Scaling Analog Values - Siemens SIMATIC ET 200AL System Manual

2.3

Scaling analog values

Scaling
It is often necessary to calculate the actual process value rather than increments
(e.g., 10 V = 27648 increments) for further processing of digitalized analog values. The
conversion of a value range (e.g., -27648 to +27648 increments) to the original physical
quantity (e.g., 0 to 500 liters) is referred to as scaling.
Scaling blocks
STEP 7 offers the suitable program block for scaling of analog values. The SCALE block
is included in the STEP 7 scope of delivery and permits input of a high and low limit
(e.g., 0 to 500 liters).
Example
In the example below the fill level of a tank with a capacity of 500 liters is to be measured.
The transducer used measures a voltage of -10 V with an empty tank and a voltage of +10 V
with a full tank. The analog module converts the voltage range from -10 V to +10 V into the
value range -27648 to +27648 and converts this range into the original quantity of 0 to
500 liters with the SCALE program block.
Figure 2-5
The analog value at input IN is read directly by the module or is transferred by a data
interface in INT format. The input LO_LIM is used to determine the low limit (0 liters) and
HI_LIM to determine the high limit (500 liters) of the physical quantity. The output OUT
outputs the scaled value (physical quantity) as floating-point number
(LO_LIM <= OUT <= HI_LIM). You can assign parameters via the input BIPOLAR to specify
if only positive or only negative values are to be converted. If the parameter receives an
operand with the state '0', there is a unipolar scaling. An operand with the state '1' results in
a bipolar scaling. The output RET_VAL outputs an error code in case of an error (e.g.,
overflow) and the value '0' in case of an error-free execution.
Additional information on the SCALE block can be found in the STEP 7 online help.
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
Scaling in the SCALE program block
What you should know about analog technology
2.3 Scaling analog values
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