What signals are we talking about?
Analog signals are electrical voltage and current signals. To map a changing physical quantity, a sensor can either generate an electrical voltage or change the voltage drop in the measuring circuit.
Typically, the following variables are measured in system and process technology:
- Level flow rate
- Deformation with regard to load measurement
- Gas concentration
- Electrophysical variables such as voltage, current, field strength, etc.
Stations of the measuring signal processing
The electrosensory acquisition, conditioning, and evaluation of status data referring to the environment or an industrial system are considered to be the core areas in the field of MCR technology.
These three areas are the main focus:
- Signal generation in the field, as the monitored area to be controlled is called
- Signal conditioning on the interface level or directly on the field level by means of electronic components for amplification, conversion, and protection from interference on the signal path
- Components for amplification, conversion, and protection against signal path interference.
The analog or digital signal processing on the control level by an evaluation or control unit.
Signal conditioners and measuring transducers
The electronic components for signal conditioning are collectively referred to as signal conditioners, signal isolators, or measuring transducers. You can have one or more of these functions:
- Electrical isolation
- Electrical supply of connected components
- Line monitoring
1. Signal amplification
Signal amplification is always required if a signal is too weak and its acquisition by the evaluation unit will only be distorted or attenuated.
Example: Without the amplifier, the load of 320 Ω connected to the measuring sensor would be higher than its maximum allowable load of 300 Ω. The measuring sensor is not able to drive this load, which would result in a distorted measuring signal.
By adding an amplifier, the load connected to the measuring sensor is 70 Ω and thus lower than the maximum permissible load of 300 Ω. Furthermore, the input resistance of the evaluation unit of 300 Ω does not overload the amplifier output, as it is intended to drive a load of up to 500 Ω. The measuring signal will not be distorted.
2. Conversion to a standard signal
Depending on the measuring task, analog sensor signals can be converted into one of the standard signals in an interface block. The conversion result should be proportional to the measured input value to avoid a distortion of the measurement results.
Example: The sensor or transmitter provides a standard signal of 4 to 20 mA. The evaluation unit requires a signal of 0 to 10 V. The standard signal converter connected between the transmitter and the evaluation unit makes the required adjustment.
In lines for measured value transmission, interference voltages can occur due to effects such as electromagnetic induction or high-frequency signals in industrial environments of frequency converters, for example. Interference is particularly strong if voltage signals are affected.
Example: The signal conditioner with filter function detects and suppresses interference voltages in a wide frequency spectrum.
It is also helpful to use additional twisted or shielded wires. Twisted wires help to reduce the induced interference voltage, and shielded wires additionally reflect and absorb electric fields. To further avoid interference resulting from the causes mentioned above, the voltage signal should be converted into a current signal.
4. Electrical isolation
An electrically isolated signal connection is referred to as a floating connection because there are no compensating currents flowing between potential differences. Electrical isolation of field circuits and control circuits has become an established standard in the plant and process industry.
The problem: The transmitter and evaluation unit are grounded, but they have different ground potentials. A compensating current Ig flows through the resulting ground current loop and thus distorts the measuring signal I1.
After integrating a galvanic signal isolator, such as a transmitter, into the connecting cables for the measuring signal, the lg compensating current no longer flows. The measured signal is I2, which is identical to the measured signal I1.
5. Line monitoring
Line monitoring is integrated in many interface blocks as an additional function. The monitoring function for line interruptions and short circuit is specified in greater detail in the NE 21 NAMUR recommendations provided by the User Association of Automation Technology in Process Industries.
The figure shows schematically how line monitoring can be used on the entire signal transmission path from the sensor to the evaluation unit.
The 400 to 2 kΩ resistance ensures a maximum current with closed switch that is smaller than the short-circuit current. The 10 kΩ resistance provides a closed-circuit current when the switch is open. In the event of a line break, the current = 0.
Electrical supply and isolation of signal paths
Depending on whether the connected sensor or transmitter has its own power supply or is supplied via sensor signal cables, there is a difference made between a passive and active input at the input terminals of a signal conditioner or evaluation unit.
The only function of the signal input is to receive the signal. In the example, the signal conditioner and the evaluation unit have passive inputs. The active sensor or transmitter (with four connections) supplies the passive input of the signal conditioner. The active output of the signal conditioner supplies the passive input of the evaluation unit.
The signal input has two functions: one is to receive the signal, and the other is to supply power to the signal generator.
In the example, the signal conditioner has an active input. It supplies the 2 or 3-wire sensor or transmitter. The active output of the signal conditioner supplies the passive input of the evaluation unit (as in the previous example). Components that need an electric power source can be supplied with separate power supplies or via the signal lines.
Passive isolation, input loop-powered
Signal conditioner supply via a signal input through the transmitter (input loop-powered).
The signal paths between the active (4-wire connection) sensor/transmitter and the signal conditioner are not electrically isolated from the transmitter supply here. In this case, the active sensor/transmitter takes over the supply of the signal conditioner.
The sensor/transmitter needs to drive the total load of the signal conditioner and the evaluation unit input.
Only suitable for 4 to 20 mA signals.
Passive isolation, output loop-powered
In this case, the isolation amplifier supply is provided via the signal output by the evaluation unit (output loop-powered).
The signal path between sensor or transmitter (4-wire connection) and signal conditioner is separated from the transmitter supply.
The signal path between signal conditioner and evaluation unit is not electrically isolated from the supply for the evaluation unit. In this case, the evaluation unit takes over the supply of the signal conditioner. Only suitable for 4 to 20 mA signals.