MXPA97009323A - Two alamb level transmitter - Google Patents

Abstract

The present invention relates to a level transmitter coupled to two process circuits of two wires to measure the height of a product in a tank comprising: a microwave antenna directed to the tank, a low power microwave source for send a microwave signal through the microwave antenna in the tank, a low-power microwave receiver to receive the microwave signal reflected from the tank, a measurement circuit assembly coupled to the source and the receiver to initiate the transmission of the microwave signal and determine the height of the product based on the received signal, the set of output circuits coupled to a two-wire process control circuit for the transmission of the information related to the height of the product towards the circuit, and; the set of power supply circuits coupled to the control circuit of the two-wire process to receive the energy from the circuit to give energy to the transmission

Description

TWO WIRE LEVEL TRANSMITTER Background of the Invention The present invention relates to the measurement of level in industrial processes. More specifically, the present invention relates to measuring the height of the product level in a storage tank of the type that is used in industrial applications using a micro-wave level indicator.

The instrumentation for measuring the level of the product (whether liquid or solid) in storage vessels is being developed from contact measurement techniques, such as a tape or float, to non-contact techniques. A technology that holds considerable promise is based on the use of microwaves. The basic premise involves the transmission of microwaves to the surface of the product and the reception of microwave energy reflected from the surface. The microwave reflected are analyzed to determine the distance they have traveled. The knowledge of the distance traveled and the height of the storage vessel allows the determination of the level of the product. Since it is known that microwaves travel at the speed of light, the distance that a microwave travels can be determined if the time of travel is known. The travel time can be determined by measuring the phase of the return wave and knowing the frequency of the microwave that was transmitted. In addition, travel time can be measured using well-known digital sampling techniques.

A standard in the process control industry is the use of 4-20mA process control circuits. According to this standard, a 4 mA signal represents a zero reading and a 20 mA signal represents a full scale reading. In addition, if a transmitter in the field has sufficient low power requirements, it is possible to power the transmitter using current from the two-wire circuit. However, microwave level transmitters in the process control industry have always 'required from a separate power source. The level transmitters were large and their operation required more power than could be sent using the industrial standard of 4-20 mA. In this way, the transmitters of the typical prior art microwave level required additional wiring in the field to provide the unit's energy. This additional wiring was not only expensive but also a source of power failure.

Compendium of the Invention A level transmitter measures the height of the product in a tank like those used in industrial process applications. The level transmitter is coupled with the two-wire process control circuit, which is used both for transmitting the level information provided by the level transmitter and for the power that is provided to the level transmitter. The level transmitter includes a microwave antenna directed to the tank. A low-energy microwave source sends a microwave signal through the antenna in the tank. The set of measurement circuits coupled to the low energy microwave source and the low energy microwave receiver initiates the transmission of the microwave signal and determines the height of the product based on the reflected signal received by the receiver. The output circuitry coupled to the control circuit of Two wire process transmits information related to the height of the product to the circuit. The set of power supply circuits coupled to the control circuit of the two-wire process receives power from the circuit to energize the level transmitter.

In one embodiment, the measurement set includes a first clock coupled to the source to periodically start the microwave signal at a rate of the first clock. A second clock coupled to the receiver periodically inputs the received signal into a second clock rate.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram of a microwave level transmitter according to the invention.

Figure 2 is a block diagram showing the set of electrical circuits of the level transmitter of Figure 1.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES Figure 1 is a diagram showing the mircoonde level transmitter 10 maneuverablely coupled to the storage tank 12. The storage tank 12 is of the type that is typically used in the application of the process and contains fluid (product) 14. As used herein, the product can be a liquid, a solid or a combination of both. The level 10 transmitter includes the housing 16 and a supply horn 18. The transmitter 10 is coupled to the two wire circuit 20. The two wire circuit 20 is a process control circuit 4-20mA. According to the invention, the transmitter 10 transmits the information related to the height of the product 14 above the circuit 20. In addition, the transmitter 10 is completely powered by the energy it receives from the circuit 20. In some installations, the transmitter 10 complies with the intrinsic safety requirements and is capable of operating in a potential explosive environment without the danger of igniting. For example, the housing 16 is strongly sealed to contain any ignition, and the circuitry in the housing 16 is designed to reduce the stored energy, with which the potential ignition is reduced.

Figure 2 is a block diagram of the coupled level 10 transmitter for a process control room 30 above the two wire process control circuit 20. The control room 30 is modeled as a resistor 32 and a source of voltage 34. The transmitter 10 controls the current 1 flowing through the circuit 20 in response to the height of the product 14 in the tank 12.

The electrical circuit assembly that is transported in the housing 16 of the transmitter 10 includes a voltage regulator 40, the microprocessor 42, the memory 44, the digital-to-analog converter 46 coupled to the output analog circuit assembly 48, the clock of the system 50 and a reset circuit assembly 52. The microprocessor 42 is connected to the UART 54 that controls the digital circuit 1/0 56 and is coupled to the current circuit 20 through the blocking capacitors DC 58. The UART 54 may also be a part of the microprocessor 42. The microprocessor 42 is also coupled to the display module 60 to provide an output of the image and the transmission and reception circuit assembly 70.

The transmitter housing 16 includes a microwave transceiver circuit assembly 70 that includes a clock 1 72 and a clock 2 74. The clock output 1 72 is coupled to the stepped generator 76 providing one. input signal to the microwave distributor 78. The microwave distributor 78 is coupled to the antenna 18 and provides an input to the receiver of the pulse 80. The pulse receiver TC also receives an input of the clock 2 74 and provides an input to the converter. analog to digital 82.

In operation, the transmitter 10 is in caustic position with the quarter of 1 30 above the circuit 20 and receives the energy above the circuit 20. The voltage regulator 40 provides the regulated voltage inputs to the electronic circuit assembly as a whole. the transmitter 10. The transmitter 10 operates in accordance with the instructions stored in the memory 44 under the control of the microprocessor 42 at a rate of the clock determined by the system clock 50. A fault reset and discovery circuit 52 monitors the supply of voltage to the microprocessor and memory. While having power, the circuit 52 provides a reset signal to the microprocessor 42 once the voltage supply has reached a level sufficient to allow the operation of the microprocessor. Additionally, the microprocessor 42 periodically provides an "ick" (pulse) signal to the fault finding circuit 52. If these kick impulses do not receives the circuit 52, this circuit 52 provides a reset output to the microprocessor 42 and thereby resumes the microprocessor 42.

The microprocessor 42 receives information from the circuit set 70 through the analog-to-digital converter 82 to determine the height of the product level. The clock 1 72 operates at a first clock frequency fi and the clock 2 74 operates at a second frequency f2. Clock 1 72 acts as a "start transmission" clock and clock 2 74 functions as a "receiving door" clock and the clocks are displaced slightly in frequency. That is to say This provides a digital sampling technique described in the ISAA document entitled "Smart Transmitter Using Microwave Pulses to Measure The Level of Liquids and Solids in Process Applications" (Intelligent Transmitter that uses microwave pulses to measure the level of liquids and solids in process applications) by Hugo Lang and Olfgang Lubcke of Endress and Hauser GmbH and Company, Maulburg, Germany. The height of the product is calculated by determining which clock cycle 2 74 matches a received microwave pulse. In one embodiment, the clock 1 72 is set for a frequency between 1 MHz and 4 MHz, depending on this condition in the installation as the maximum distance being measured and the current consumption requirements of the circuitry. The clock 2 74 is synchronized to the clock 1 72, but varies in frequency between about 10 Hz and 40 Hz. The difference in frequency (Δ f which gives a difference in clock ratios) between clocks 72 and 74 determines the proportion of the transmitter 10. It is possible to obtain a higher signal level received by integrating pulses received during several cycles at the expense of the updated reduced proportions.

The signal of the clock 2 74 provides an input window that draws through the input signals at a rate determined by? F. The pulse receiver 80 inputs the incoming microwave signal using the signal f2 of the clock 2 74. The output of the pulse receiver 80 is a series of pulses. These impulses will vary in amplitude depending on the noise or false or adulterated reflections contained in the received signal. When the microwave echo receiver from the surface of the product matches the input pulse of the clock 2 74, it results in a larger amount of output pulses and is converted into a higher value by the analog-to-digital converter 82. The microprocessor 42 calculates the distance by determining which clock cycle 2 74 provided the highest output pulse from the receiver 80. The microprocessor 42 determines the distance knowing that the input pulse caused the greatest amount of output pulses from the receiver 80 as determined by the analog-to-digital converter 82. The height of the product was determined by means of the equation: Level - Tank Height - Distance of Impulse Travel Ec.l Level - Tank Altitude - R-? F. C Ec.2 fi 2 - f? One-Way Distance of Impulse Travel R -Af. C Ec.2 fi 2 - f? where: fi = clock frequency 1 I2 = clock frequency 2? f = f2 - fi R = The sample pulse that detected the return to the echo is received (R = 0 to fi /? f) Analog to digital converter 82 should have a fairly fast conversion rate, for example Q. H.H , when the transmission ratio (clock 1) is 2 MHz, since a sample must be taken after each transmission pulse to see if an echo is present; the converter must have a sampling ratio that is at least equal to the clock frequency 1 -72. • An example of this type of analog-to-digital converter is the sigma-delta converter described in the U.S. Patent Application co-pending with Serial No. 08 / 060,448 entitled SIGMA DE1TA CONVERTER FOR VORTEX FLOWMETER (Sigma-Converter). delta for the vortex flow meter). The resolution of the analog-to-digital converter 82 is not particularly critical because only the presence or absence of a pulse is significant.

For a further improvement in the operation of the transmitter 10, the receiver and transmitter circuits in the circuit set 70 are electrically separated from one another. This is important so that the transmitter pulses are not detected incorrectly by the receiver, as is the case with echo pulses. The use of microwave distributor 78 allows exact control of the impedance of the source and the receiving impedance. The use of the microwave distributor provides the isolation between the reception and transmission circuitry. In addition, the distributor 78 prevents the transmitter pulse from causing the received circuit to sound. An example of a distributor is the three-port device that only allows signals from the transmitter circuit (step generator 76) to reach the antenna 18 and the input signals from the antenna 18 to reach the reception circuitry 80. The isolation Electricity between the transmission and reception circuits can be obtained by other techniques known to those skilled in the art. For example, the distributor 78 can be removed and a separate transmission and reception antenna can be implemented. In addition, circuit isolation techniques can be used, which provides isolation between the transmit and receive circuits together with a delay circuit, such that a received pulse was not received until after a signal was extinguished ( "ring") from the transmission pulse. In another embodiment, the microwave antenna 18 is replaced with a probe that extends into the tank 12 shown in Figure 1. This embodiment may also include a dispenser.

Based on the detection of an echo pulse by means of the microprocessor 41 through the analog to digital converter 82, the microprocessor determines the height of the product 14 in the tank 12. This information can be transmit digitally to the two-wire circuit 20 using the digital circuit 56 under the control of the UART 54. Alternatively, the microprocessor 42 can control the level of the current (between, for example, 4 and 20 mA) using an analog converter to 46 to control the output circuit 48 and thereby transmit the information to the circuit 20 of two wires. In one embodiment, the microprocessor 42 can be set to provide a high output (e.g., lßmA) on the circuit 20 if the product level is either above or below the threshold level stored in the memory 44.

In a preferred embodiment, the microprocessor 42 comprises a Motorola 68HC11. This is a low-energy microprocessor that also provides high-speed operation. Another suitable microprocessor is Intel 80C51. Low-power memory devices are preferred. In one mode, a 24 Kbyte EPROM is used for the program memory; 1 kbyte RAM is used for working memory and a non-volatile EEPROM memory of 256 bytes is provided. A typical system clock for the microprocessor is between approximately 2MHz and 4MHz. However, a slower clock requires less energy, but also yields a Smaller updated ratio. Typically, the power supply 40 provides efficient conversion of energy from the control circuit within a supply voltage,. For example, if the supply of the input power is 12 volts and the electronics of the level measurement require 4 mA, the power supply must efficiently convert these 48 mwatts into a usable supply voltage, such as 5 volts. The invention provides a number of significant advances in the art. For example, the transmitter 10 has the full power it receives from the two-wire current circuit 20. This reduces the amount of wiring required to place the transmitter 10 at a remote location. The microprocessor 42 is also capable of receiving control of the current circuit of two wires 20 sent from the control room 30. This is according to the digital communications protocol, for example, the HART® or, preferably, a protocol of digital communications that have a voltage of averaging zero. Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention.

Claims (11)

1. A level transmitter coupled to two process circuits of two wires to measure the height of a product in a tank comprising: a microwave antenna directed to the tank; a low power microwave source to send a microwave signal through the microwave antenna in the tank; a low power microwave receiver to receive the microwave signal reflected from the tank; a measurement circuit assembly coupled to the source and the receiver to initiate the transmission of the microwave signal and determine the height of the product based on the received signal; the set of output circuits coupled to a two-wire process control circuit for transmitting the information related to the height of the product to the circuit, and; the power supply circuitry coupled to the two wire process control circuit to receive the energy from the circuit to give power to the transmitter.

2. The transmitter of the level of claim 1, wherein the measurement circuitry measures a time delay between the transmission of a microwave signal and the reception of a reflected microwave signal.

3. The level transmitter of claim 1, wherein the measurement circuitry includes: a first clock coupled to the source to periodically start the microwave signal in a first clock ratio.

4. The level transmitter of claim 3, wherein the measurement circuitry further includes: a second clock coupled to the receiver to periodically input the received signal into a second clock ratio, and; wherein the measurement circuit set determines the height of the product based on the reception of the signal received and a difference between the first and second proportion of the clock.

5. The transmitter of the level of claim 4, wherein the first clock ratio is generated based on the first clock ratio plus a difference in ratio.

6. The level transmitter of claim 4, wherein the first clock ratio is between about 1.0 MHz and about 4.0 MHz and the difference between the first and the second clock ratio is between about 10 Hz and about 40 Hz.

7. The level transmitter of claim 1, wherein the process control circuit is a process control circuit of 4-10 mA.

8. The level transmitter of claim 1, including a second microwave antenna coupled to the microwave receiver.

9. The level transmitter of claim 4, including a pulse receiver that receives the received signal from microwave and that provides an output in response to the second clock ratio.

10. The level transmitter of claim 1, including a secure housing that intrinsically contains the circuitry of the level transmitter.

11. The level transmitter of claim 1, wherein the microwave antenna comprises an elongated microwave probe that extends into the tank to carry the microwave signal therethrough.