Measures

A unit-of-measure library, measures.
This enables you to write type-checked expressions in terms of SI units.

There is also a representation of Thevenin and Norton equivalent circuits for doing electrical calculations.

Using Measures

If the types agree then the expression is dimensionally correct:

  let v1 = Amp(0.1) * Ohm(10.0);             // v1 has type Volt (Ohm's law)
  let x  = v1 + Amp(10.0);                   // error: can't add volts to amps.
  let x: Volt = v1 / Ohm(10.0);              // error: result has type Amp, not Volt
  let r1 = Second(450.0*u) / Farad(100.0*n); // OK: compute the resistor value for an RC network

(There is an emphasis on electrical units.)

The types provided, called measures, represent unscaled SI units.
They include: Volt, Amp, Ohm, Farad and Second.

Standard scaling constants are also provided: M, k, m, u, n, p.
So 10kΩ can be written Ohm(10.0*k), 10.0*Ohm(k), Ohm(10.0)*k and so on.
Not quite the natural notation but simple and predictable.

Formatting

Measures can be formatted in a natural notation, that is, engineering notation. The formatted value is scaled to the appropriate range and the SI unit symbol is appended. For example:

println!("R = {}", Second(450.0*u) / Farad(100.0*n));
// R = 4.50kΩ

Declaring Measures

Measures are declared with a macro:

measure!(Candela, "cd"); // the measure of luminous intensity

This declares a type, Candela and some traits for the type:

• multiply operators between Candela and dimensionless f64 values.
• divide operator on Candela by f64.
• add, subtract and divide operators between Candela values.
• comparison operators between Candela values.
• formatting for Candela using the given unit name "cd".

Expressions

The library knows something about how different measures can be combined. Two macros are used to express relationships between measures.

A product rule establishes a three-measure relationship, such as Ohm's law.
An inverse rule establishes a two-measure relationship such as time and frequency.

product!(Amp, Ohm, Volt); // Ohm's law.
inverse!(Second, Hertz);  // Time and frequency.

These rules define further operators between the types:

• multiply between Amp and Ohm
• divide Volt by Ohm or Amp
• multiply between Second and Hertz
• divide dimensionless f64 by Second or Hertz

Equivalent Circuits.

A Cct type is provided to help with simple DC circuit calculations. It represents a Thevenin or Norton equivalent circuit. Ref: wikipedia

A Thevenin Cct is created by combining a Volt and an Ohm measure: v + r. Here + is the series operator.

A Norton Cct is created by combining an Amp and a Siemen measure: i | g. Here | is the parallel operator.

A Cct of either form can be combined with another element in series parallel using + or |. The added element can be an Ohm or a Siemen measure or another Cct.

In this way larger networks can be built up. The result Cct will be represented internally in Thevenin or Norton form depending on its antecedents and its equivalent conductance. Norton is favoured for very low conductance and Thevenin otherwise.

An example voltage divider circuit:

    let vcc = Volt(5.0);
    let r1 = Ohm(10.0*k);
    let r2 = Ohm(5.0*k);
    
    let circuit =
        vcc + r1    // A voltage source vcc in series with resistor r1 forms a Cct,
        | r2;       // which is extended by resistor r2 in parallel
    
    let v1 = circuit.v_open(); // v1 = 1.67V is the voltage produced

A Cct can be queried with methods .v_open(), .i_short(), .r_equiv() or g_equiv() for its open circuit voltage, short circuit current, equivalent resistance or conductance.

Care must be taken with very low resistance or conductance:

let vs = Volt(x) + Ohm(0.0);    // OK: zero resistance voltage source
let is = Amp(x) | Siemen(0.0);  // OK: zero conductance current source
let i = vs.i_short();           // error: infinite current in short circuit
let v = is.v_open();            // error: infinite voltage in open circuit
let p = vs | vs;                // error: can't parallel pure voltage sources
let s = is + is;                // error: can't place pure current sources in series

Finally, it is possible to reverse the direction of a Cct with -, the negate operator.
The result has the same resistance (conductance) but the voltage (current) source polarity is reversed.

The following brash one-liner uses the circuit defined above and seeks to represent a Wheatstone bridge:

let w =  - circuit + circuit;  

TODO

• More measures.
• Complex impedances. AC sources.
• More tests.