## Overview
A proportional-integral-derivative (PID) controller.

Inspired by [pid-rs](https://github.com/braincore/pid-rs) 

With cleaner API and assumptions (constant time delta and symmetrical limits) dropped.

## Features
* Discrete time PID controller
* Defined for generic float types
* Attempts to conform to rust [API Guidelines](https://rust-lang.github.io/api-guidelines/about.html)
* ```#![no_std]```
* Limits for each of p, i, d terms and output
* Calculates derivative term using measurement over error (no derivative kick on new setpoint)
* Clamps time interval to between ```Float::epsilon()``` and ```Float::infinity()```

## Installation
```cargo add pid-ctrl```

## Examples
```rust
use pid_ctrl;
use num_traits;

fn main() {
    let mut pid = pid_ctrl::PidCtrl::new_with_pid(3.0, 2.0, 1.0);

    let setpoint = 5.0;
    let prev_measurement = 0.0;
    // calling init optional. Setpoint and prev_measurement set to 0.0 by default.
    // Recommended to avoid derivative kick on startup
    pid.init(setpoint, prev_measurement);

    let measurement = 0.0;
    let time_delta = 1.0;
    assert_eq!(
        pid.step(pid_ctrl::PidIn::new(measurement, time_delta)), 
        pid_ctrl::PidOut::new(15.0, 10.0, 0.0, 25.0)
    );

    // changing pid constants
    pid.kp.set_scale(4.0);
    assert_eq!(
        pid.step(pid_ctrl::PidIn::new(measurement, time_delta)), 
        pid_ctrl::PidOut::new(20.0, 20.0, 0.0, 40.0)
    );

    // setting symmetrical limits around zero
    pid.kp.limits.set_limit(10.0);
    assert_eq!(
        pid.step(pid_ctrl::PidIn::new(measurement, time_delta)), 
        pid_ctrl::PidOut::new(10.0, 30.0, 0.0, 40.0)
    );

    let time_delta = 0.5;
    assert_eq!(
        pid.step(pid_ctrl::PidIn::new(measurement, time_delta)), 
        pid_ctrl::PidOut::new(10.0, 35.0, 0.0, 45.0)
    );

    // setting upper limits returns error if new value conflicts with lower limit
    pid.ki.limits.try_set_upper(28.0).unwrap();  
    assert_eq!(
        pid.step(pid_ctrl::PidIn::new(measurement, time_delta)), 
        pid_ctrl::PidOut::new(10.0, 28.0, 0.0, 38.0)
    );

    // time_delta gets clamped to Float::epsilon() - Float::infinity()
    let measurement = 1.0;
    let time_delta = -7.0;
    pid.kd.set_scale(num_traits::Float::epsilon());
    assert_eq!(pid.step(
        pid_ctrl::PidIn::new(measurement, time_delta)), 
        pid_ctrl::PidOut::new(10.0, 28.0, -1.0, 37.0)
    );

    // configure setpoint directly
    pid.setpoint = 1.0;
    assert_eq!(pid.step(
        pid_ctrl::PidIn::new(measurement, time_delta)), 
        pid_ctrl::PidOut::new(0.0, 28.0, 0.0, 28.0)
    );
}
```
## Contribute
Feel free to raise issues. 

Would like to make ```#![no_std]``` optional so types can impl Display trait.