An analog value is continuous not discreet as shown in the analog and
continuous values graph in the previous unit of instruction techniques were
discussed for designing logical control systems that had inputs and outputs that
could only be on or off
these systems are less common than the logical control systems but they are
very important in this unit will examine analog inputs and outputs and their use
in continuous control systems typical analog inputs and outputs for plc talk
to input analog voltage into a PLC the continuous voltage value must be sampled
and then converted to a numerical value by an a/d converter the graph shown on
this screen shows the continuous voltage changing over time
there are three samples shown on the graph the process of sampling the data
is not instantaneous
so each sample has a start and stop time the time required to acquire the sample
is called this sampling time
ad converters can only acquire a limited number of samples per second
the time between samples is called the sampling period t and the inverse of the
sampling period is this sampling frequency also called the sampling rate
the sampling time is more often much smaller than this sampling . the
sampling frequency is specified when buying hardware but for a typical plc a
maximum sampling rate might be 20 Hertz
a more realistic drawing of sample data is shown here the parameters defined on
the previous screen can be used to calculate values for the a/d converters
if the voltage being sampled changes too fast
false readings may occur this device is an 8-bit ADD instruction... SCL or scale
instruction is used to scale data from an analog module and bring it into the
limits prescribed by the process variable this process variable may be
either an arbitrary number or engineering units such as pounds per
square inch degrees Celsius ounces etc when run conditions are true
this instruction multiplies the source by a specified rate the rounded result
is added to an offset value and placed in the destination the equations used in
calculating the linear relationship between the input and the scale that put
our scale value equals input value times rate plus offset rate equals ... Scale
with parameters instruction is an output instruction that contains six parameters
these parameters may be integer floating point or immediate data values or
constants the SCP instruction is only available in SLC 503 processors and
above as well as micrologix 1200 and 1500 processors all of the calculations
needed for the scl instructions are done by the processor
when using the SCP instruction the parameters needed by the SCP instruction
are below input the value to be scaled this may be a word address or an address
of floating point data elements
input men the minimum value for the input low end of range
input max the maximum value for the input high end of range scaled men the
minimum scaling value representing the low end of the scale range of the input
the scaling relationship is linear scale max the maximum scaling value
representing the high end of the scale range of the input the scaling
relationship is linear output the address for the scale value that has
returned after the instruction is executed if any floating point file
types or floating point constants are encountered in any of the previously
described parameters the entire instruction is treated as floating point
and all immediate integer values are converted to immediate floating point
data values the SCP instruction here is exactly the same as described previously
the exception being that the output word that is specified is an analog output
address rather than an integer word address again it's important to note
that all of the calculations are done behind the scenes by the processor
See BIN95.com