant to such long periods that the unit will respond to only the very slow changes in the earth’s g-field levels. This is best done with the output jack brought out to an external meter and capacitor load, using the output jack provided. With the judicious selection of the operating parameters it should be possible to set the unit to read (32) or (98) on a meter scale to conform to a g-factor of 32 ft/sec2 or 980 cm/sec2 . Thus the unit would be a direct reading g-factor gravimeter for many purposes. This type of operation will also disclose to the experimenter the subtle daily changes in the earth’s gravity field which are presently affecting our weather patterns (not always to the good!)
While the current requirements for this unit are very low, the experimenter should use ‘good’ batteries for best results. The unit is a very good introduction to the new field of gravitational astronomy for the serious experimenter or researcher. Good experimenting to all!
Greg Hodowanec
Figure (1): Portable Impulse Gravimeter Circuit #2005 ~
Notes: (1) Gravimeter may read up or down scale for an increase in the g-field, depending on the set-control position; (2) Use gain control to set sensitivity levels; (3) Place filter on for Local Galaxy responses; off for distant responses; (4) Built-in meter is roughly +/- 0.5 volts.