Module mlx9064x::calculations[−][src]

Expand description

Calculations for turning sensor output into temperatures

The MLX90640 and MLX90641 datasheets have roughly a third of their pages dedicated to mathematical formulas, which can be a little intimidating. Fortunately most of the formulas can be simplified by assuming that treating some raw bits as a signed integer is a “free” operation, and that shifting bits to the left or right is simpler to write using << and >>. In other words:

If you see an operation similar to $$ \mathrm{If } K_{Foo} \gt (2^n - 1) \to K_{Foo} = K_{Foo} - (2^{n + 1}) $$ It is converting an unsigned integer to a signed one. The portions in parentheses are typically written as the actual value instead of a formula, so for example instead of 2⁷ - 1 and 2⁸, 127 and 256 will be used.
Masking a value off with a logical AND, followed by a division by a power of two. This is just extracting some of the bits out of a larger word (the word size for these cameras is 16 bits).
Building off of the previous item, multiplication and division by powers of two are used extensively in the datasheet to perform operations that would typically be written as a logical bit shift to the left (division) or right (multiplication). Be aware that not all divisions by a power of two can be rewritten as bit shifts; sometimes they’re converting a fixed-point value to a floating-point one.

An example of these points from section 11.1.1 of the MLX90640 datasheet:

\begin{align*} K_{Vdd} =& \frac{\eeprom{0x2433} \And \text{ 0xFF00}}{2^8} \newline &\mathrm{If } K_{Vdd} \gt 127 \to K_{Vdd} = K_{Vdd} - 256 \end{align*}

Can be written in Rust as:

fn read_eeprom(address: u16) -> u16 {
    unimplemented!();
}
let raw_k_v_dd = ((read_eeprom(0x2433) & 0xFF00) >> 8) as u8;
let k_v_dd = i8::from_be_bytes(raw_k_v_dd.to_be_bytes());

Non-standard integer widths (like 4 or 10) can easily be converted to signed integers with sign extension.

Glossary

$\alpha$, alpha: Sensitivity coefficient.
CP: Compensation pixel.
$\varepsilon$, emissivity: A measure of how well a material emits IR radiation. For a better explanation, see Wikipedia.
K: Prefix for constants.
PTAT: Proportional to ambient temperature
$T_a$: Ambient temperature of the camera.
$T_{a_{0}}$: Ambient temperature reference, 25.0 ℃. If it looks like 0, it's probably "o" as this value is really only used in one place.
$T_o$: Object temperature, meaning the temperature an individual pixel has detected for an object.
$V_{DD}$: Pixel supply voltage.
$V_{DD_{25}}$: Pixel supply voltage reference at 25.0 ℃.