WO1994017367A1

WO1994017367A1 - Universal measurement device

Info

Publication number: WO1994017367A1
Application number: PCT/US1994/000855
Authority: WO
Inventors: Martin J. Burns
Original assignee: Honeywell Inc.
Priority date: 1993-01-22
Filing date: 1994-01-21
Publication date: 1994-08-04
Also published as: AU6094994A

Abstract

A universal measurement device for receiving inputs from at least one external source, the universal measurement device having at least one input receiving means for receiving an input from an external sensor. The input means provides an output equivalent to the input from the external sensor to a selecting means which selects one of the input receiving means and provides the output from the input receiving means to a buffer amplifier. The buffer amplifier is electrically connected to at least one conversion means. Each of the conversion means then converts the selected output from the input receiving means and converts the signal. Each of the conversion means provides its output to a second selecting means. A second selecting means selects the appropriate conversion means output, this signal then being the appropriate converted sensor input for use.

Description

UNIVERSAL MEASUREMENT DEVICE

BACKGROUND OF THE INVENTION This invention is in the field of signal conditioning devices. More specifically, this invention is in the field of signal conditioning devices which must be capable of conditioning signals from varying sensor inputs and converting those signals to usable outputs.

Various sensors can become extremely reliable and reproducible for the measurement of varying conditions. In a standard heating, ventilation and air conditioning system (HVAC), a system must be capable of measuring many different conditions. An example would be the system must be capable of measuring the discharged air temperature from a furnace, damper positions, room temperature, outside air temperature, and numerous other conditions. In a standard system, each of these measurements would be done by utilizing a single sensor and the proper conversion circuitry to provide a signal to a microprocessor so that the microprocessor can evaluate this information. However, this requires numerous piece parts to be added to the system in addition to the individual sensor. For example, resistance temperature devices (RTD) are extremely reliable and reproducible transducers for the measurement of temperature. A shortcoming is that the variation of resistance is small with respect to temperature and this requires special circuitry to amplify and linearize the resistance produced. Thus, when one utilizes an RTD, one must utilize secondary circuitry to scale the inputs. As each RTD requires conditioning circuitry, it is desirable that the same circuitry be utilized for all RTDs. However, since RTDs are not the only sensor inputs, the system must be capable of conditioning many variable inputs.

SUMMARY OF THE INVENTION To accommodate a wide array of sensors, the universal measurement device utilizes highly precise .1% tolerance resistors with common ground at the circuitry for the input. In addition, a switchable 5-volt DC source was provided to power sensors briefly to allow measurement. To provide for the wide range of currents to be sensed, several amplifiers can be utilized to adjust the range of input voltages generated by currents flowing through the precision resistor. The inputs to the universal measurement device are converted to a voltage by providing the input to a precision resistor electrically connected to ground. The voltage drop across the resistor provides voltage input to the buffer amplifier. In practice, a desired input is conditioned selectin the desired input and routing the input to a buffer amplifier via an analog selector. The buffered signal is provided to all of the conditioning amplifiers. The desired buffer and scaling amplifier is chosen based on the sensor type. The voltage at this point is sampled and combined with a reference leg made of two precision resistors. Since the RTD, 200 ohm resistor and a buffer amp are chosen with negligible error, the bridge circuit can accurately scale the voltage range of interest. The selected amplified and scaled output is routed to the analog-to-digital converter. To calibrate the system, measurements are taken using reference resistors and a regression is performed and the results stored in EEPROM on the universal measurement device mother board. Thus, errors due to component initial values can be compensated for and used for each measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates the schematic for the universal measurement device. Fig. 2 illustrates one embodiment of an input circuit. Fig. 3 illustrates a switchable 5 -volt supply.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment comprises at least one input circuit 10, at least a first multiplexer 30, buffer 40, amplifying circuit 50, a first scaling circuit 51 and a second scaling circuit 52. The universal measurement device also comprises a second multiplexer 60 and an A-to-D converter 70. The input circuit 10 comprises an input node 15A which is electrically connected to a sensor (not shown). Node 15A is electrically connected to resistor 11 A which is also connected to ground. In this manner, when a sensor provides an input signal to node 15 A, resistor 11 A converts that input signal to a voltage. The preferred embodiment is designed to operate with an input range of 0-20 milliamps. For this reason, a 200 ohm resistor has been selected for a resistor 11 A. In this manner, the scaled inputs at resistor 11 A will be between 0 volts to 4 volts. The voltage drop across resistor 11 A is provided to multiplexer 30 through resistor 12 A. Resistor 12A must be a resistor of a sufficient value to prevent a significant current drained through resistor 12. In the preferred embodiment, multiple input circuits 10A through ION are illustrated. As this system is only limited by the number of multiplexer 30 inputs, there is no limit on the number of sensor inputs which can be conditioned through this circuit. An outside source, such as a microprocessor, provides a first input 80 to multiplexer 30 to determine which one of the inputs the outside source wishes to investigate. For this embodiment a 74HC4051 multiplexer is utilized and is available from National Semiconductor. The selected input signal is provided to buffer 40 and the output of buffer 40 is provided to multiple calibration amplifiers. As an example in the preferred embodiment, the buffered signal is provided to multiplier 50 and conversion amplifiers 51 and 52. By providing all of the conditioning amplifiers after the signal is buffered and selected by multiplexer 30. it is possible to decrease the number of components necessary to utilize numerous sensors. By providing one amplifying circuit to be utilized with numerous sensors, the designer need only select which conditioning circuits are appropriate and may then add as many sensors as the operator wishes without having to provide a new conditioning circuit for each sensor. The inputs from the conditioning circuits 50, 51 and 52 and the input directly from buffer amplifier 40 are provided directly to multiplexer 60. A second input 82 from the outside source determines which one of the conditioned signals is provided to the A-to-D converter 70. A-to-D converter 70, when receiving this signal, converts it to a digital signal which is provided to a microprocessor or other source which requires the sensed signal.

Fig. 2 illustrates a second embodiment of input circuit 10. Fig. 2 still utilizes resistor 11; however, a transient suppression diode 17 is electrically connected to node 15 and ground. In this manner, transients, which may be harmful to the system, are suppressed. Due to the simplistic input circuit utilized and the use of a high impedance input to the buffer, it is possible to avoid transient problems with a minimal parts count. This helps in achieving the overall goal of the invention of great versatility with a minimum parts count. Sensor 16 provides an input current to input node 15. For this embodiment, an RTD sensor is used. An RTD sensor may be a Honeywell C7100A averaging discharge sensor, or a Honeywell C7170A encapsulated temperature sensor, or the system may utilize a thermistor. The only requirement for a sensor is that it provide a 0 to 20 milliamp input. For this embodiment, a 5 -volt source is provided at the input to the RTD sensor 16.

In order to prevent overheating, a switched 5-volt source is provided to the sensors to allow a microprocessor or some other source to turn sensor 16 ON and OFF, thus, preventing continuous operation which would cause self-heating errors.

Therefore, a switchable 5-volt source, as illustrated in Fig. 3, is provided. The switchable 5-volt source comprises a transistor 92 with the source electrically connected to a 5.3 volt DC source. The 5.3 volt DC source is also electrically connected to resistor 93 which is electrically connected to the base of transistor 92. The emitter of transistor 92 is electrically connected through diode 91 to output node 90. Output node 90 provides a 5-volt source to all of the external sensors. Transient protection is provided by diode 91 and zenor diode 96. The base of transistor 92 is also electrically connected to resistor 94 which is electrically connected to an external source. External source 95 may be a signal provided by a microprocessor or it may be some other form of an external switch. When external source signal 95 is electrically connected to ground, transistor 92 is energized and node 90 provides a 5-volt source. When external source signal 95 is electrically high or a high impedance, transistor 92 is de-energized and node 90 does not provide any voltage and thereby shuts down all external sensors. In this manner, self-heating is prevented.

Claims

1. A universal measurement device for receiving the inputs from at least one external sensor wherein said universal measurement device comprises: at least one input receiving means for receiving an input from an external sensor, said input means providing an output equivalent to the input from said external sensor; a first selecting means for selecting one of said input receiving means outputs; at least one conditioning means, said conditioning means receiving said selected output from said selected input means, said conditioning means conditioning said output and providing a conditioned signal; a second selecting means, said conditioning means providing said conditioned signal to said second selecting means, said second selecting means selecting one of said conditioned signals.

2. The universal measurement device of claim 1 wherein said input means comprises a first and a second resistive means having a first and a second terminal, said first terminal of said first resistive means electrically connected to one of the external sensors said second terminal of said second resistive means electrically connected to ground, said first terminal of said second resistive means electrically connected to said first terminal of said first resistive means, said second terminal of said second resistive means electrically connected to said first selecting means.

3. The universal measurement device of claim 2 wherein said input means further comprises a transient suppression diode electrically connected in parallel to said first resistive means.

4. The universal measurement device of claim 1 further comprising a buffer, said buffer buffering said selected output from said first selecting means and providing a buffered signal to said conditioning means, said conditioning means conditioning said buffered signal and providing a conditioned signal.

5. The universal measurement device of claim 3 further comprising a buffer, said buffer buffering said selected output from said first selecting means and providing a buffered signal to said conditioning means, said conditioning means conditioning said buffered signal and providing a conditioned signal.

6. The universal measurement device of claim 1 further comprising a switchable power source for providing power to the external sensors, said switchable power source providing power to said external sensors when said first selecting means for selecting one of said input receiving means outputs selects one of said input receiving means outputs.

7. The universal measurement device of claim 5 further comprising a switchable power source for providing power to the external sensors, said switchable power source providing power to said external sensors when said first selecting means for selecting one of said input receiving means outputs selects one of said input receiving means outputs.