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Robotic arm simulator: angles, torque and a pick-and-place you code

Two angles decide where the hand goes. Your equations decide if it can lift.

A robotic arm with a rotating base, a shoulder, an elbow and a gripper, driven by servos. Students work out where the hand will be before they move it, check the torque on the shoulder before they lift, and code a pick-and-place mission in 3D.

Open the Robotic Arm labAll robots

First room free · full arm with a plan · Runs in the browser · Class 9 to 12

How the robotic arm works

The base turns the arm left and right; it never changes the height. The shoulder and elbow angles together decide how far out and how high the hand is. Each move is three numbers: base, shoulder, elbow.

Before lifting, the code asks its own torque(): the block's weight times the horizontal distance from the shoulder. If that is more than the servo can give, it does not try. The grip must squeeze hard enough that friction carries the weight: μ × F ≥ m × g.

Parts you build it from

The code

In the lab the library only attaches the servos and waits for each joint to arrive. It does not work anything out: the student writes the equations and the mission. An excerpt of the starting code:

// Robotic Arm - SPYRAL Robotics Lab (an excerpt)
// The library attaches the servos; YOU write the equations and the mission.
#include "SpyralArm.h"
SpyralArm arm;
//   arm.L1  7.02 cm   shoulder to elbow
//   arm.LE  7.63 cm   elbow to the jaws
#define BLOCK_MASS  0.050     // kg

// Friction has to carry the weight:  mu * F  >=  m * g
float gripForce(float m){
  return m * 9.81 / 0.55;
}
// A moment is force times distance; only the horizontal part counts against gravity
float torque(float m, float L, float th){
  return m * L * cos(radians(th));
}

void loop(){
  // Ask your own torque() first: is this even liftable?
  float load = torque(BLOCK_MASS, arm.LE, 0);
  if (load > arm.SERVO_STALL) {
    Serial.println("too much for this servo. Not lifting.");
    return;
  }
  arm.moveTo(40, 60, 92);   // base, shoulder, elbow: over the block, high
  arm.waitUntilStill();
  arm.moveTo(40, 30, 95);   // down to the table
  arm.waitUntilStill();
  arm.close(BLOCK_MASS);    // squeeze, using YOUR gripForce()
  arm.waitUntilStill();
  arm.moveTo(40, 60, 92);   // lift it clear
}

Code it with blocks first

Rooms for Physics, Maths, Coding and "Move it". The Maths room asks for the reach, the height and the straight line to the hand before the arm moves, with blocks like move it to base 90 shoulder 90 elbow 180 and square root of.

Try this in the lab

1. Predict where the hand will be at shoulder 60°, elbow 90°. Then move it and measure.
2. Raise the block mass until the arm refuses to lift. What was the torque?
3. Change the friction in gripForce() and watch the block slip.
4. Code a pick-and-place that moves a block to a new spot and comes home.

Open the Robotic Arm lab

Robotic Arm: common questions

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Read more: robotic arm simulation online · what a robotic arm costs