Overview
The process simulation logic consists of a block of configuration that
simulates the process by accepting controller outputs and returning calculated
process variables (PV's). The simulation logic replaces the I/O and uses the
control logic and graphics with all revisions that are made during normal
operation. Analog indication points are manually set to normal operating values
with a signal generator block or calculated from other process variables as
required for training.
Descriptions of how each element of the process is simulated
are shown below :
Analog Control Logic Simulation
Discrete Control Logic Simulation
Analog Control Logic
Simulation
Flow Control Loops
Process variables for non-interacting PID loops (i.e. flow into an open tank)
are simulated by applying the PID controller output to a function generator
block to simulate the process gain. The output of the function generator is fed
to a lead/lag block to simulate the process time constant. The time constant in
the lead/lag block is set based upon the loop type and valve size to provide a
response similar to the process being simulated. Note that the function
generator block allows the output signal to be inverted to simulate air to close
valves by simply swapping the output coordinates. Flow loops can include a
status signal to set the flow to 0 if the supply pump is off or a discrete block
valve is closed.
The PV for the outside loop controller in a cascade configuration, is
calculated based on the PV of the inner loop controller. This allows for a
second process time constant which is typical for second order processes that
require cascade control.
For dual outputs to split range valves, the two outputs are summed together
with gain constants on each output equal to one-half of the process gain.
A rigorous calculation of pressure values would require a complete
thermodynamic model of the process which would include values that may not be
measured. However , the pressure values can be simulated with an energy balance
using measured values and estimates of non-measured variables. While this will
not yield the actual pressures that will result from all operating conditions,
it will provide the proper response of pressures to changes in operating
conditions and can be tuned to provide normal operating points for operator
training. In general, any pressure P, is simulated as the integral of the
differential energy input as shown below. Energy flowing into a confined space
(E1 & E2) will raise the temperature and pressure. Energy flowing out of a
confined space (E3 & E4) will lower the temperature and pressure.
For simulation, a measured variable for each energy source is identified and
this signal is applied to a function generator to normalize the energy input to
0-100 % of maximum energy. For sources that are not directly measured, estimates
are calculated based on valve positions and differential pressures. A summer is
used to add the sources together and to scale each energy source according to
it's relative contribution to total energy flow. The resulting normalized signal
is integrated and a gain parameter is set to represent the rate of pressure
increase (P/E). The higher the gain, the faster the pressure will change. Upper
and lower limits are set on the integrator block for the minimum and maximum
pressure expected.
For in-line process tanks, the levels are calculated as the integral of the
difference between volumetric flow in and flow out of the tank. The flow rates
include the stock flows and dilution flows into and from the tank. For flow
rates that are not directly measured, estimates are calculated based on valve
positions and differential pressures.
Mass flows are integrated to calculate the total mass in the vessel. Specific
volume, density or consistency can then be calculated based upon the total
volume and total mass. For tanks with a high consistency, a mining zone can be
simulated by considering a volume with a separate mining dilution water flow
into the zone. High density flow from the tank into the mining zone is the total
flow out of the tank minus the dilution water into the tank.
Motor Simulation
The motor simulation logic consists of logic to map motor start and stop
outputs to inputs to simulate motor starter auxiliary inputs. Motor ready inputs
where used, are simulated by forcing the Ready input on to simulate normal
conditions. The ready inputs may be selectively forced off to simulate a tripped
motor overload relay. The motor start/stop function is mapped to provide the
feedback signal from the motor "running" signal.
Discrete Valve
Simulation
Valve outputs are mapped to inputs to simulate valve limit switches.
