DiscreteTime Integrator
Perform discretetime integration or accumulation of signal
 Library:
Simulink / Commonly Used Blocks
Simulink / Discrete
HDL Coder / Discrete
HDL Coder / HDL Floating Point Operations
Description
Use the DiscreteTime Integrator block in place of the Integrator block to create a purely discrete model. With the DiscreteTime Integrator block, you can:
Define initial conditions on the block dialog box or as input to the block
Define an input gain (K) value
Output the block state
Define upper and lower limits on the integral
Reset the state with an additional reset input
Output Equations
With the first time step, block state n = 0
, with either initial output
y(0) = IC
or initial state x(0) = IC
,
depending on the Initial condition setting parameter
value.
For a given step n > 0
with simulation time
t(n)
, Simulink^{®} updates output y(n)
as follows:
Forward Euler method:
y(n) = y(n1) + K*[t(n)  t(n1)]*u(n1)
Backward Euler method:
y(n) = y(n1) + K*[t(n)  t(n1)]*u(n)
Trapezoidal method:
y(n) = y(n1) + K*[t(n)t(n1)]*[u(n)+u(n1)]/2
Simulink automatically selects a statespace realization of these output
equations depending on the block sample time, which can be explicit or triggered.
When using explicit sample time, t(n)t(n1)
reduces to the
sample time T
for all n > 0
.
Integration and Accumulation Methods
This block can integrate or accumulate a signal using a forward Euler, backward Euler, or
trapezoidal method. Assume that u
is the input,
y
is the output, and x
is the state. For a
given step n
, Simulink updates y(n)
and x(n+1)
. In
integration mode, T
is the block sample time (delta
T
in the case of triggered sample time). In accumulation
mode, T = 1
. The block sample time determines when the output is
computed but not the output value. K
is the gain value. Values
clip according to upper or lower limits.
Forward Euler method (default), also known as forward rectangular, or lefthand approximation
The software approximates 1/s
as
T/(z1)
. The expressions for the output of the block at step
n
are:
x(n+1) = x(n) + K*T*u(n) y(n) = x(n)
The block uses these steps to compute the output:
Step 0: y(0) = IC (clip if necessary) x(1) = y(0) + K*T*u(0) Step 1: y(1) = x(1) x(2) = x(1) + K*T*u(1) Step n: y(n) = x(n) x(n+1) = x(n) + K*T*u(n) (clip if necessary)
Using this method, input port 1 does not have direct feedthrough.
Backward Euler method, also known as backward rectangular or righthand approximation
The software approximates 1/s
as
T*z/(z1)
. The resulting expression for the output of the
block at step n
is
y(n) = y(n1) + K*T*u(n).
Let x(n) = y((n)1)
. The block uses these steps to compute
the output.
If the parameter Initial condition setting is set to
Output
orAuto
for triggered and functioncall subsystems:Step 0: y(0) = IC (clipped if necessary) x(1) = y(0)
If the parameter Initial condition setting is set to
Auto
for nontriggered subsystems:Step 0: x(0) = IC (clipped if necessary) x(1) = y(0) = x(0) + K*T*u(0) Step 1: y(1) = x(1) + K*T*u(1) x(2) = y(1) Step n: y(n) = x(n) + K*T*u(n) x(n+1) = y(n)
Using this method, input port 1 has direct feedthrough.
For this method, the software approximates 1/s
as
T/2*(z+1)/(z1)
.
When T
is fixed (equal to the sampling period), the
expressions to compute the output are:
x(n) = y(n1) + K*T/2*u(n1) y(n) = x(n) + K*T/2*u(n)
If the parameter Initial condition setting is set to
Output
orAuto
for triggered and functioncall subsystems:Step 0: y(0) = IC (clipped if necessary) x(1) = y(0) + K*T/2*u(0)
If the parameter Initial condition setting is set to
Auto
for nontriggered subsystems:Step 0: x(0) = IC (clipped if necessary) y(0) = x(0) + K*T/2*u(0) x(1) = y(0) + K*T/2*u(0) Step 1: y(1) = x(1) + K*T/2*u(1) x(2) = y(1) + K*T/2*u(1) Step n: y(n) = x(n) + K*T/2*u(n) x(n+1) = y(n) + K*T/2*u(n)
Here, x(n+1)
is the best estimate of the next output. It is
not the same as the state, in that x(n)
is not equal to
y(n)
.
Using this method, input port 1 has direct feedthrough.
WhenT
is a variable (for example, obtained from the
triggering times), the block uses these steps to compute the output.
If the parameter Initial condition setting is set to
Output
orAuto
for triggered and functioncall subsystems:Step 0: y(0) = IC (clipped if necessary) x(1) = y(0)
If the parameter Initial condition setting is set to
Auto
for nontriggered subsystems:Step 0: x(0) = IC (clipped if necessary) x(1) = y(0) = x(0) + K*T/2*u(0) Step 1: y(1) = x(1) + T/2*(u(1) + u(0)) x(2) = y(1) Step n: y(n) = x(n) + T/2*(u(n) + u(n1)) x(n+1) = y(n)
Define Initial Conditions
You can define the initial conditions as a parameter on the block dialog box or input them from an external signal:
To define the initial conditions as a block parameter, set the Initial condition source parameter to
internal
and enter the value in the Initial condition text box.To provide the initial conditions from an external source, set the Initial condition source parameter to
external
. An additional input port appears on the block.
When to Use the State Port
Use the state port instead of the output port:
When the output of the block is fed back into the block through the reset port or the initial condition port, causing an algebraic loop. For an example, see the
sldemo_bounce_two_integrators
model.When you want to pass the state from one conditionally executed subsystem to another, which can cause timing problems. For an example, see Building a Clutch LockUp Model.
You can work around these problems by passing the state through the state port rather than the output port. Simulink generates the state at a slightly different time from the output, which protects your model from these problems. To output the block state, select the Show state port check box. The state port appears on the top of the block.
Limit the Integral
To keep the output within certain levels, select the Limit output check box and enter the limits in the corresponding text box. Doing so causes the block to function as a limited integrator. When the output reaches the limits, the integral action turns off to prevent integral windup. During a simulation, you can change the limits but you cannot change whether the output is limited. The table shows how the block determines output.
Integral  Output 

Less than or equal to the Lower saturation limit and the input is negative  Held at the Lower saturation limit 
Between the Lower saturation limit and the Upper saturation limit  The integral 
Greater than or equal to the Upper saturation limit and the input is positive  Held at the Upper saturation limit 
To generate a signal that indicates when the state is being limited, select the Show saturation port check box. A new saturation port appears below the block output port.
The saturation signal has one of three values:
1 indicates that the upper limit is being applied.
0 indicates that the integral is not limited.
1 indicates that the lower limit is being applied.
Reset the State
The block resets its state to the specified initial condition, based on an external signal. To cause the block to reset its state, select one of the External reset parameter options. A reset port appears that indicates the reset trigger type.
The reset port has direct feedthrough. If the block output feeds back into this port, either directly or through a series of blocks with direct feedthrough, an algebraic loop results. To resolve this loop, feed the output of the block state port into the reset port instead. To access the block state, select the Show state port check box.
Reset Trigger Types
The External reset parameter lets you determine the attribute of the reset signal that triggers the reset. The trigger options include:
rising
– Resets the state when the reset signal has a rising edge. For example, this figure shows the effect that a rising reset trigger has on backward Euler integration.falling
— Resets the state when the reset signal has a falling edge. For example, this figure shows the effect that a falling reset trigger has on backward Euler integration.either
— Resets the state when the reset signal rises or falls. For example, the following figure shows the effect that an either reset trigger has on backward Euler integration.level
— Resets and holds the output to the initial condition while the reset signal is nonzero. For example, this figure shows the effect that a level reset trigger has on backward Euler integration.sampled level
— Resets the output to the initial condition when the reset signal is nonzero. For example, this figure shows the effect that a sampled level reset trigger has on backward Euler integration.The
sampled level
reset option requires fewer computations, making it more efficient than thelevel
reset option.Note
For the DiscreteTime Integrator block, all trigger detections are based on signals with positive values. For example, a signal changing from 1 to 0 is not considered a rising edge, but a signal changing from 0 to 1 is.
Behavior in Simplified Initialization Mode
Simplified initialization mode is enabled when you set Underspecified
initialization detection to Simplified
in
the Configuration Parameters dialog box. If you use simplified initialization mode,
the behavior of the DiscreteTime Integrator block differs from
classic initialization mode. The new initialization behavior is more robust and
provides more consistent behavior in these cases:
In algebraic loops
On enable and disable
When comparing results using triggered sample time against explicit sample time, where the block is triggered at the same rate as the explicit sample time
Simplified initialization mode enables easier conversion from ContinuousTime Integrator blocks to DiscreteTime Integrator blocks, because the initial conditions have the same meaning for both blocks.
For more information on classic and simplified initialization modes, see Underspecified initialization detection.
When you use simplified initialization mode with Initial condition
setting set to Output
for triggered
and functioncall subsystems, the enable and disable behavior of the block is
simplified as follows.
At disable time t_{d}
:
y(t_{d}) = y(t_{d}1)
At enable time t_{e}
:
If the parent subsystem control port has States when enabling set to
reset
:y(t_{e}) = IC.
If the parent subsystem control port has States when enabling set to
held
:y(t_{e}) = y(t_{d}).
The following figure shows this condition.
When using simplified initialization mode, you cannot place the DiscreteTime Integrator block in an iterator subsystem block.
In simplified initialization mode, Iterator subsystems do not maintain elapsed time. Thus, if a DiscreteTime Integrator block, which needs elapsed time, is placed inside an iterator subsystem block, Simulink reports an error.
Behavior in an Enabled Subsystem Inside a FunctionCall Subsystem
Suppose you have a functioncall subsystem that includes an enabled subsystem, which contains a DiscreteTime Integrator block. The following behavior applies.
Integrator Method  Sample Time Type of FunctionCall Trigger Port  Value of delta T When
FunctionCall Subsystem Executes for the First Time After Enabled  Reason for Behavior 

Forward Euler  Triggered 
 When the functioncall subsystem executes for the first
time, the integrator algorithm uses 
Backward Euler and Trapezoidal  Triggered 
 When the functioncall subsystem executes for the first
time, the integrator algorithm uses 
Forward Euler, Backward Euler, and Trapezoidal  Periodic  Sample time of the functioncall generator  In periodic mode, the DiscreteTime
Integrator block uses sample time of the
functioncall generator for 
Ports
Input
Output
Parameters
Model Examples
Block Characteristics
Data Types 

Direct Feedthrough 

Multidimensional Signals 

VariableSize Signals 

ZeroCrossing Detection 

Extended Capabilities
See Also
Topics
 C Code Generation Configuration for Model Interface Elements (Simulink Coder)
 C Code Generation Configuration for Model Interface Elements (Simulink Coder)
 Organize Parameter Data into a Structure by Using Struct Storage Class (Embedded Coder)