[Bug] Incorrect motor loop time for ev3dev

[laurensvalk]

Describe the bug

Variation is expected, but it would be good to match the model, on average.

If we can't get the loop time down to 5 ms, we should instead discretize the model constants to match the average, e.g. 8 ms.

To reproduce

• Install the latest build for pybricks-micropython to override the default.
• Make sure latest pybricksdev is installed
• Run:

~/git/pybricks-micropython$ tests/motors/run_test.py tests/motors/open_loop.py --target ev3dev --address 192.168.133.244 --show

Screenshots

[laurensvalk]

This makes it a lot better:

diff --git a/bricks/ev3dev/pbinit.c b/bricks/ev3dev/pbinit.c
index b5bc099d2..46674bd85 100644
--- a/bricks/ev3dev/pbinit.c
+++ b/bricks/ev3dev/pbinit.c
@@ -39,7 +39,7 @@ static pthread_t task_caller_thread;
 static void *task_caller(void *arg) {
     struct timespec ts;
     ts.tv_sec = 0;
-    ts.tv_nsec = PBIO_CONFIG_CONTROL_LOOP_TIME_MS * 1000000;
+    ts.tv_nsec = 1000000;
 
     while (!stopping_thread) {
         MP_THREAD_GIL_ENTER();
diff --git a/lib/pbio/src/motor_process.c b/lib/pbio/src/motor_process.c
index d258f4742..1d7719b76 100644
--- a/lib/pbio/src/motor_process.c
+++ b/lib/pbio/src/motor_process.c
@@ -28,6 +28,9 @@ PROCESS_THREAD(pbio_motor_process, ev, data) {
     for (;;) {
         PROCESS_WAIT_EVENT_UNTIL(ev == PROCESS_EVENT_TIMER && etimer_expired(&timer));
 
         // Update battery voltage.
         pbio_battery_update();
 
@@ -36,9 +39,6 @@ PROCESS_THREAD(pbio_motor_process, ev, data) {
 
         // Update servos
         pbio_servo_update_all();
 
         // Reset timer to wait for next update
-        etimer_restart(&timer);
+        etimer_reset(&timer);
     }
 
     PROCESS_END();

And there is also etimer_reset, but it appears that the increased frequency in the background thread is needed both ways. EDIT: I knew this seemed familiar, see here. Using etimer_reset is perhaps better after all.

[laurensvalk]

This has a huge impact on performance (in a good way - motors work way better) and may even fix #953.

It also fixes speed estimation.

[laurensvalk]

Linked PR in pybricks/pybricks-micropython#146

[laurensvalk]

pybricks/pybricks-micropython#146 is now merged, so closing.