Fix hall sensor velocity estimation

The hall sensor velocity estimation could end up returning a nonzero
value if the timer "wrapped" between runs, even if the motor was
actually stationary. If using the velocity regulator the motor may then
fail to start.

To solve this:
- Switch to using unsigned types so that wrapping is defined.
- When timing out the pulse value, also reset it so that the velocity
  won't be briefly treated as valid again when the timer wraps.

Seen on a platform with 32 bit long, so running the motor and then
waiting ~40 minutes is enough to observe this bug.
Note that the pulse_diff variable is reset if the hall sensor direction
changes, which may happen naturally when the motor stops, depending on
what it is attached to.

Author: ahughesattomra

src/sensors/HallSensor.cpp

@@ -122,10 +122,19 @@ float HallSensor::getSensorAngle() {
 */
 float HallSensor::getVelocity(){
   noInterrupts();
-  long last_pulse_timestamp = pulse_timestamp;
-  long last_pulse_diff = pulse_diff;
+  unsigned long last_pulse_timestamp = pulse_timestamp;
+  unsigned long last_pulse_diff = pulse_diff;
   interrupts();
-  if (last_pulse_diff == 0 || ((long)(_micros() - last_pulse_timestamp) > last_pulse_diff*2) ) { // last velocity isn't accurate if too old
+  if (last_pulse_diff == 0) {
+    return 0;
+  } else if ((_micros() - last_pulse_timestamp) > last_pulse_diff*2) {
+    // Last velocity isn't accurate if too old.
+    // Reset to avoid the diff being used again when the time wraps.
+    noInterrupts();
+    if (pulse_diff == last_pulse_diff) {
+        pulse_diff = 0;
+    }
+    interrupts();
     return 0;
   } else {
     return direction * (_2PI / (float)cpr) / (last_pulse_diff / 1000000.0f);

src/sensors/HallSensor.h

@@ -90,7 +90,7 @@ class HallSensor: public Sensor{
     // function pointer for on sector change call back
     void (*onSectorChange)(int sector) = nullptr;
 
-    volatile long pulse_diff;
+    volatile unsigned long pulse_diff;
 
 };