4.3.1 Insect Locomotion
Take a second and just think about how complex a task like walking really is. Most people simply take it for granted because it seems so effortless for humans, animals, and insects to just move their legs to go anywhere they want over rough, uneven landscapes. But if one really looks at locomotion, it is readily seen that it is not a trivial task. It takes a great deal of coordination between the different muscle groups to fire in just the right order, at just the right time to synchronize the leg movements. If the legs are not synchronized to work with each other, then the animal will fall down or simply thrash around. Uneven terrain also presents a formidable challenge in that the animal has to adapt its locomotion on the fly. If an insect is walking along and a rock is in its way, and it steps normally like it always does, then it may cause some of the other legs to no longer be touching the ground. This could mean that it would have trouble taking the next step, or could even fall over when it picks up its legs on the other side for the next step.
2. Dynamic Stability
Human locomotion is typically more complex than insect locomotion. Humans use what are known as dynamically stable gates. When the center of mass is plotted as a human walks, it looks like an inverted pendulum motion. Potential energy at the peak is converted into kinetic energy in the next step, the the kinetic energy is recovered and converted back into potential energy again. This is visible in figure 1. Even when a human is standing still they are not very stable. They are using their muscles to insure that they remain balanced on their feet. If they did not, then the slightest perturbation would knock them over and they would fall. So humans maintain stability by constantly adjusting the muscles of their body to insure that they do not fall. This is what is meant by dynamic stability.
3. Static Stability
Some insects also have a measure of dynamic stability in their gates. However, they typically use statically stable gates. Most insects have six legs, sometimes more, and sometimes a lot more. The statically stable gate uses these other legs to make certain that the insect is always stable while it is moving. Unlike in dynamic stability where it is constantly adjusting its body and legs to maintain stability, statically stable is always stable because at least three of the legs are always kept on the ground in a configuration that will keep the insect from falling. This is similar to the stability that is attained by the tripod stand for a camera or of a stool. There are two primary gaits that insects use for walking. These are the metochronal wave and the tripod gate. These are really just extreme ends of a continuum of gates. The metochronal wave gate is used for slower walking speeds and the tripod gate is used for faster speeds. The insect can also mix them to walk at different speeds in between the two types of gaits. Figure 2 shows a timeline that displays both of the gates. The metachronal wave gait is the slowest of the gates for an insect. It's seen when a "wave" of leg movements ripples down each side of the insect. A good example of this type of gait is a caterpillar. The tripod gait is the fastest of the gaits. In it, the insect always has two legs on the ground on one side and one leg on the ground on the other side such that it forms a tripod. This can be seen in Figure 3. So three of the legs are on the ground and moving backwards while the other three legs are raised and moving forward. As the feet on the ground are moved back, this causes the body of the insect to move forward. Then when the raised legs are all the way forward they lower to make contact, and the legs that were down are raised and the whole pattern is repeated.
4. Central Pattern Generators
One of the underlying concepts that have been found for all animal locomotion is that of the central pattern generator. The CPG is the set of interconnected neurons that are responsible for generating the rhythmic motor patterns that animals use in locomotion. So for instance, in the tripod gate each leg would have its own CPG that would control the rhythm of moving its leg back and forth, and raising and lowering the leg in the correct sequence. CPG's have been proven to exist for animals, and there has been a good deal of research on their properties and even on how they are used by cockroaches to generate their tripod gates. An important part of using the CPG to actually make an insect walk is the sensory feedback from the legs. The information from the legs and body feed into the CPG's to allow them to adapt to the uneven and rugged terrain in the environment. So a CPG by itself will produce a fairly predictable, rhythmic firing pattern. But when it receives sensory information the output of the CPG changes based on what those sensors are telling it.
5. Simulator Locomotion
The insect simulator system has two levels of systems that control the walking of the insect. The lowest level is the leg controller. The leg controllers emulate the CPG's described above. There is one of these controllers for each of the legs. It is responsible for the generation and timing of the muscle firing patterns to make the leg take a step. The next level is the locomotion controller. Its purpose is to synchronize the separate leg controllers in order to make them work together so that the insect will actually walk rather than just thrash around on the ground. Both of these systems will be discussed in more detail in the following sections.
Sometimes it is fun to just sit and watch an ant as it goes about its business of trying to find food. It is constantly encountering obstacles, rugged terrain, leaves, sticks, and pebbles in its way. But it appears to navigate over or around all of these obstacles in an effortless way. It seems so simple, but it is really complex. The ant must have a number of CPG's working for each leg, and each of them must be synchronized to work together. If they are not, then the ant could try and raise 4 or 5 of its leg all at the same time and it would fall. Or it could try and move all of the legs forward at the same time, and so on. There are uncountable ways to mess up and end up falling or just thrashing around. But getting everything synchronized correctly so that the legs all work with a unified purpose of propelling the ant forward or backward is much more difficult. The following sections begin to explain some of the details of how the insect simulator is able to accomplish this.