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Queen Ant - Autonomous hexpod without a microprocessor

For my 2nd year project at university, my team and I set out to create a photophobic walking robot that did not use any processors. Queen Ant was the result.

The project specification was to create an autonomous, 6 legged walking robot, which has the ability to seek heat sources. The project achieves this without the use of a micro-processor to demonstrate how a Central Pattern Generator (CPG) can be effectively used to achieve an otherwise complex behaviour.

A controlled walking and light-seeking behaviour was successfully achieved using only the simplest of components.

You can watch a video of the completed bot walking - Queen Ant Video

The full report is available here - Queen Ant report

The Background

Traditionally robots have been created which utilise wheels for locomotion; the humble wheel can provide fast and simple locomotion which can be easily setup and controlled. However in many real world situations where a robot may be required on uneven terrain, for example in earthquake disaster zones, the wheel fails to provide effective locomotion due to the limitation of vertical motion. It is for this reason that engineers have long sought to create effective robotic locomotion utilising legs. One of the most successful insects in terms of biodiversity and numerical dominance is the ant, with over 11,800 known species1. The ant locomotes with six legs using a form of locomotion much like our own bipedal locomotion, however instead of raising a single leg, it raises a group of three in a tripod formation. By moving in this way the ant will always have a steady tripod on the ground.

Design

The design, works by allowing the twisting of the central section of the robot, forcing a tripod to be created. When the tripod has been formed, the leg motors move the raised legs forward, the central section is now twisted in the opposite direction, reversing the grounded tripod, and the process can repeat. In order to maintain a fluid motion, the two processes occur simultaneously. The central section beings to twist, raising the tripod, and concurrently the legs being forward motion, at the point when the legs are at 50% of the full forwards motion, the central section begins to reverse the twist.

In order to create the duty cycles for the motors, a Central Pattern Generator (CPG) will be used. The central pattern will take the form of an oscillating signal, and will then be replicated in each section of the robot. By delaying or inverting the signal being replicated to each section it is possible to create the required duty cycle. By utilising simple NV neurons (below)) it is possible to create an oscillating signal. The NV neuron inverts a given input at a time constant set by the resistor and capacitor, by grouping two neurons together, an oscillating signal will establish itself. This setup is commonly referred to as the Bicore.

nv neuron

The final circuit (below)) requires buffering of each local signal in order to drive the motors. Signal lines from the CPG to the waist sections require series resistance to delay the signal. By using variable resistors for this purpose it is possible to fine tune the walking pattern. The CPG also uses a variable resistor to change the period of oscillation, by doing this we can ensure the robot walks even with low power.

The mechanics of the robot must ensure that the oscillations remain in step. This is achieved for motors 1, 3 & 5, by the use of elastic attaching each individual leg to its respective segment. The elastic balances the legs by providing resistance if the legs are in any position other than neutral (centered). Motors 2 and 4, controlling vertical motion, are balanced by gravity.

Light seeking behaviour is created using two Light Dependant Resistors (LDRs) mounted on the front of the robot, which feed into two differential amplifiers. By using two differential amplifiers a differential signal is created which will directly drive a motor bi-directionally in accordance with the strongest light source. Turning is realised by manipulating the mechanics of the robot. Attaching the elastic of the front two legs to a motor instead of the front segment, allows the neutral position of the legs to be changed, leading to a natural turn. The motor is controlled by the light seeking circuit (below).

The chips used in the light-seeking circuit are Audio op-amps designed to drive loudspeakers. They take two inputs and measure the difference between the two, then amplifier the smaller signal up to the same level as the higher signal. Using two of these chips and bridging a motor between the outputs allows for the voltage to swing both ways giving bi-directional movement. When the incident light on both LDR’s is the same then the out puts of both chips will be equal and no voltage will be dropped across the motor.

The Build

Electronically the build went very smoothly due to the project being well researched before hand and the simplicity of the design. The CPG was built and modelled using LED’s very early on in the lab and the duty cycles ran seamlessly without much tweaking. The same can be said about the circuit for the light seeking circuit, we simply altered the resistor value in order to achieve the desired sensitivity.

When it came to the mechanics of the robot it was a different story. We always new that this would be the toughest challenge, but in the end after a lot of testing of configurations and materials we built a stable and well balanced chassis. With the chassis and CPG in place we then had to add legs and set up an effective
walking gait. The challenge was to gain a good balance between rolling the front and rear legs to achieve lift and forward motion of the legs to push and pull the robot forward. All the while the robot had to remain balanced through all of the strides. It has taken nature millions of years to perfect leg mechanics and walking gaits, unfortunately we didn’t have that kind of time, but we did achieve a stable and
smooth walk.

An important decision we made was to make all of the circuits “plug and play”. We used single inline pins as motor and power terminals, this meant that we could troubleshoot very easily and that we could alter the robots behaviour quickly and easily.

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