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 $\mathtt{\text{E}}=\mathtt{\text{on}}$) in the following cases:

F1: Hardware error. The input signal $\mathtt{\text{HWF}}=\mathtt{\text{on}}$.

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

$$\mathtt{\text{du}}=\left\{\begin{array}{cc}\hfill \frac{\mathtt{\text{vmax}}-\mathtt{\text{vmin}}}{{\mathtt{\text{2}}}^{\mathtt{\text{nbits}}}},& \text{for }\mathtt{\text{nbits}}\in \left\{8,9,...,16\right\}\hfill \\ \hfill 0,& \text{for }\mathtt{\text{nbits}}\notin \left\{8,9,...,16\right\}.\hfill \end{array}\right.$$

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 $\mathtt{\text{nc}}=0$, 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,

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

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 $\mathtt{\text{u}}\in ⟨\mathtt{\text{vmin}},\mathtt{\text{vmax}}⟩$). The block includes a SPIKE filter with fixed parameters $\mathtt{\text{mingap}}=\frac{\mathtt{\text{vmax}}-\mathtt{\text{vmin}}}{100}$ and $\mathtt{\text{q}}=2$ 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 $\mathtt{\text{nr}}=0$, 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 $⟨\mathtt{\text{vmin}},\mathtt{\text{vmax}}⟩$. If the parameter nv is set to $\mathtt{\text{nv}}=0$, 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 $\mathtt{\text{R}}=\mathtt{\text{off}}$, 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:

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 $\mathtt{\text{nb}}\ge 5$ to allow the SPIKE filter initial conditions to fade away.

This block does not propagates the signal quality. More information can be found in the 1.4 section.

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