Transmitter for transmitting a process variable to a programmable logic controller

Abstract

The invention relates to a transmitter for transmitting a process variable to a programmable logic controller. In a current mode a 4-20 milliamp current is fed into a burden and in a voltage mode 0-10 Volt signals are generated. The transmitter includes a process value input, a current stage, a voltage stage and a U/I output for the current or the voltage signal, and a U/I control input for switching between the two modes. The transmitter is configured to feed a current which is dependent on the voltage across the burden into the current stage via a resistor and thus to compensate for a current loss at the voltage stage which is dependent on the voltage across the burden.

Claims

1. A Transmitter for transmitting a process variable to a programmable logic controller, wherein in a current mode a 4-20 mA current is fed into a burden (1) and in a voltage mode 0-10 V signals are generated, comprising a process value input (2), a current stage (3), a voltage stage (4) and a U/I output (5) for the current or the voltage signal, and a U/I control input (6, 6a, 6b) for switching between the two modes, wherein the transmitter is configured to feed a current which is dependent on the voltage across the burden (1) into the current stage (3) via a resistor (7) and thus to compensate for a current loss at the voltage stage (4) which is dependent on the voltage across the burden (1).

2. The transmitter according to claim 1, wherein the resistor (7) is disposed between the U/I output (5) and the input of the current stage (3).

3. The transmitter according to claim 1, wherein the transmitter is configured to avoid a mutual interference of the operating modes by switching off the current stage (3) by means of interrupting its supply voltage in the voltage mode, and by short-circuiting the input of the voltage stage (4) in the current mode.

4. The transmitter according to claim 2, wherein the transmitter is configured to avoid a mutual interference of the operating modes by switching off the current stage (3) by means of interrupting its supply voltage in the voltage mode, and by short-circuiting the input of the voltage stage (4) in the current mode.

5. The transmitter according to claim 1, wherein the U/I control input (6, 6b) is connected to an output of a microcontroller.

6. The transmitter according to claim 2, wherein the U/I control input (6, 6b) is connected to an output of a microcontroller.

7. The transmitter according to claim 3, wherein the U/I control input (6, 6b) is connected to an output of a microcontroller.

8. The transmitter according to claim 4, wherein the U/I control input (6, 6b) is connected to an output of a microcontroller.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained by way of example with reference to the attached drawings based on preferred exemplary embodiments, wherein the features shown below both individually and in combination may represent an aspect of the invention. In the drawings:

(2) FIG. 1 shows schematically the components of the transmitter in accordance with the present invention.

(3) The present invention will be described in connection with a preferred embodiment, however, it will be understood that there is no intent to limit the invention to the embodiment described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by this specification and the attached drawings and claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) For a general understanding of the present invention, reference is made to the drawings. The present invention will be described by way of example, and not limitation. Modifications, improvements and additions to the invention described herein may be determined after reading this specification and viewing the accompanying drawings; such modifications, improvements, and additions being considered included in the spirit and broad scope of the present invention and its various embodiments described or envisioned herein.

(5) The present invention relates to a Transmitter For Transmitting a Process Variable to a Programmable Logic Controller.

(6) The essential idea of the invention is to make the output resistance of the voltage stage, which is mainly determined by a voltage divider, as low-ohmic as required for signal technology, and to compensate the current loss by a resistor supplied with the (unknown) burden voltage. This is advantageously achieved by feeding the current determined by said resistor and the voltage across the burden into the current stage, since this makes it proportional to the current losses, which are also dependent on the burden voltage, thus making the above-mentioned complex measuring and control devices dispensable.

(7) As has been found, it is particularly advantageous to dispose the compensation resistor between the U/A output and the input of the current stage.

(8) An advantageous further development of the invention is to prevent a mutual interference between the two operating modes, namely the current mode and the voltage mode, by switching off the current stage by interrupting its supply voltage in the voltage mode, and by short-circuiting the input of the voltage stage in the current mode.

(9) Furthermore, it is advantageous to connect the control input necessary for said switching action to an output of a microcontroller (?C), wherein it is irrelevant whether there is a common control pin for the current stage and the voltage stage, and/or whether they are controlled by different terminals of the ?C.

(10) The invention will now be explained in more detail by means of an exemplary embodiment with reference to the drawing.

(11) FIG. 1 shows schematically the components of the transmitter according to the present invention.

(12) Shown is a temperature measuring device with a constant series resistance Rc and a temperature-dependent resistor ? in an extremely simplified representation.

(13) The burden 1 consists of an ohmic resistor and a parallel capacitance. It is normally part of a programmable logic controller (PLC).

(14) The process value input 2 receives an analog measurement signal (measured value) generated by an amplifier V, wherein the amplifier V can be controlled by a microcontroller.

(15) Furthermore, a current stage 3 and a voltage stage 4 are shown, which convert the analog measurement value applied to the process value input 2 into a current signal (4-20 mA) or into a voltage signal (0-10 V).

(16) Their outputs are combined as a common U/I output 5 for current or voltage. The mutual interference of these two operating modes is avoided on the one hand by switching off the current stage 3 by interrupting (cutting off) its supply in the voltage mode, and on the other hand by short-circuiting the input of the voltage stage 4 in the current mode. This is done by means of a U/I control input 6 shown here in two parts (two inputs 6a and 6b), wherein via 6a the supply voltage of the operational amplifier 8 controlling the output transistor is interrupted (cut off), and via 6b the input of the voltage stage 4 is short-circuited.

(17) As is easily obvious to a person skilled in the art, 6a and 6b can be combined to form a U/I control input 6, but this deprives the possibility of a separate control of the two output stages 3 and 4.

(18) According to the invention, the output resistance of the voltage stage 4 determined by the voltage divider 9 is designed as low-ohmic as required for signal technology, and the resulting current loss is compensated by a resistor 7 supplied with the (unknown) burden voltage. This is achieved by feeding the current determined by the resistor 7 and the voltage across the burden 1 into the current stage 3, since this current is thereby proportional to the current losses, which are also dependent on the burden voltage, which makes the above-mentioned complex measuring and control devices dispensable.

(19) As has been found, it is particularly advantageous to dispose the compensation resistor 7 between the U/I output 5 and the input of the current stage 3.

(20) The described transmitter can advantageously be used for detecting a process variable in a production plant of the food industry or also of the chemical industry for detecting pressure, temperature, flow rate or filling level (level).

LIST OF REFERENCE SYMBOLS

(21) 1 burden 2 process value input 3 current stage 4 voltage stage 5 U/I output 6, 6a, 6b U/i control input 7 compensation resistor 8 operational amplifier 9 voltage divider

(22) While the various objects of this invention have been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of this specification and the attached drawings and claims.