Unfiltered Derivative
The FixPt Derivative realization is a masked subsystem
that performs discrete-time differentiation. For the this method, differentiation
is approximated by the z-domain transfer function
where Ts is the sampling period. The transfer
function yields the difference equation
where k is the sample time, y(k)
is the current output, u(k) is the current input, and u(k
- 1) is the input from the previous time step.
Parameters and Dialog Box
-
Sample time
-
The time interval, Ts, between samples
-
Base data type
-
The processor's base data type
-
Accumulator data type
-
The processor's accumulator data type
Model Design Review
A brief review of the model design is given below.
The design criteria reflect these rules.
-
Using the Accumulator data type for the FixPt Sum block would rarely
be advantageous. Both inputs are given by the Base data type with
identical scaling so using the same data type for the output makes sense.
Also, the subsequent block is a gain; and its input should be the Base
data type or smaller. The input values to this block should be close
so the subtraction can be safely carried out using the Base data type.
-
The gain involves multiplication which is a size-growing operation. In
most cases, it is desirable for gain and input to use the word size given
by the Base data type or smaller. The output can be left at the
Accumulator data type for extra precision in subsequent operations.
Alternatively, if the output were stored to RAM, or used by a size-growing
operation, it could be reduced to the Base data type.
-
The FixPt Conversion casts the output to the Base data type before
storage in RAM (i.e., before input to the unit delay).
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