Robot arm with joysticks on an Arduino Uno

// Robot arm with two joysticks: a four-servo desktop arm driven straight
// from the Uno with the Servo library.
//
//   arm      BASE pin 3, SHOULDER pin 5, ELBOW pin 6, GRIP pin 9
//            VCC and GND from a separate 5 V supply, GND joined to the Uno's
//   left     joystick VRX A0 (base), VRY A1 (shoulder)
//   right    joystick VRX A2 (elbow), VRY A3 (claw)
//
// The sticks are rate controls, the way the kit sketches do it: push a stick
// and its joint keeps moving while you hold it, faster the further you push;
// let go and the stick springs back to the middle and the joint stays put.
// A small dead band round the middle stops a stick that does not center
// perfectly from creeping the arm.
//
// Four SG90s moving at once can pull more than an amp, which is more than an
// Uno's 5 V pin gives from USB. That is why the arm has its own supply on the
// bench and here, with the grounds joined so the signals mean something.

#include <Servo.h>

Servo base, shoulder, elbow, claw;
Servo *joints[4] = {&base, &shoulder, &elbow, &claw};
const int SERVO_PINS[4] = {3, 5, 6, 9};
const int STICKS[4] = {A0, A1, A2, A3};

// Where each joint starts, and how far it may go. The claw shuts at 20 and
// is wide open at 120; the shoulder and elbow stop short of the ends so the
// arm does not fold into its own base.
int angle[4] = {90, 90, 90, 70};
const int LOW_END[4] = {0, 40, 40, 20};
const int HIGH_END[4] = {180, 160, 160, 120};

const int DEAD_BAND = 60;  // of 1023, either side of the middle

void setup() {
  for (int i = 0; i < 4; i++) {
    joints[i]->attach(SERVO_PINS[i]);
    joints[i]->write(angle[i]);
  }
  Serial.begin(115200);
  Serial.println("robot arm: push a stick to move a joint");
}

void loop() {
  bool moved = false;
  for (int i = 0; i < 4; i++) {
    int offset = analogRead(STICKS[i]) - 512;
    if (offset > DEAD_BAND || offset < -DEAD_BAND) {
      // Up to 3 degrees a pass at full deflection.
      angle[i] += offset / 170;
      if (angle[i] < LOW_END[i]) angle[i] = LOW_END[i];
      if (angle[i] > HIGH_END[i]) angle[i] = HIGH_END[i];
      moved = true;
    }
    joints[i]->write(angle[i]);
  }
  if (moved) {
    Serial.print("base ");
    Serial.print(angle[0]);
    Serial.print("  shoulder ");
    Serial.print(angle[1]);
    Serial.print("  elbow ");
    Serial.print(angle[2]);
    Serial.print("  claw ");
    Serial.println(angle[3]);
  }
  delay(20);
}