Merge pull request #3 from emersion/master

Adds update interval support, minor fixes

Author: Philmod

README.md

@@ -12,9 +12,10 @@
 Let's take the example of a car cruise control.  We want the car driving at 120km/h.
 
 ### Create a Controller instance
+`k_p`, `k_i` and `k_d` are the proportional, integral and derivative terms. `dt` is the interval of time between two measures. If not set, it will be automatically calculated.
 ```js
 var Controller = require('node-pid-controller');
-var ctr = new Controller(0.25, 0.01, 0.01); // k_p, k_i, k_d
+var ctr = new Controller(0.25, 0.01, 0.01, 1); // k_p, k_i, k_d, dt
 ```
 
 ### Set the target
@@ -46,4 +47,4 @@ mocha test
 
 ## Author
 
-Philmod <[email protected]>
+Philmod <[email protected]>

lib/index.js

@@ -1,11 +1,16 @@
 /** 
  *  PID Controller.
  */
-var Controller = function(k_p, k_i, k_d) {
-  this.k_p = k_p || 1;
+var Controller = function(k_p, k_i, k_d, dt) {
+  // PID constants
+  this.k_p = (typeof k_p === 'number') ? k_p : 1;
   this.k_i = k_i || 0;
   this.k_d = k_d || 0;
 
+  // Interval of time between two updates
+  // If not set, it will be automatically calculated
+  this.dt = dt || 0;
+
   this.sumError  = 0;
   this.lastError = 0;
   this.lastTime  = 0;
@@ -17,12 +22,27 @@ Controller.prototype.setTarget = function(target) {
   this.target = target;
 };
 
-Controller.prototype.update = function(current_value) {
-  this.current_value = current_value;
-
-  var error = (this.target - this.current_value);
-  this.sumError = this.sumError + error;
-  var dError = error - this.lastError;
+Controller.prototype.update = function(currentValue) {
+  this.currentValue = currentValue;
+
+  // Calculate dt
+  var dt = this.dt;
+  if (!dt) {
+    var currentTime = Date.now();
+    if (this.lastTime === 0) { // First time update() is called
+      dt = 0;
+    } else {
+      dt = (currentTime - this.lastTime) / 1000; // in seconds
+    }
+    this.lastTime = currentTime;
+  }
+  if (typeof dt !== 'number' || dt === 0) {
+    dt = 1;
+  }
+
+  var error = (this.target - this.currentValue);
+  this.sumError = this.sumError + error*dt;
+  var dError = (error - this.lastError)/dt;
   this.lastError = error;
 
   return (this.k_p*error) + (this.k_i * this.sumError) + (this.k_d * dError);

test/index.js

@@ -12,13 +12,13 @@ var Controller = require('../')
  */
 describe('pid-controller', function(){
 
-  var k_p = 0.5
-    , k_i = 0.1
-    , k_d = 0.2
-    ;
+  var k_p = 0.5,
+      k_i = 0.1,
+      k_d = 0.2,
+      dt  = 1;
 
   // Create the controller
-  var ctr = new Controller(k_p, k_i, k_d);
+  var ctr = new Controller(k_p, k_i, k_d, dt);
 
   it('should have set the coefficient', function() {
     ctr.k_p.should.equal(k_p);
@@ -45,4 +45,17 @@ describe('pid-controller', function(){
     ctr.lastTime.should.equal(0);
   });
 
+  it('should return the correction for the given update interval', function(){
+    ctr.dt = 2; // 2 seconds between updates
+    var correction = ctr.update(115);
+    correction.should.equal(4);
+  });
+
+  it('should return a null correction', function(){
+    var ctr = new Controller(0, 0, 0);
+    ctr.setTarget(120);
+    var correction = ctr.update(110);
+    correction.should.equal(0);
+  });
+
 });