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Process Data In
Process Control Out
Programming Model
Chapter 3
code that implements a control algorithm. You define the algorithm for a
control loop by selecting one of the HP E1415's two standard PID
algorithms, or by downloading a custom algorithm you have created in the
HP E1415's Algorithm Programming Language (a 'C' like language). Once
defined, the control loop algorithm becomes a subprogram function that is
executed each time the module receives a trigger signal and after all input
signal channels have been scanned.
The HP E1415 provides advanced data acquisition capability which
includes on-board signal conditioning and Engineering Unit (EU
conversion. The signal conditioning means accurate signal values from a
wide range of process sensors. The EU conversion means that signal values
measured at process sensors will be returned in standard engineering units
such as volts, Ohms, degrees C, or micro strain. Further, custom EU
conversions can be defined to convert signal values from standard sensors,
to values more closely related to the process variables being measured. For
instance, voltage from a pressure sensor can be converted to PSI. The input
data appear to the control algorithm as program constants. They are
constants only in that the algorithmic program cannot change their values.
These values are updated each time a trigger causes the input channels to be
scanned. After all input channels are scanned, each of the defined and
enabled control algorithms is executed.
Control output to the process is determined by the executing algorithms. In
general the algorithm assigns a value to one of 64 special "output channel"
identifiers. If the algorithm executes the statement:
O107 = control_out_var;
the value of the variable "control_out_var" is placed in the Output Channel
Buffer entry for channel 7. After all active algorithms have been executed,
the buffer values (one for each assigned channel) are sent to the output
Signal Conditioning Plug-ons (SCP) at those channel positions. The
characteristic of the actual output quantity is determined by the type of
output SCP that is installed at the specified channel. For instance, if an
HP E1532 Current Output SCP were installed at the specified channel, the
parameter value could range from -0.01 to +0.01 Amps (±10 mA). A
Voltage Output SCP at the channel would allow a parameter value of -16 to
+16 Volts.
You is configure, start, stop, and communicated with the HP E1415 using its
SCPI command set. The module can be in one of two states; either the "idle"
state, or the "running" state. The INITiate[:IMMediate] command moves the
module from the "idle" state to the "running" state. We will call these two
states "before INIT", and "after INIT". See Figure 3-2 for the following
discussion.
Before INIT the module is in the Trigger Idle State and its DSP chip (the
on-board control processor) is ready to accept virtually any of its SCPI or
Common commands. At this point, you will send it commands that
configure SCPs, link input channels to EU conversions, configure digital
input and output channels, configure the trigger system, and define control
algorithms.
Programming the HP E1415 for PID Control
57
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