BECKHOFF DK9222-0112-0059 User Manual

I/O

Application Note DK9222-0112-0059

Measuring analog signals

Keywords

Loop-powered
2-wire transmitter
True Zero
4-20 mA
KL3458
Analog
Single-ended
Current loop
Loop power
Live Zero
KL3454
KL3054
Difference
Input

Analog signal transmission: Current interface 4…20 mA
(Live Zero principle)

This application example describes the facets of analog signal recording and their forms of transmission in

general. In particular, this document covers the recording of measured values using the KL3054, KL3454 and

KL3458 analog input Bus Terminals from Beckhoff. When used in conjunction with 2 or 3-wire sensors, these

terminals enable the detection of cable breakage and sensor failures internally in the circuit according to

the Live Zero principle.

Analog signal recording

Analog measured variables such as pressure, temperature, flow, speed, etc. are converted by sensors into analog values and,

depending on the sensor, are also linearized inside the sensor. Sensors for analog measured variables usually consist of two

functional elements – the measuring sensor and the transducer – and determine the change in the measured variable by

means of a physical principle. The digitized analog signal transmitted to the controller normally corresponds to a standardized

level and is always composed of the type (voltage, current, resistance, etc.) and the value (0 – 10 V, ± 1 V, 0 – 20 mA, 500 mΩ,

etc.)

For application notes see disclaimer on the last page

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Application Note DK9222-0112-0059

Process

Sensor

Physical quantity

Digital variable

Process data format

Measured
value

Analog
value

Analog
standard signal

Signal
transmission

Analog
input

Control

2-wire sensor

3-wire sensor

sensor

signal

GND

U

B

sensor

signal -

signal +

Measuring analog signals

Fig. 1 The path of an analog process value: from the field into the controller

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In control technology, a sensor can also contain further functional elements (bus connection, integrated scaling of the

measured value, pattern recognition, etc.). It is then usually called a “smart” sensor.

Types of analog sensor connections

Sensors that directly connect to the analog inputs of the controller (‘classic analog sensors’) can be manufactured with a 4-, 3-

or 2-wire configuration, depending on the power supply concept and signal transmission method.

4-wire sensors conduct signal and power each via two wires. The wiring expenditure is high with this type of connection.

3-wire sensors use two wires for the power supply and output the signal via a separate wire. The reference potential is the

GND wire.

2-wire sensors conduct the signal and power supply via one (common) supply wire. The 2-wire technique requires little wiring

expenditure and represents the state of the art in measuring transducers for field and sensor head mounting.

For application notes see disclaimer on the last page

Fig. 2 2-wire sensor with power and signal on one wire, 3-wire sensor with separate wires for power and signal

Methods of transmission of analog process values

In control technology the signal levels for the transmission of the sensor signals are standardized in order to ensure the

most extensive compatibility of sensors and evaluation units possible, without restricting the level of freedom in product

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Voltage Current

Types of analog signals

±2 V, ±10 V –bipolar

0…20 mA

0…2 V, 0…10 Vtrue zero

1…10 V 4…20 mA

live zero

+

–

0

+

0

+

0

Application Note DK9222-0112-0059

Measuring analog signals

development. Voltage or current-based methods of transmission are mainly used (table 1). In the transmission of analog

process values, one must also keep in mind that the digitalized signal has a tendency to dither as the resolution increases

(the more finely it represents the process value). If a high-resolution signal is demanded, the transmission must be free from

superimposed interference. Current-based transmission via so-called “current interfaces” is particularly recommended for such

applications. In comparison with voltage signals they are significantly less sensitive to electromagnetic interference. In general

it can be concluded that, in the case of current interfaces, power-related voltage drops (resulting from the internal resistance

of the supply line) hardly affect the quality of the signal transmission if at all: The length of the cable is limited only by the

maximum available supply voltage of the power source.

Tab. 1 Types of analog signals

Bipolar signals

Bipolar signals alternate around a voltage or current level that is usually 0 (“zero”). Depending on the application and the

specification of the sensor, an offset can be applied to the level. With this type of signal it must be ensured that the sensor

and the evaluation electronics are also suitable for AC voltages. The current-based transmission of bipolar signals is not very

common.

True Zero (0…2/10 V | 0…20 mA)

This type of signal is only conditionally suitable for transmission via 2-wire connections, since signals with the classification

“True Zero” always require external auxiliary power at the start of the measuring range so that the sensor remains “viable.” In

addition, a wire breakage or sensor failure can be detected reliably only with external monitoring.

The adequate detection of a wire breakage or a sensor defect is problematic because the value “0” can be interpreted both as

the end value of the measuring range and as an error. In practice, therefore, an additional external sensor monitor is frequently

used.

For application notes see disclaimer on the last page

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