Machina Speculatrix


Over fifty years ago W. Grey Walter started building three wheeled, turtle like, mobile robotic vehicles. These vehicles had a light sensor, touch sensor, propulsion motor, steering motor, and a two vacuum tube analog computer. Even with this simple design, Grey demonstrated that his robots exhibited complex behaviors. He called his creation Machina Speculatrix after their speculative tendency to explore their environment. The Adam and Eve of his robots were named Elmer and Elsie ( ELectro MEchanical Robots, Light Sensitive. ) The following is a photo of Elsie without her shell.

The following is a spectacular photograph taken by Eric Long of the second generation Grey Walter turtle in the collection of the Smithsonian Institution. It is presented here by permission from Peter Liebhold of the Smithsonian Institution.

His robots were unique because, unlike the robotic creations that preceded them, they didn't have a fixed behavior. The robots had reflexes which, when combined with their environment, caused them to never exactly repeat the same actions twice. This emergent life-like behavior was an early form of what we now call Artificial Life.

In the 1950's Grey wrote two Scientific American articles and a book "The Living Brain." You can probably find them in a local library. There are several excellent online articles about Grey Walter and his robot turtles that include photographs of them searching for lights and avoiding obstacles. I have obtained permission from Scientific American and the original illustrator Bernarda Bryson to reproduce some of the illustrations from the 1950 article. The first shows Grey's turtle (he called it a tortoise) seeking a light while the second shows the reaction of the tortoise to a short obstacle.


I have started to recreate the basic structure of the robots using LEGO Mindstorms. Grey's turtles used a front wheel drive tricycle style chassis. The propulsion motor sits in the fork of the front wheel assembly and moves with it. The steering motor is mounted to the body and rotates the steering assembly trough a worm gear. The light sensor is rigidly fixed to the steering assembly so that it is always pointing in the direction of travel. This is my third generation design and I have reduced the construction to the bare necessities to make it easier to build.

Grey's turtles had a steering assembly that rotated 360 degrees and in only one direction. This requires slip rings to carry electricity to the drive motor and the light sensor. Since there is no LEGO part like this, I added a switch to detect when the steering assembly is pointed backwards. This condition is used to reverse the steering motor direction creating a windshield wiper action. The post that supports the light sensor also pushes the switch. The small wheel on the front is used for debugging and to set the initial direction.

The propulsion motor is mounted in the fork of the steering assembly. It drives the front wheel through pulleys and a rubber band. This gears down the speed and also smoothes out the motion. Because the zero power level of the RCX was still too fast for some of the modes of operation I used an on-for-a-while off-for-a-while scheme to slow the motor down.

The touch sensor of Grey's turtles was a stick-in-ring switch connected to a plastic shell that covered the entire robot making it look like a turtle. This produced an all around touch sensitivity that is difficult to reproduce with Mindstorms. I was forced to make a homebrew stick-in-ring sensor by mounting a 1/4" inside diameter, 12-10 wire gauge, connector (Radio Shack #64-3120) on a short piece of copper rod for the ring and wrapping a LEGO cross axle with copper foil for the post. The Shell is made as illustrated with the help of some extra #5201 Angle Pieces.

The turtles had four modes of operation: Search, Move, Dazzle and Touch. The first three modes are determined by light level. Dark is search, moderate light is Move and bright light is Dazzle. Touch causes an oscillation between Search and Dazzle. In Search mode the steering motor is on full and the propulsion motor is on half speed. In Move mode the steering motor is off and the propulsion motor is on full speed. And in Dazzle mode the steering motor is on half and the propulsion motor is on full speed.

At first the Dazzle mode may seem superfluous, but it really helps to create unique behaviors. For one, it prevents the moth drawn to the flame phenomenon. When there are two light sources, the turtle is drawn first to one light and then the other rather than being stuck between the two. When lights are attached to the turtles themselves, they are attracted and then repelled from each other creating a dance something like mating and territorial aggression. The following are illustrations used by permission from Scientific American and the original illustrator Bernarda Bryson from the 1950 Scientific American article showing the Dazzle mode in action.


Here is an example run with my Mindstorms turtle. I made it by attaching a white board marker to the back and letting it record the path it took while it drove over a large sheet of plastic. The path was then transferred to graph paper and scanned into an image. I think the behavior is pretty close to the ones that Grey recorded with time lapse photography. Examples of the Grey's turtle paths can be found on many of the links listed below.


I programmed the Mindstorms turtle using Visual Basic through the Spirit.OCX. After initialization the program is broken down into four tasks: Normal State Machine, Steering Sweeping, Driving Motor Speed and Stuck Override. The Normal State Machine task takes care of reading the light level and determining which of the two motors should be on and at what speed. The Steering Sweeping task watches the steering switch and reverses the direction when it is pushed. The Driving Motor Speed task is necessary because the zero power level of the RCX causes the turtle to move too much for the search mode. The Stuck Override task watches the stick-in-ring sensor and when it is touching it turns off the Normal State Machine, gives the turtle a short burst of reverse propulsion and veers the steering.

So far this program doesn't even reproduce all of the functions Grey managed to make with only two vacuum tubes. As time allows, I plan to duplicate the Walter turtle behavior more exactly. It is a real testament to W. Grey Walter's genius that 50 year after his experiments a state-of-the-art product is sold that allows people to essentially duplicate his work.


Walter, W. Grey, "The Living Brain," W. W. Norton, New York, 1963.


Walter, W. Grey, "An Imitation of Life," Scientific American, May 1950, p42-45.

Walter, W. Grey, "A Machine that Learns," Scientific American, August 1951, p60-63.

Morris, Brian, "The World of Robots," Multimedia Products, 1985, p23.


Long, Eric, American Heritage of Invention & Technology, Vol. 14, Number 4, Spring, 1999, Back Cover.

More biographical information about Grey Walter can be found in Mark Ward’s book on Virtual Organisms.


A great Biographical Sketch of Grey Walter by Walter Freeman in PDF form.

Robot Builder has an article on W. Grey Walter and His Turtle Robots.

Near the bottom of this page is the text of the Time Magazine article from March 27, 1950, p. 74:

Holland, Owen E., "Grey Walter: The Pioneer of Real Artificial Life, Artificial Life V : Proceedings of the Fifth International Workshop on the Synthesis and Simulation of Living Systems , Christoper Langton Editor, MIT Press, Cambridge, 1997, ISBN# 0-262-62111-8, p34-44.

Levy, Steven, "Artificial Life" Vintage Books, 1992, p 282-284.


This type of robot is similar to Braitenberg Vehicles.

Or BEAM Robots.


Web Links: 

Here is where you can buy LEGO Mindstorms online.


The following link is from Owen E. Holland:

There is also a Grey Walter Picture Archive.

New Scientist article:

John Purbrick has written a simulation of Grey Walter's turtle that can be downloaded.

More general robot information can be found on the Robotbooks homepage.

Professor Antonio D'Angelo at an Italian University has class notes on Grey Walter Tortoises.

Other Grey Walter sites:

My General LEGO Mindstorms site: