Four Lego Delta Robots

Years of development, months of building and programming.  Here it is.

About the Lego Quad Delta Robot System.

This system uses four Lego parallel robots which are fed by two conveyor belts.  As items flow down the conveyor belt toward the robots, each item passes by a light/color sensor mounted on each conveyor.  When the item is detected, a signal is sent to the robots telling them information such as the color of the object, which belt the object is on and the position of the object on the belt.  The robot reaches out and grabs the item from the moving conveyor belt when each item gets close enough and moves it to a location based on the color of the item.

The cell is capable of picking and placing objects at a rate of 48 items per minute.  Each robot can move 12 items per minute, or it can move an item in 5 seconds!

DELTA ROBOTS

Delta Robots, also known as Parallel robots are commercially available from several manufacturers.  They go by names such as ABB Flexpicker, Bosch Paloma D2, Fanuc M-1iA, Kawasaki YF03N, and Adept Quattro s650H.  They are known for moving small objects very quickly, usually at two hundred or more moves per minute.  Parallel robots are often used in many industries such as the food industry where the payload is small and light and the production rates are very high.  Many times a series of parallel robots are used to do things like assemble cookies, package small items, stack pancakes and much, much more.

THE ROBOTS

Each robot operates independently.  The robots receive a signal from the master, which in this case is the NXT that controls the light sensors.  The signal contains information about the color, lane, and position of each object.  When the signal is received, the data is stored in a chronological array.  When the object gets close enough, the robot goes through a preprogrammed series of movements based on the information in the array. 

STARTING UP

At the beginning of each run, all three arms move slowly upward until they each hit a touch sensor.  After all three arms have reached the top they all move down together to a predetermined zero position and the encoders are reset.  At that point all the robots wait for the first signal which will be the master sending the belt speed signal.  The robots can automatically adjust movements such as where they pick up the objects based on the belt speed.

Immediately after the belt speed information has been received, each NXT brick will sound off in a timed sequence with their respective brick number.  This is an error checking technique.  If the operator doesn’t hear the full “ONE, TWO, THREE, FOUR, FIVE, SIX” there is a problem and the run should be terminated and restarted.

THE SIGNAL

The signal is an eight bit binary light signal that takes about 170 milliseconds to transmit.  The master NXT blinks the LEDs that are attached to each robot on and off at an interval about 20 milliseconds each flash.  Each robot is equipped with a Lego light sensor that easily sees the short flashes.  The same signal is sent to all the NXT bricks, but data encoded in the signal determines which robot will move the item.  The robot’s NXT brick decode the message and sends that information to a procedure that does the appropriate movements.

The binary signal is converted to a three digit number such as 132 or 243.  The first digit is the lane.  Possible values are 1 and 2 corresponding to conveyor 1 and conveyor 2 respectively.  The second digit is the robot number and the possible values are 1 through 4 corresponding to each of the four robots.  The third digit is the color of the object.  The possible values are 1 through 6, i.e.  BLACK=1, BLUE=2, GREEN=3. YELLOW=4. RED=5, WHITE=6.  The position of the brick is noted by the time that the light signal is received.  The robots calculate the position of each object by using the time when the signal was received relative to the current, dynamic time.  The belt moves precisely at 100 inches per minute so based on this, the position of the item on the belt can be precisely calculated.

A few signals other than brick information and belt speed are programmed to be sent.  The master can send an emergency shut down message in which all robots immediately stop what they are doing, drop their bricks and go to their home position as well as stop the conveyors.  Signals can also be sent to make the robots dance, play sound files and music files concurrently.

THE MOVEMENTS

The precise kinematics for the movements of the robots are dynamically calculated using detailed formulas that convert the Cartesian coordinates (x,y,z) of the location of the brick into the angles of the servo motors (theta1, theta2 and theta3) and vice versa.   This is the heart and soul of the robot.  Without precise calculations, this project would be nearly impossible.

As the gripper or “end effector” is moved around, it becomes necessary to calculate the best route for it to move.  The best route is usually a straight line.  This is done by locating the start point (x1, y1, z1) and the end point (x2, y2, z2) and then calculating a discrete number of points that lie on the line between the two points.  For each and every movement, the robot first creates an array for all the points in between and then moves nonstop from point to point to point through the array until it reaches the end point.

As the robot moves around, each motor speed is adjusted relative to the other motors speed in a manner that all three motors arrive at their target position at the same time.  This makes all the movements very smooth and the robot doesn’t shake too much.  The motor speeds are adjusted so that the robot moves as fast as possible.

Since the objects on the conveyors are moving at all times, the robot actually moves to a position where the object will be rather than where the object is actually at.  Also, when the robot grasps an object, it doesn’t lift it straight up, but up and forward slightly so that any objects behind the object on the conveyor belt won’t hit the object that is being moved.

It is possible for the robot to be overwhelmed by having too many objects to pick up.  Once an object goes past a limit point where it is too far to reach, it is removed from the queue and will not be picked up by any robot.

As the robots place items in the bins, the release point is shifted slightly so that the items won’t pile up.

THE GRIPPERS

The grippers are each driven using a single pneumatic cylinder.  The cylinder is cycled by a valve equipped with a medium PF motor connected to an IR receiver.  Each NXT is equipped with a HiTechnic IRLink sensor.  The NXT controls the gripper by sending a signal to the motor through the IRLink sensor.  The motor then rotates clockwise or counterclockwise for one quarter of a second to switch the pneumatic valve.  This is a very effective way of controlling Lego pneumatics with a NXT.

THE AIR SYSTEM

The air system must be robust because the pneumatic cylinders on the grippers move about 96 times a minute.  This requires a great deal of air.  The air compressor consists of six pumps (with the springs removed) turned by three XL PF motors.  The pressure is measured using a MindSensors Pressure sensor.  The pressure is kept between 10 and 13 psi to maintain good operational speed and gripping capacity.  The whole system will not start until air pressure is up to a minimum of 8 psi, and an audible alarm sounds if the pressure drops below 8 psi.  At this point, the operator can help the compressor by manually pumping up the system to the required pressure.

The three XL-PF motors are powered using a 9v train controller.  This is done so that consistent power is transmitted to the motors.  Air compressors tend to use batteries very quickly and using a train controller avoids that cost.

There are also six air tanks for storage, a manual pump, a pressure gage, and a pressure release valve to purge the system of pressure.  The manual pump is primarily used to assist the compressor if it can’t keep up.

The compressor motors are turned on and off using a Lego servo motor and a PF switch.  As the pressure sensor senses the pressure going above or below the thresholds, the motor moves the switch back and forth to add air or turn off the compressor.

THE CONVEYORS

The conveyors are controlled by a dedicated NXT brick.  The timing and speed of the conveyors is critical so that the items will be positioned accurately.  The speed of the conveyors is governed by a proportional controller.  They were originally controlled using a PID controller, but it turns out that a proportional control was adequate.   The speed of the conveyor can be vary from zero inches per minute up to two hundred inches per minute, but one hundred inches per minutes is the best for all the robots.

The NXT brick that controls the conveyors reads the same light signal information as all of the robots, but ignores most of the signals.

Each conveyor is ten feet long.

LIGHT CURTAIN/COLOR READER

The light/color sensors mounted on the conveyor do double duty.  Their default mode is as an ambient light sensor but they are frequently changed to color sensor.  A PF LED light is mounted opposite to the light sensor to give a high value of light detected.  When an item passes between the LED and the light sensor, a low light condition is detected and the sensor immediately switches mode to a color sensor.  This can be seen when the sensor briefly emits an RGB light as a brick passes in front of the sensor.  As soon as the color is correctly read, it immediately switches mode back to an ambient light sensor and waits for the next item.  When the color is determined, the brick then sends a signal to all of the slave bricks and an audible color sound is played.

There is a condition when two bricks pass by both light sensors at the same time.  It is impossible to send two signals at the same time, so the first item to be detected takes priority and the second brick signal is sent 400 milliseconds later.  A special signal is sent to tell the robot to adjust the position timing to account for the 400 ms delay when the brick comes to be picked up.

THE STRUCTURE

The frame structure holding the robots is highly engineered.  The combination of the weight of all the robots as well as the constant movement is a considerable problem.  The main horizontal member is achieved by layering Technic bricks with plates.  This configuration is very strong and has very little sag.  Movement is also minimized, but not completely eliminated.

The two main posts in the middle carry most of the weight and do a great deal to stop the structure from moving while the robots are operating.  The four outside posts help, but are mostly for support.  The diagonal braces are quite small relative to the size of the other members, but actually do a great deal to stop movement.

All of the posts are made from standard Lego bricks with Technic beams attached around to lock them together.  The structure is completely tied together as one piece, but can be broken down into eight parts for transport.

DEVELOPMENT

I have a personal fascination with this type of robot.  I find the movements mesmerizing and extremely interesting. The movements of the actual robots are extremely fast and accurate and defy belief.  I especially like the fact that the location of the end effector can be precisely calculated from the angular location of the three servo motors positioned at one hundred and twenty degrees from each other. 

This is not the first parallel robot that I have built.  My first delta robot was built in 2004 using the Mindstorms RCX and was very crude and not very useful.  After several more attempts, I finally found a design using the Mindstorms NXT system that worked well.  At that time I still hadn’t worked out the kinematics but I found a way to fake the movements by positioning the end effector by hand and reading the encoder values.  Then I used those values to create a series of movements that closely resembled an actual robot.

I have researched for about six years and built this project many times.  This project took about five months to build and program.  It was purely a labor of love for this robot.

I don’t know how to improve on the current design.  As you can tell if you have read this description of the robot, I have exhaustively researched and built to every goal I have.  Sadly, I believe that I have reached the limit of what can be built using only Lego building elements.