CONTINUOUS CONDITION MEASURING SYSTEM

• US 3,781,672 A
• Filed: 05/10/1971
• Issued: 12/25/1973
• Est. Priority Date: 05/10/1971
• Status: Expired due to Term

• Current Assignee: Frederick L. Maltby, Jonathan Kramer
• Original Assignee: Frederick L. Maltby, Jonathan Kramer
• Inventors: Maltby, Frederick L., Kramer, Jonathan
• Primary Examiner(s): Queisser, Richard C.
• Assistant Examiner(s): Shoon, Frederick
• Application Number:
  US05/141,839
• Time in Patent Office: 960 Days
• Field of Search: 73/34C 324/61R 330/17
• US Class Current: 324/663
• CPC Class Codes: G01F 23/263 by measuring variations in capacitance of capacitors
• CPC Class Codes: G01N 27/24 Investigating the presence of flaws

Abstract

In a system which continuously measures the composition, position or size of materials, the probe has a guard shield which is effective for continuous measurement. The measurement system includes a zero adjusting impedance connected with the variable probe impedance as two adjacent arms of a balanceable bridge. A span impedance is connected between the junction of the adjacent arms and a low impedance AC source. The buffer amplifier has its input connected across the span impedance so that changes in the input impedance of the amplifier do not change the operating point of the system. A guard shield amplifier has its input connected to the output of the buffer amplifier. The guard shield amplifier maintains the guard shield at the same potential as the voltage on the probe electrode continuously for all points of operation, without significantly changing the voltage.

Claims

1. A system responsive to the condition of materials comprising:

a conductive electrode, a conductive member, the variable impedance between said electrode and said conductive member being determined by the condition of said materials, a guard shield interposed between said electrode and said conductive member, a source of alternating voltage having first, second and third terminals, said source generating a low amplitude alternating voltage between said first and said second terminals and a high amplitude alternating voltage between said first and said third terminals, another impedance, said variable impedance and said other impedance forming a voltage divider across said source between said first and third terminals with a junction between said variable impedance and said other impedance, amplifier means having an input connected between said junction and said second terminal so that the output follows the voltage at said junction, the output of said amplifier indicating said condition of materials, said output being connected to said guard shield.

2. The system recited in claim 1 wherein said other impedance is a zero adjusting impedance which comprises:

an adjustable capacitor, a first fixed capacitor connected in series with said adjustable capacitor as a voltage divider, said adjustable capacitor being connected to said third terminal, said first fixed capacitor being connected to said second terminal, and a second fixed capacitor connected between the junction of said first fixed and said adjustable capacitors and said variable impedance so that a change in said adjustable capacitor produces a smaller change in the effective capacitance of said network in the ratio of said voltage divider.

3. The system recited in claim 2 further comprising:

a variable resistance connected across said network for balancing out the resistive component of the signal from said probe.

4. The system recited in claim 1 wherein said amplifying means comprises a buffer amplifier having an output representing the condition of said material, and a guard shield amplifier having its input connected to the output of said buffer amplifier, the output of said guard shield amplifier being connected to said guard shield.

5. The system recited in claim 4 wherein said buffer amplifier comprises:

a two stage amplifier having high open circuit gain and closed circuit unity gain, said input being connected to said high gain amplifier, and an amplifier having substantially 100 percent feedback capacitively coupled to the output of said high gain amplifier to supply power to said high gain amplifier from a source which is at the same A.C. potential as the output of said buffer amplifier.

6. The system recited in claim 4 wherein said buffer amplifier and said guard shield amplifier are supplied with a D.C. voltage which is common to the center connection of said bridge but is isolated from the normal common of said circuit.

7. The system recited in claim 1 wherein said guard shield completely surrounds said conductive electrode except in a direction in which impedance is being measured.

8. The system recited in claim 1 wherein said conductive Electrode and said conductive member lie in a common plane opposing said material, the variable impedance between said electrode and said conductive member being a function of the fringe field which varies with the relative position between said material and said plane.

9. The system recited in claim 1 wherein said conductive electrode and said guard shield lie in a common plane and wherein said conductive member lies in an opposed plane so that the material whose condition is to be measured can be passed therebetween and wherein said variable impedance is a measure of the weight per unit length of the material.

10. The system recited in claim 1 wherein said electrode, said guard shield and said conductive member all lie in a curvilinear arc so that material whose composition can be measured can be passed over said arc with minimal imposition of air between said arc and said material, said variable impedance being a measure of the composition of said material.

11. The system recited in claim 1 wherein the condition being measured is the level of said material, said material being between the conductive electrode and said conductive member with the surface of said material being substantially parallel to the surface of said conductive electrode, and wherein said variable impedance is a measure of level of said material.