Flow Measuring Apparatus With Analog, Essentially Linear Output

The present invention relates to instrumentation for measurement of flow velocity and mass flow rates.

The measurement of gas flows with solid particles therein (and certain fluids which behave similar thereto) is a substantial, unresolved problem in several industries--e.g., chemical processing, food handling, transport loading and unloading, filtration, aeronautics, combustion fuel feed. The state of the art is limited to complicated sample and weigh and costly in-line, real time wave energy scanning devices (ionization, x-ray, ultrasound, back-scattering, seeding with nuclear or optical emitting traces and read-out by corresponding scanners). The art has also used standard fluid flow meters with a density correction on account of contained solids.

It is a principal object of the invention to provide a relatively simple, low cost (capital and operational), low maintenance, sensor of flow giving up essentially linear, analog input that is accurate and available in real time.

The object is satisfied by instrumentation utilizing triboelectric effects. A metal probe is inserted in the stream of flowing solids. Those particles in the flow which strike the probe, impart a charge transfer, due to the phenomena known collectively as the triboelectric effect. The charge which is transferred to the probe returns to earth ground through the electronics, constituting a input direct current to the electronics. The rate of charge transfer is proportional to the flow rate of solids impacting the probe.

Triboelectric monitoring has been described in U.S. Pat. No. 3,359,796 to R. C. Dimick granted Dec. 26, 1967. The industrial arts were not advanced by this disclosure because a triboelectric probe in a gas/solid stream encounters a variety of unusual conditions and problems recognized and overcome through the present invention.

In accordance with the present invention, a flow measurement instrument utilizes a triboelectric sensor probe in (or coupled to) the flow.

The current signal from the sensor probe is conducted to the electronics through an ultra low noise coaxial cable. An over voltage protector is installed at the probe to prevent voltage built-up in the event of cable disconnection. The first stage of the electronics is a current to voltage conversion circuit with conversion factor adjustable from 1.times.10.sup.8 V/A to 1.times.10.sup.12 A/V in X10 steps with 10:1 continuous adjustments. A current limiting barrier resistor is placed between the input summing junction and the remainder of the circuitry to insure that the sensing probe and cable are intrinsically safe, thus allowing the probe to be installed in hazardous areas. This safety feature forms no part of the measurement means, per se.

Since either polarity of charge transfer may occur at the probe, for different species of particles striking the probe and/or constituting the flow stream, an absolute value circuit must be employed to provide a unipolar signal. An adjustable low pass filter follows the absolute value circuit to provide damping of high frequency signal components.

Due to offset voltage and current characteristics of the current to voltage conversion circuit, the output of the circuit will vary with ambient temperature. To eliminate the temperature effect, an auto-zero circuit is connected so as to re-zero the circuit periodically (about once per minute). An asymmetric timing ciruit disconnects the amplifier input by energizing the input relay and connects an integrator circuit, so as to provide negative feedback to zero the current to voltage conversion circuit. The integrator maintains the zeroing level. A sample and hold circuit is used to hold the present output signal during the zeroing cycle. A time delay circuit causes the output to be held until the converter has settled after the zeroing cycle.

The 0-8 Volt output signal is converted to 4 to 20 mAmp to provide a standard instrumentation output.

A meter is provided to display the flow signal and as an aid in adjusting the instrument.

An adjustable set point relay output is also provided so that an alarm or control function can be initiated at a selected flow level. The meter is also used to set the relay set point.

Other objects, features, and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawing in which:

FIGS. 1-3 are circuit diagrams for elements of the apparatus of a preferred embodiment of the invention.

FIG. 1 is a schematic of the electronics which amplifies and conditions the signal from a triboelectric probe 1 inserted into a flow pipe Fp. The electronics provides a standard instrumentation current output 19, and a 0 to 8 V. voltage output, drives a meter 20, and inputs a signal to a threshold circuit 21 (comprising an op amp in two stages with differential adjustment, FIG. 1A), which actuates a relay 24.

The single ended signal is directed through a low noise coaxial cable 2, a low noise current limiting resistor 3, then through a relay 5 contact closure to an op amp 10 configured as a voltage to current converter. An over voltage protection device 4 protects against excessive voltages, together with the current limiting resistors they provide an intrinsically safe network, allowing the probe to be used in hazardous areas. Over voltage devices 1A and 1B limit the maximum voltage within the probe itself.

A low noise, low drift op amp 10 directs the signal current from the probe 1 through resistor 6 and 7. This configuration produces a low output impedance signal voltage at the op amp output 8. The signal voltage equals the signal current multiplied by the effective resistance of resistor 6 whose voltage drop is modified by a voltage divider which has gain adjustment (step) means (not shown).

Since the signal current may be in either (direction), the voltage signal at op amp 10 output 8 can be either a positive or negative voltage. An absolute value circuit 12 produces a positive voltage at its output 12A equal in magnitude to the positive or negative voltage magnitude at its input 12B. FIG. 2 shows a more detailed schematic of this circuit. If a positive voltage is applied to the input 34 of op amp 35, the op amp 35 drives its output 35A positive forward biasing the diode 36, reverse biasing diode 41, and driving the input 38 of op amp 39 positive, which in turn drives the output 40 positive. Diode 41 is reversed bias. Resistors 43 and 44 provide feedback to input 42 of op amp 35 to cause the whole absolute value circuit to act as a follower with a gain of +1. When the signal is a negative voltage applied to input 34, the output 35A drives negatively thereby reverse biasing diode 36 and forward biasing diode 41. Op amp 35 acts to cause the input 42 to go negatively until it equals the negative signal input. The negative voltage at input 42 causes a current through resistor 43. The input 38 of op amp 39 is held at zero volts by resistor 37, and the output 40 drives positive enough to supply the current through resistor 44 to equal the current through 43 and hold the input 45 at zero volts. Resistors 43 and 44 are equal ensuring the positive voltage at output 40 equals the negative voltage value at input 42 and also input 41--the signal input. The gain is -1. The circuit thus produces an output positive voltage equal in magnitude to the positive or negative voltage magnitude input.

Referring back to FIG. 1, the positive voltage at the absolute value circuit output 12A is directed, through switch 13, to a one pole low pass filter composed of a capacitor 15 and an adjustable resistor 14, which allows the time constant to be trimmed. A voltage follower buffer op amp 16 receives the filtered signal at node 16A and drives: a milliammeter 20 scaled to read appropriately by resistor 21; a comparator 21 which closes relay contacts available to the user and lights a red LED when a selected threshold is exceeded; and a voltage to current converter circuit 18 suitable as an instrumentation output.

FIG. 3 is a more detailed schematic of the voltage to current circuit 18. A voltage signal 0 volts to 8 volts is input at node 46 and creates a current through resistor 47 into input 49 of op amp 51. This current is balanced by the current through resistors 61 and 48 holding input 49 at 0 volts. Op amp 51 drives its output 50 to a voltage necessary to produce the balance at input 49. The output 50 voltage is applied to a voltage divider formed by resistors 52 and 53 and the divided voltage is applied to the input 54 of op amp 55. Op amp 55 responds by turning on FET 59, driving a current through resistor 57 and the FET 59 to a point of use 60. This current increases to where the voltage across resistor 57 at the input 56 equals the voltage at input 54. The value of resistor 57 determines the current value delivered through FET 59. By varying resistor 61, its current changes and the output 50 of op amp 51 changes to maintain the current balance at input 49. By this mechanism the subsequent current output throgh FET 59 can be trimmed to a convenient level to represent a zero or full scale signal (4 mA). As the signal at 46 goes positive, the voltage across 57 increases thereby increasing output current at 60 up to 20 mA at 8 volts.

Referring back to FIG. 1, the output of op amp 10, the input current to voltage converter, is temperature sensitive and it is automatically zeroed each minute. In FIG. 1, when switch 11 is closed the integrating amplifier 27 drives a current through resistor 26 to input 10A forcing the output 8 to zero volts. During this time the input current from the probe 1 is decoupled by input relay 5, so the integrating amplifier 27 is balancing out temperature caused offsets. The integrating amplifier 27 stores the voltage necessary to maintain the offset balancing current when switch 11 is opened and relay 5 closes.

Relay 5 also connects the signal input to 10A during the zeroing sequence to prevent charge build-up on cable 2.

The auto zeroing would cause all the outputs to go to their zero levels. However, switch 13, resistor 14 and capacitor 15 act as a track and hold circuit. The last signal, before auto zeroing, is represented by the voltage on capacitor 15, and before the auto zero can change this voltage, switch 13 opens. This holds the voltage on capacitor 15 and the outputs to their last values while auto zeroing occurs. When auto zero is complete switch 11 opens, and, after the new signal levels are established through to the absolute value circuit output 12A, switch 13 closes. The requisite delay is formed in delay circuit 30. Timing circuit 33 provides the timing signals necessary to auto zero approximately each minute and lights a yellow LED when the auto zero is active.

A capacitively coupled probe 1P with OVP, which produces an AC signal, can be used with the aforementioned circuitry. With this probe 1P and switch, with sections 29, 32, and 17, places the auto zero function in continuous operation and directs the signal from the current to voltage converter op amp output 8 directly to the voltage to current converter 18.

The active components are, typically:

10--Burr, Brown OPA 104 CM

11/13--Harris Semiconductor HI1-200-5

16, 21, 27, 35, 39,--RCA CA3240E

51, 55--NATIONAL SEMICONDUCTOR LM1458

The passive components are, typically:

3--150k (RL20)

6--100 Meg. Ohms

7--0 to 10k

26--1 Meg.

25--100 ohms

14--0 to 1 Meg.

15--22 microfarads

37--4.37k

43--10K

44--10K

47--10k

48--10k

61--5k

52--4k

53--1k

It will now be apparent to those skilled in the art that other embodiments, improvements, details, and uses can be made consistent with the latter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.