SAI – Safety analog input

Block SymbolLicensing group: ADVANCED

Function Description
The SAI block tests the input signal u and assesses its validity. The input signal u is considered invalid (the output $E = on$ ) in the following cases:

F1: Hardware error. The input signal

HWF = on

F2: The input signal u varies too little. The last nc samples of the input u lies within the interval of width du,

du = <\begin{matrix} \frac{vmax - vmin}{2^{nbits}}, & f o r nbits \in {8, 9, . . ., 16} \\ 0, & f o r nbits \notin {8, 9, . . ., 16}, \end{matrix}

where vmin and vmax are the lower and upper limits of the input u, respectively, and nbits is the number of A/D converter bits. The situation when the input signal u varies too little is shown in the following picture:

If the parameter nc is set to $nc = 0$ , the condition F2 is never fulfilled.

F3: The input signal u varies too much. The last nr samples of the input u filtered by the SPIKE filter have a span which is greater than rate,

rate = prate \frac{vmax - vmin}{100},

where prate defines the allowed percentage change in the input signal u within the last nr samples (with respect to the overall range of the input signal $u \in ⟨ vmin, vmax ⟩$ ). The block includes a SPIKE filter with fixed parameters $mingap = \frac{vmax - vmin}{100}$ and $q = 2$ suppressing peaks in the input signal to avoid undesirable fulfilling of this condition. See the SPIKE block description for more details. The situation when the input signal u varies too much is shown in the following picture:

If the parameter nr is set to $nr = 0$ , the condition F3 is never fulfilled.

F4: The input signal u is out of range. The last nv samples of the input signal u lie out of the allowed range

⟨ vmin, vmax ⟩

If the parameter nv is set to $nv = 0$ , the condition F4 is never fulfilled.

The signal u is copied to the output y without any modification when it is considered valid. In the other case, the output y is determined by a substitute value from the sv input. In such a case the output E is set to on and the output iE provides the error code. The input R resets the inner error flags F1–F4. For the input R set permanently to on, the invalidity indicator E is set to on for only one cycle period whenever some invalidity condition is fulfilled. On the other hand, for $R = off$ , the output E is set to on and remains true until the reset (rising edge R: off $\to$ on).

The table of error codes iE resulting from the inner error flags F1–F4:


F1	F2	F3	F4	iE

0	0	0	0	0
0	0	0	1	1
0	0	1	0	2
0	0	1	1	3
0	1	0	0	4
0	1	0	1	5
0	1	1	0	6
0	1	1	1	7
1	*	*	*	8

The nb parameter defines the number of samples which are not included in the validity assessment after initialization of the block (restart). Recommended setting is $nb \geq 5$ to allow the SPIKE filter initial conditions to fade away.

Inputs

u	Analog input of the block	Double (F64)
sv	Substitute value to be used when the signal u is marked as invalid	Double (F64)
HWF	Hardware error indicator	Bool
	off .. The input module of the signal works normally on ... Hardware error of the input module occurred
R	Reset inner error flags F1–F4	Bool

Outputs

y	Analog output of the block	Double (F64)
yf	Filtered analog output signal y, output of the SPIKE filter	Double (F64)
E	Output signal invalidity indicator	Bool
	off .. Signal is valid on ... Signal is invalid, $y = yf = sv$
iE	Reason of invalidity	Long (I32)
	0 .... Signal valid 1 .... Signal out of range 2 .... Signal varies too little 3 .... Signal varies too little and signal out of range 4 .... Signal varies too much 5 .... Signal varies too much and signal out of range 6 .... Signal varies too much and too little 7 .... Signal varies too much and too little and signal out of range 8 .... Hardware error

Parameters

nb	Number of samples which are not included in the validity assessment of the signal u after initialization of the block $⊙$ 10	Long (I32)
nc	Number of samples for invariability testing (the F2 condition) $⊙$ 10	Long (I32)
nbits	Number of A/D converter bits $⊙$ 12	Long (I32)
nr	Number of samples for variability testing (the F3 condition) $⊙$ 10	Long (I32)
prate	Maximum allowed percentage change of the input u within the last nr samples (with respect to the overall range of the input signal $vmax - vmin$ ) $⊙$ 10.0	Double (F64)
nv	Number of samples for out-of-range testing (the F4 condition) $⊙$ 1	Long (I32)
vmin	Lower limit for the input signal u $⊙$ -1.0	Double (F64)
vmax	Upper limit for the input signal u $⊙$ 1.0	Double (F64)

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