General operating principle

The following sections provide a generation description of the function blocks in the TwinCAT controller toolbox.

Discretization

The continuous transfer functions of the transfer elements assembled in this library are transformed to discrete values using the trapezoidal rule (Tustin formula).

The Tustin formula:

General operating principle 1:

Function block inputs

eMode

The operation mode of the majority of function blocks can be selected with this input. This makes it possible to select one of the following operation modes:

eCTRL_MODE_PASSIVE	The output or outputs of the function block are set to zero, but the internal states are retained.
eCTRL_MODE_ACTIVE	The function block is executed in accordance with its description, and appropriate output values are calculated (normal operation).
eCTRL_MODE_RESET	In this operation mode all internal states are reset and the error bit is cleared.
eCTRL_MODE_MANUAL	The value of the input value `fManSyncValue` is provided at the output (manual operation).

stParams

The necessary parameters are passed to the function block with this structure. The variables tTaskCycleTime and tCtrlCycleTime are contained in all the parameter structures. These parameters function in the following way:

The parameter tTaskCycleTime specifies the cycle time with which the function block is called. If the function block is called in every cycle this corresponds to the cycle time of the calling task. If it is only called in every second cycle, the time must be doubled accordingly. The parameter tCtrlCycleTime indicates the control loop's sampling time. This time must be greater than or equal to the parameter tTaskCycleTime. If the sampling time is set equal to tTaskCycleTime then the function block is executed with every call. If the selected value is larger by a factor of 5, the function block is only processed in every fifth call. This makes it possible to implement slow control loops even in a fast task.

The parameters tTaskCycleTime and tCtrlCycleTime are of type TIME and therefore do not permit inputs of less than 1ms. In order to use the controller in a fast PLC task with a cycle time of less than 1ms, a global base time can be specified as reference for the specified cycle times. The use of the base time is explained in the following examples.

It is assumed that the function block is called in every task cycle.

Task: Configuration	Parameter: `tTaskCycleTime`	Parameter: `tCtrlCycleTime`	Action
T#10ms	T#10ms	T#10ms	The control loop is processed using a 10 ms sampling time.
T#10ms	T#10ms	T#50ms	The control loop is processed using a 50 ms sampling time.
T#100ms	T#100ms	T#100ms	The control loop is processed using a 100 ms sampling time.
T#100ms	T#100ms	T#50ms	ERROR, execution is not possible!
T#100ms	T#50ms	T#50ms	ERROR, although the function block has been executed, incorrect output values have been calculated!

The outputs of the function blocks

eState

This output indicates the current internal state of the function block.

eCTRL_STATE_IDLE	The function block has successfully been reset, and is now waiting for selection of the operation mode.
eCTRL_STATE_PASSIVE	The function block is in the passive state in which no calculations are carried out.
eCTRL_STATE_ACTIVE	The function block is in the active state, which is the normal operating state.
eCTRL_STATE_RESET	A reset request is being processed, but the reset has not yet been completed.
eCTRL_STATE_MANUAL	The function block is in the manual state, and the output value can be manually specified at the appropriate input.
eCTRL_STATE_...	If there are any other internal states, they are described together with the corresponding function blocks.
eCTRL_STATE_ERROR	An error has occurred; the function block is not executed when in this state. See eErrorId for further information.

bError

An error in the function block is indicated by a TRUE at this boolean output.

eErrorId

The error number is provided at this output if the bError output is TRUE.

Using the global base time

In order to be able to use the function blocks of a PLC task with a cycle time of less than 1ms, it is possible to interpret the specified cycle times as ticks of a base time. In this special parameterization, the time unit of 1ms is interpreted as 1 tick. This approach is equivalent to setting a PLC cycle time of less than 1ms in the TwinCAT System Manager.

The switchover and declaration of the base time is done with the global structure stCtrl_GLOBAL_CycleTimeInterpretation for all function blocks of the toolbox.

VAR_GLOBAL
    stCtrl_GLOBAL_CycleTimeInterpretation :
    ST_CTRL_CYCLE_TIME_INTERPRETATION;
END_VAR

TYPE ST_CTRL_CYCLE_TIME_INTERPRETATION :
STRUCT
 bInterpretCycleTimeAsTicks  : BOOL; (* e.g. 2ms -> 2ticks *)
 fBaseTime                   : FLOAT; (* Base time in seconds, e.g. 200µs -> 200E-6s *)
END_STRUCT
END_TYPE

In order to interpret the specified cycle times as ticks, the variable bInterpretCycleTimeAsTicks in the global structure stCtrl_GLOBAL_CycleTimeInterpretation is set to TRUE. Within this structure, the base time unit has to be set in variable fBaseTime.

By setting the flag bInterpretCycleTimeAsTicks, the interpretation of the parameters with the names

tTaskCycleTime
tCtrlCycleTime

is changed. The interpretation and effect of all other parameters of type TIME remains unaffected.

Sample

The base time unit of the TwinCAT system is 200µs. The PLC task, and therefore the function blocks of the Toolbox, are called cyclically every 400µs.

Setting the global structure:

stCtrl_GLOBAL_CycleTimeInterpretation.bInterpretCycleTimeAsTicks := TRUE;
stCtrl_GLOBAL_CycleTimeInterpretation.fBaseTime := 200E-6;

Parameterization of a function block from the Toolbox:

stParams.tTaskCycleTime := T#2ms; (* 2*200µs=400µs *)
stParams.tCtrlCycleTime := T#4ms; (* 4*200µs=800µs *)
stParams. ...

The TaskCycleTime specified on the function blocks is "2•200E-6s = 400µs" and thus corresponds to the set PLC cycle time. The CtrlCycleTime is set to "800µs = 4•200E-6s" so that the control loop operates with a sampling time of 800µs, i.e. it is processed in every second PLC cycle.