Models: what the instrument measures

A query with respuesta: "{tension}" returns what was configured. A measurement is something else: the current a supply delivers depends on the voltage, the load and whether the output is on. That is a model.

modelos:
  corriente_medida:
    tipo: formula
    cuando: { salida: true }
    expr: "min(tension / carga_ohm, limite_corriente) + gauss(0, 1e-5)"
    fallback: 0.0
    formato: fijo:5

And the query that uses it:

  - patron: "MEASure[:SCALar]:CURRent[:DC]"
    query: true
    modelo: corriente_medida

Fields

Field	What it is
`tipo`	Always `formula`.
`expr`	The formula.
`cuando`	Conditions on the state. If any is not met, `fallback` is answered. If there are several, all must hold.
`fallback`	The answer when `cuando` is not met. Defaults to `0.0`. Number or text.
`formato`	How the number is written. Optional.

What goes in `expr`

State variables, by name: tension, carga_ohm.
Numbers: 10, 0.5, 1e-3, 2.5E+2.
+ - * / with the usual precedence, parentheses and unary minus.
Functions:

Function	Result
`gauss(mean, sigma)`	Gaussian noise. It changes on every read, but the series repeats the same each time you start Crucible, so automated tests are reproducible.
`abs(x)`	Absolute value.
`sqrt(x)`	Square root.
`min(a, b)`, `max(a, b)`	Minimum and maximum.

There are no conditionals inside the formula: cuando only chooses between the formula and a fixed fallback.

The supply’s model, running

Against a freshly started bench:

$ printf 'MEAS:CURR?\n' | nc -q1 127.0.0.1 5025
0.00000
$ printf 'OUTP?\n' | nc -q1 127.0.0.1 5025
0
$ printf 'VOLT 5;:OUTP ON;:MEAS:CURR?\n' | nc -q1 127.0.0.1 5025
0.50001
$ printf 'VOLT 20;:MEAS:CURR?;:OUTP?\n' | nc -q1 127.0.0.1 5025
0.99999;1
$ printf 'MEAS:VOLT?\n' | nc -q1 127.0.0.1 5026
+3.300139E+00

Step by step:

With the output off cuando is not met, and the measurement is the fallback, written in the model’s format: 0.00000.
OUTP? answers 0, not false: it is answered by the salida_como_numero model, with formato: entero, like a real instrument.
At 5 V across 10 Ω, half an ampere plus noise.
At 20 V it would be 2 A, but min clips it to the 1 A limit: the supply goes into constant current.
The multimeter writes scientific notation, like most do.

`formato`

Your software expects numbers written the way the real instrument writes them:

`formato`	Example answer	Typical use
(not set)	`1.0`, `4.501385029307777`	—
`entero` (integer)	`1`, `0`, `-3`	States (`OUTP?`) and counters.
`fijo:N` (fixed)	`fijo:4` → `4.5014`	Supplies that answer with fixed decimals.
`cientifico:N` (scientific)	`cientifico:6` → `+4.501385E+00`	Multimeters and most SCPI measurements.

The format also applies to fallback when it is numeric.

Queries with respuesta: "{variable}" return the value as it is (5.0). If your software expects another form, use a model whose expr is the variable, with the right formato.

Errors in formulas

Syntax errors —an unclosed parenthesis, a function that does not exist— show up when validating or starting, with the position:

dispositivo:
  modelo: DMM-ABC
  idn: "ACME,DMM-ABC,SN12345,FW2.01"

protocolo: scpi

estado:
  tension_entrada: 0.0

comandos:
  - patron: "MEASure:VOLTage[:DC]"
    query: true
    modelo: lectura

modelos:
  lectura:
    tipo: formula
    expr: "tension_entrada + gaus(0, 0.0001)"

$ crucible --validar perfiles/formula_rota.yaml

error: perfiles/formula_rota.yaml: perfil inválido: modelo 'lectura': error de evaluación: fórmula 'tension_entrada + gaus(0, 0.0001)': función 'gaus' desconocida; existen: gauss, abs, sqrt, min, max (posición 23)

The message says the function gaus is unknown, lists the ones that exist and gives the position.

A variable that does not exist in the state, however, is only detected when the query is made: no answer arrives and SYST:ERR? returns -200,"Execution error;…la variable '…' no existe en el estado…" (the variable does not exist in the state). Declare in estado every variable your formulas use.

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