Position-controlled arms

Trigger position now sets arm ANGLE rather than power, via
setAbsoluteMotorPosition with BrakingStyle::HOLD. Encoders are zeroed on
hub1 connect, so both arms must be down at that moment.

Spans measured with tools/arm_calibrate: roughly +280 left, -275 right,
both comfortably under one motor revolution so the absolute encoder is
unambiguous. Set to +/-250 to keep backlash clear of the end stop.

L3/R3 are no longer bound - releasing the trigger is lowering.
Also documents the async-init trap in serviceHub.

receiver/receiver.ino

@@ -9,6 +9,8 @@
  *
  * Core 3.x will not build Legoino - you get 'std::string does not name a type'
  * and a ReadUInt32LE declaration mismatch. Stay on 2.0.17 for this board.
+ * Do NOT select the esp32_bluepad32 package here either: it starts BTstack
+ * before setup() runs and NimBLE aborts with ESP_ERR_INVALID_STATE.
  *
  * Wiring to the transmitter board:
  *   RX GPIO16  <-  TX GPIO17 on transmitter
@@ -21,12 +23,19 @@
  *   Pins output 3.3V, not 3V. ~12mA per pin is comfortable, 40mA is the hard
  *   limit. Anything drawing more than ~20mA per circuit needs a transistor.
  *
+ * ARMS ARE POSITION CONTROLLED:
+ *   The triggers set an ANGLE, not a power level. Trigger released holds the
+ *   arm at 0, fully depressed holds it at ARM_SPAN_*. The encoders are zeroed
+ *   on connect, so BOTH ARMS MUST BE DOWN when the hub connects.
+ *   Measured travel was about 275 motor degrees each way; the spans below are
+ *   set short of that deliberately, so backlash cannot drive the motor into
+ *   its end stop while holding.
+ *
  * FILE ORDER MATTERS:
  *   The Arduino IDE injects generated function prototypes immediately before
  *   the FIRST function definition in the file. Any type used in a function
  *   signature must be declared above that point - which is why HubLink,
- *   PortState and Frame all live in the types block. Move a function above
+ *   PortState, ArmState and Frame all live in the types block.
- *   them and you get "'Frame' has not been declared".
  *
  * Created by: Jess Rogerson (yelling commands at Claude.AI)
  */
@@ -45,7 +54,6 @@
 static const char *HUB0_ADDR = "90:84:2b:61:f2:d7";
 static const char *HUB1_ADDR = "90:84:2b:61:e6:8c";
 
-// Port numbers are just bytes in the LEGO protocol, same on every hub type.
 static const byte PORT_A = 0x00;
 static const byte PORT_B = 0x01;
 static const byte PORT_C = 0x02;
@@ -56,11 +64,9 @@ static const byte PORT_D = 0x03;
 static const int LED_1_PIN = 25;    // Circle toggles this pair
 static const int LED_2_PIN = 26;    // Triangle toggles this pair
 
-// Technic / Control+ motors are tacho motors -> leave this at 1.
 #define USE_TACHO_MOTORS 1
 
-// Set to 1 to log every motor command that goes out. Useful for proving which
+// Set to 1 to log every command that goes out. Noisy - turn it back off.
-// port a command actually lands on. Noisy - turn it back off afterwards.
 #define DEBUG_MOTORS 0
 
 static const int DEADZONE = 40;     // raw stick counts ignored around centre
@@ -71,32 +77,34 @@ static const int DIR_RIGHT_TRACK = -1;
 static const int DIR_BODY_LIFT   =  1;
 static const int DIR_HEAD_TILT   =  1;
 static const int DIR_HEAD_TURN   =  1;
-static const int DIR_LEFT_ARM    =  1;
-static const int DIR_RIGHT_ARM   =  1;
 
-// Per-axis power caps. Everything except the tracks runs into a mechanical end
+// Arm travel in MOTOR degrees, measured with tools/arm_calibrate.
-// stop and there is no position feedback, so holding a direction at a stop
+// Raw measurements were roughly +280 (left) and -275 (right). These are set
-// stalls the motor. Lower these if an axis feels forceful.
+// ~10% short so backlash cannot stall the motor against the top stop.
+// The sign carries the direction - there is no DIR_ constant for the arms.
+static const int32_t ARM_SPAN_LEFT  =  250;
+static const int32_t ARM_SPAN_RIGHT = -250;
+
+static const int  ARM_SPEED     = 60;   // how fast it travels to the target
+static const byte ARM_MAX_POWER = 40;   // torque cap - keeps a jam survivable
+static const int  ARM_STEP_DEG  = 8;    // ignore target changes smaller than this
+
+// Per-axis power caps for the velocity-controlled axes.
 static const int TRACK_MAX = 100;   // LEGO speed range is -100..100
 static const int HEAD_MAX  = 45;
 static const int LIFT_MAX  = 60;
-static const int ARM_UP_MAX   = 45;
-static const int ARM_DOWN_MAX = 30;    // gravity helps on the way down
 
 // Track speed, toggled by Cross.
 static const int SPEED_SLOW = 50;
 static const int SPEED_FAST = 100;
 
-// Bluepad32 button masks. Verify against your own pad with DEBUG_BUTTONS in
+// Bluepad32 button masks. Verify with DEBUG_BUTTONS in the transmitter.
-// the transmitter sketch if any of these seem wrong.
 static const unsigned BTN_A  = 0x0001;   // Cross
 static const unsigned BTN_B  = 0x0002;   // Circle
 static const unsigned BTN_X  = 0x0004;   // Square
 static const unsigned BTN_Y  = 0x0008;   // Triangle
 static const unsigned BTN_L1 = 0x0010;
 static const unsigned BTN_R1 = 0x0020;
-static const unsigned BTN_L3 = 0x0100;   // left stick click
-static const unsigned BTN_R3 = 0x0200;   // right stick click
 
 static const unsigned DPAD_U = 0x01;
 static const unsigned DPAD_D = 0x02;
@@ -133,6 +141,11 @@ struct PortState {
     unsigned long lastSentAt;
 };
 
+struct ArmState {
+    int32_t lastTarget;
+    unsigned long lastSentAt;
+};
+
 struct Frame {
     int lx, ly, rx, ry;
     unsigned buttons, dpad;
@@ -147,10 +160,13 @@ static HubLink gHubs[2] = {
 };
 
 // Indexes follow physical ports, not functions:
-//   0 hub0/A  1 hub0/B  2 hub0/D
+//   0 hub0/A  1 hub0/B  2 hub0/D  3 hub1/B  4 hub1/C
-//   3 hub1/A  4 hub1/B  5 hub1/C  6 hub1/D
+static PortState gPort[5] = {{999, 0}, {999, 0}, {999, 0}, {999, 0}, {999, 0}};
-static PortState gPort[7] = {{999, 0}, {999, 0}, {999, 0}, {999, 0},
+
-                             {999, 0}, {999, 0}, {999, 0}};
+// Arms are position controlled, so they get their own state.
+static ArmState gArmLeft  = {INT32_MIN, 0};
+static ArmState gArmRight = {INT32_MIN, 0};
+static bool gArmsZeroed = false;
 
 static unsigned long lastFrameAt = 0;
 static bool failsafeEngaged = true;
@@ -173,13 +189,13 @@ static int stickToSpeed(int raw, int maxSpeed) {
     return sign * (int)scaled;
 }
 
-// Analog trigger, 0..1023, to a 0..maxSpeed power with a deadzone at the bottom.
+// Analog trigger, 0..1023, to a target angle between 0 and span.
-static int triggerToSpeed(int raw, int maxSpeed) {
+static int32_t triggerToAngle(int raw, int32_t span) {
     if (raw <= TRIGGER_DEADZONE) return 0;
-    long scaled = ((long)(raw - TRIGGER_DEADZONE) * maxSpeed) /
+    long travel = (long)raw - TRIGGER_DEADZONE;
-                  (1023L - TRIGGER_DEADZONE);
+    long full = 1023L - TRIGGER_DEADZONE;
-    if (scaled > maxSpeed) scaled = maxSpeed;
+    if (travel > full) travel = full;
-    return (int)scaled;
+    return (int32_t)((travel * span) / full);
 }
 
 static void driveMotor(HubLink &hl, byte port, int speed, PortState &st) {
@@ -189,8 +205,8 @@ static void driveMotor(HubLink &hl, byte port, int speed, PortState &st) {
     bool stopping = (speed == 0 && st.lastSpeed != 0);
 
     // Stops always go out immediately. Everything else is rate limited twice:
-    // per port, and per hub - hub 1 has four motors on it, and the per-port
+    // per port, and per hub - the per-port limit alone lets through more than
-    // limit alone lets through more than the hub will swallow.
+    // a hub with several motors on it will swallow.
     if (!stopping) {
         if (speed == st.lastSpeed) return;
         if ((now - st.lastSentAt) < MOTOR_MIN_GAP_MS) return;
@@ -212,18 +228,52 @@ static void driveMotor(HubLink &hl, byte port, int speed, PortState &st) {
     hl.lastCmdAt = now;
 }
 
+// Position control. HOLD keeps the motor actively at the target rather than
+// letting gravity drag the arm back down.
+static void driveArm(HubLink &hl, byte port, int32_t target, ArmState &st) {
+    if (!hl.hub.isConnected() || !gArmsZeroed) return;
+
+    unsigned long now = millis();
+    if (labs((long)target - (long)st.lastTarget) < ARM_STEP_DEG) return;
+    if ((now - st.lastSentAt) < MOTOR_MIN_GAP_MS) return;
+    if ((now - hl.lastCmdAt) < HUB_MIN_GAP_MS) return;
+
+#if DEBUG_MOTORS
+    Serial.printf("TX %s port %u angle %ld\n", hl.label, port, (long)target);
+#endif
+
+    hl.hub.setAbsoluteMotorPosition(port, ARM_SPEED, target, ARM_MAX_POWER,
+                                    BrakingStyle::HOLD);
+
+    st.lastTarget = target;
+    st.lastSentAt = now;
+    hl.lastCmdAt = now;
+}
+
 static void stopEverything() {
     driveMotor(gHubs[0], PORT_A, 0, gPort[0]);
     driveMotor(gHubs[0], PORT_B, 0, gPort[1]);
     driveMotor(gHubs[0], PORT_D, 0, gPort[2]);
-    driveMotor(gHubs[1], PORT_A, 0, gPort[3]);
+    driveMotor(gHubs[1], PORT_B, 0, gPort[3]);
-    driveMotor(gHubs[1], PORT_B, 0, gPort[4]);
+    driveMotor(gHubs[1], PORT_C, 0, gPort[4]);
-    driveMotor(gHubs[1], PORT_C, 0, gPort[5]);
+
-    driveMotor(gHubs[1], PORT_D, 0, gPort[6]);
+    // Arms: a plain speed command overrides the position hold and goes limp.
+    // Reset the cached targets so the next trigger movement re-commands.
+    if (gHubs[1].hub.isConnected()) {
+        gHubs[1].hub.setTachoMotorSpeed(PORT_A, 0);
+        gHubs[1].hub.setTachoMotorSpeed(PORT_D, 0);
+    }
+    gArmLeft.lastTarget = INT32_MIN;
+    gArmRight.lastTarget = INT32_MIN;
 }
 
 // Connect the hubs one at a time. Kicking off two scans at once upsets the
 // shared NimBLE scanner and you end up with one hub connected and one sulking.
+//
+// Note the 'initialised' one-shot. init() starts an ASYNCHRONOUS scan, so
+// immediately afterwards isConnected() and isConnecting() are both still
+// false. Guarding on those alone re-enters NimBLEDevice::init() thousands of
+// times a second and the Bluetooth controller aborts.
 static void serviceHub(HubLink &hl) {
     if (hl.hub.isConnected()) return;
 
@@ -247,6 +297,23 @@ static void serviceHub(HubLink &hl) {
     }
 }
 
+// Define "arms down" as angle zero. Has to happen after hub1 connects, and
+// the arms must physically be at the bottom of their travel.
+static void zeroArms() {
+    if (gArmsZeroed || !gHubs[1].hub.isConnected()) return;
+
+    delay(500);   // let the hub finish reporting its ports
+    gHubs[1].hub.setAbsoluteMotorEncoderPosition(PORT_A, 0);
+    delay(200);
+    gHubs[1].hub.setAbsoluteMotorEncoderPosition(PORT_D, 0);
+    delay(200);
+
+    gArmsZeroed = true;
+    gArmLeft.lastTarget = INT32_MIN;
+    gArmRight.lastTarget = INT32_MIN;
+    Serial.println("Arms zeroed at current position");
+}
+
 static uint8_t xorChecksum(const char *s, size_t len) {
     uint8_t c = 0;
     for (size_t i = 0; i < len; i++) c ^= (uint8_t)s[i];
@@ -307,29 +374,25 @@ static void applyFrame(const Frame &f) {
     int headTurn = ((f.buttons & BTN_R1) ? HEAD_MAX
                  :  (f.buttons & BTN_L1) ? -HEAD_MAX : 0) * DIR_HEAD_TURN;
 
-    // D-pad: up/down lifts the body, left/right tilts the head. Both move
+    // D-pad: up/down lifts the body, left/right tilts the head.
-    // while held.
     int bodyLift = ((f.dpad & DPAD_U) ? LIFT_MAX
                  :  (f.dpad & DPAD_D) ? -LIFT_MAX : 0) * DIR_BODY_LIFT;
     int headTilt = ((f.dpad & DPAD_R) ? HEAD_MAX
                  :  (f.dpad & DPAD_L) ? -HEAD_MAX : 0) * DIR_HEAD_TILT;
 
-    // Arms: analog trigger raises proportionally, stick click lowers.
+    // Arms: trigger position IS arm angle. Released means "go to zero", which
-    int leftRaise  = triggerToSpeed(f.l2, ARM_UP_MAX);
+    // gravity is already doing, so the motor mostly just catches it.
-    int rightRaise = triggerToSpeed(f.r2, ARM_UP_MAX);
+    int32_t leftTarget  = triggerToAngle(f.l2, ARM_SPAN_LEFT);
-
+    int32_t rightTarget = triggerToAngle(f.r2, ARM_SPAN_RIGHT);
-    int leftArm  = ((f.buttons & BTN_L3) ? -ARM_DOWN_MAX : leftRaise)
-                   * DIR_LEFT_ARM;
-    int rightArm = ((f.buttons & BTN_R3) ? -ARM_DOWN_MAX : rightRaise)
-                   * DIR_RIGHT_ARM;
 
     driveMotor(gHubs[0], PORT_A, rightTrack, gPort[0]);
     driveMotor(gHubs[0], PORT_B, leftTrack,  gPort[1]);
     driveMotor(gHubs[0], PORT_D, bodyLift,   gPort[2]);
-    driveMotor(gHubs[1], PORT_A, leftArm,    gPort[3]);
+    driveMotor(gHubs[1], PORT_B, headTurn,   gPort[3]);
-    driveMotor(gHubs[1], PORT_B, headTurn,   gPort[4]);
+    driveMotor(gHubs[1], PORT_C, headTilt,   gPort[4]);
-    driveMotor(gHubs[1], PORT_C, headTilt,   gPort[5]);
+
-    driveMotor(gHubs[1], PORT_D, rightArm,   gPort[6]);
+    driveArm(gHubs[1], PORT_A, leftTarget,  gArmLeft);
+    driveArm(gHubs[1], PORT_D, rightTarget, gArmRight);
 }
 
 // ==================================================================== main
@@ -347,6 +410,7 @@ void setup() {
     digitalWrite(LED_2_PIN, LOW);
 
     Serial.println("LEGO hub receiver starting (Johnny 5 Evolved)");
+    Serial.println("Both arms must be DOWN before hub1 connects");
 }
 
 void loop() {
@@ -354,6 +418,12 @@ void loop() {
     serviceHub(gHubs[0]);
     if (gHubs[0].hub.isConnected()) serviceHub(gHubs[1]);
 
+    if (gHubs[1].hub.isConnected()) {
+        zeroArms();
+    } else {
+        gArmsZeroed = false;   // re-zero after a reconnect
+    }
+
     bool ready = gHubs[0].hub.isConnected() && gHubs[1].hub.isConnected();
     digitalWrite(STATUS_LED_PIN, ready ? HIGH : LOW);