The Meccano Turtle was constructed in 1974 to demonstrate to my class several techniques that might be used in the making of robotic devices in partial fulfillment of the requirements of the course I was teaching. The object of the Turtle was to have a mobile device which would follow a white line along the darker tile floor. When the end of the line was reached, the device was to stop and sound an alarm.
Photo 1. This is the rear left side of the model showing its name plate and the control panel. The switches are Amplifier, Alarm, and Motor. Two batteries were used - one 6 volt for the motors and one 9 volt for the amplifier.
Photo 2. This is taken from upper front left. The object here is to show the amplifier board. This will of course be changed to a platform for the Stamp and its associated components. The skirt around the model was a light shield. The object of the model was to have it follow a line on the floor. Ambient light would upset that - so the shield.
Photo 3. The Turtle was a three wheeled vehicle with a front caster and two drive wheels at the rear. Steering was achieved by turning off one of the drive wheels causing the Turtle to pivot around the non-turning wheel. Thus, if the right hand motor was stopped, its wheel would stand still and the operating left hand motor and wheel would cause the Turtle to pivot and thus turn to the right.
The steering was controlled by a pair of photo cells (Electronics Set) which would sense the light reflected from the tape on the floor. It was soon determined that the photo cells and the relays from the Electronics Set did not have the necessary sensitivity for this purpose and an amplifier had to be added. That is the circuit board shown on the front of the Turtle. On the back of the Turtle were three switches to control the model. They were an off-on switch for the amplifier; one for the alarm, and another for the motor.
Power for the Turtle was provided by the two batteries on the top of the model. The amplifier used the 9 volt battery at the very top, while the motors used the 6 volt output of the lantern battery. The lantern battery had power far in excess of what was required for the Turtle, but served another purpose in adding "ballast" where it was needed. This added weight over the wheels gave better traction allowing the Turtle to operate over a variety of floor coverings.
The front peak of the model contained a light source to illuminate the tape line to be followed. The reflection on the tape was then sensed by the photo cells. This action required that the entire process be shielded from ambient light. Hence, a skirt was placed around the entire model. Looking at the rear view, the sign "Meccano Turtle" was engraved on a laminate material whose color closely matched the blue color of the 1970's Meccano. You can see that the relays controlling the motors were mounted on the top deck of the model with each relay controlling the wheel directly behind it.
Photo 4. Turtle without the skirts. You can see from this the detail of the motor drive and the front swivel or caster wheel. You can also see the light source in the front along wiith the two photo cells.
Photo 5. The bottom of the Turtle with the skirts removed for clarity.
It is difficult to see much of the model with the skirts attached. Therefore, several black and white photographs were made for record purposes. The side view shows the position of the light source (a # 46 Lamp) and the two hoods of the photo cells. These are sufficiently forward of the caster wheel to prevent any interference in the light pattern. The caster is a #168 Ball Thrust Bearing with the axle captured so it could not fall out when the model was lifted from the floor. The wheels were chain driven from the motors and the chains tightened to minimize backlash.
The bottom view shows more detail, in fact it tends to be cluttered. Space was tight, so the construction became rather cramped. This view does, however show the relative positions of the various components.
The Turtle was an outstanding success and was the subject of a newspaper article and a visit from the local TV station when it was running. It was being used as a messenger. I put down an adhesive tape (1" wide) track from my office and down the hall to the office. There the tape track ended. I had also mounted a basket on the top of the Turtle. Each day, I would put the Turtle on its track, turn it on, and it would travel to the secretary in the office where it would stop and ring its bell. The secretary would then put my mail in the basket, turn the Turtle around and it would return to my office with the morning mail. This was a hit with the TV crew who had to film several trips of the "Turtle". I was the brunt of much good-natured kidding by my fellow faculty. The students loved it!
The Turtle had an interesting gate. As it ran it would seek the line thus giving the impression that it was "shaking its hips". One student commented that it truly did resemble a Turtle when it was following its line.
All of this was 25 years ago. Now I am rebuilding the Turtle in order to update it to be controlled by the Basic Stamp 2. The original was controlled by relays as you can see in the schematic.
Photo 6. This is a scan of a photo copy ( 1974 technology) of my pencil drawing intended for record purposes only. But you can see just how it worked. One point, however, the left photo cell and associated amplifier operated the right hand motor and visa - versa. The transistors were general purpose NPN - #2222 as I recall. The potentiometers were used to balance the action of the two photocell - amplifiers.
What is next? I have promised Dr. Adler that I will keep him up to date with the progress on the reconstruction of the Turtle by sending him photos at frequent intervals. I have identified the components needed using the photos and am ready to assemble the chassis. In checking my "Meccano Morgue" - the box that holds old parts and special pieces - I found the original nameplate and control panel. I am ready to begin!
Fred Culpepper - May 1999
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