Derivative fix: Comments and code style

src/modules/controllib/blocks.cpp

@@ -293,10 +293,16 @@ int blockIntegralTrapTest()
 
 float BlockDerivative::update(float input)
 {
-	float output = 0.0f;
+	float output;
 	if (_initialized) {
 		output = _lowPass.update((input - getU()) / getDt());
 	} else {
+		// if this is the first call to update
+		// we have no valid derivative
+		// and so we use the assumption the
+		// input value is not changing much,
+		// which is the best we can do here.
+		output = 0.0f;
 		_initialized = true;
 	}
 	setU(input);