Well its just critical in discussing a robot that we have agreement on what some terms and conventions mean. So we’ll do some terminology talk first. There’s so many things to cover. Important is to stress that we need to understand what we are discussing. With a robot, there are multiple physical objects moving in three dimensional space. We want a way to describe this without confusion, and this requires a terminology agreement.
We certainly understand that others may have ways to discuss robots, and there’s a group we could call the “professional” robot makers. They must have their own agreements concerning what terms means what. We are not implying that they are incorrect, or that our words and concepts for robots are superior, or better, or clearer, or any of that. We just want everyone that’s reading along with us to have a high probability that they at least understand what’s being said, and in another sense of the terminology dilemma, we really don’t want a discussion going where one person or sets of persons has such a clear and real seeming set of concepts going in their discussion and everyone else, or just a few others, they’re just reading along and contemplating and neither group realizes that they are actually not thinking about the same physical motions or concepts. This is a subtle, but upsetting form of non-communication, and we’re also trying to avoid this communication problem with our discussions in this terminology section.
We want this robot to be seen and contemplated as a “people’s robot”. By this we mean we want people to be attracted to the robot, and to enjoy learning about it, and to enjoy the programming steps of getting the robot so that it can actually walk. It is important that the robot is easily visualized by just about anyone and we want people to be able to say “I understand this robot …, I could build one if I had the parts …, I could program how it moves …, and I could get it to walk”. Another advantage of the human form is that in any discussion of movements, combinations of movements, and likely results from various types of movements, one can refer to a human making these motions and have an ability to “check out” these proposed motions in advance. One doesn’t even need the robot, one can make the motions oneself or observe these motions by someone else and have an immediate feedback.
We will take just a moment here to say “Hello” to our readers with disabilities. We understand of course that throughout the world there exist people who may not be able to make the motions of human locomotion. We do wish to you that your disabilities had never happened, and we hope for all that somehow perhaps the disability will some day be gone. We of course are delighted that we may have readers with disabilities and we welcome you. We understand that throughout we may make reference to certain human motions, certain patterns of human action, perhaps with references to “stand up and try this just to see what happens or just to feel how it feels…” We understand completely that such actions may not be possible to everyone reading along, and we mean not to be hurtful or insensitive. For those where such test motions are not feasible, we welcome you to just imagine what is being discussed, or to use your powers of observation to see these actions in the people around you.
The human form has a significant advantage concerning the process of discussing the robot and its various parts. To the extent that the robot is structured along the human lines, then the parts and motions of the robot are easily described simply by using discussion with naming words borrowed from the human form. For example, if the two sides of the robot have the same sort of mirror image symmetry that humans have, then concepts like the front of the robot, the side of the robot, the left or the right side of the robot are easily understood. If the parts of the robot are assembled similar to the human form, then no harm at all comes from referring to that part of the robot in contact with the ground as the foot of the robot. If the robot has the familiar human design parameters of two vertically oriented support structures with a cylindrical type of overall shape, and they are connected and stabilized at their top by a rigid structure, and the bottom ends of these cylinders contact the ground with independently moveable structures that contact the ground at multiple points, then we can certainly use such phrases as the pelvis, the hips, the legs, the feet, the front of the feet, the back of the feet, bending of the legs, tilting the pelvis, etc.
Most likely everyone would understand what is being discussed if our terminology convention is to simply attach to the different parts of the robot names that are exactly what the names are for those parts on the human form. We note that the robot is a collection of parts that are connected together in an articulating fashion, and that these parts allow the robot to stand upright against the downward force of gravity. It is clear that this can occur only if the robot contains a rigid structural element that resists deformation under a compressive force.
We could use the phrase rigid, strong, metallic, cylindrical structural support element, but such a phrasing is complex, overly wordy, and probably confusing. So we will choose instead to simply refer to these rigid structural elements of the robot as the “bones” of the robot. The word bone is simple, easy to use, and the concept of a bone and what purpose it serves is well understood by just about everybody. We will of course emphasize one more time, these “bones” of the robot are metallic, strong, rigid structural elements that serve the purpose to allow the robot to be upright without collapsing from the force of gravity.
These “bones” of the robot are not living structures, are not derived from crystals of calcium held in a fibrous matrix, and are not constructed so that they are identical copies in form or function of the bones of a human. We will, however, take advantage of the easy familiarity that people have with the names, general shapes, and general functions of the bones of the human body and we will use these features as we assign names the to the “bones” of the robot.
We can appreciate immediately that the robot will have 2 legs, and these 2 legs act to hold up the pelvis of the robot. The pelvis will be a mostly flat rigid support structure that will ultimately hold the power source, processors, valves, and potential payload of the robot. The pelvis is connected to the 2 legs via a separate articulated joint for each leg and this joint is called the hip. The bone of the robot connected to the pelvis at the hip is called the thighbone or femur, and the lower end of this thighbone is connected via an articulated joint to a bone called the shin bone. The articulated joint connecting the thighbone to the shinbone is called the knee. The lower end of the shin bone is connected to a bone of the foot called the ankle bone, and the ankle bone is connected to the toes of the foot via articulated joints of the foot.
In order for the parts of the robot to move, there must be a device that can translate an energy input into motion. This robot uses pistons to create the motive force upon the bones of the robot. The basic energy equation for the robot will be that the robot takes an input of some sort of energy and uses this energy to create pressurized fluid that can be directed to the pistons so that the pistons can make the various parts of the robot move.
The design of the robot will explore various types of energy sources. At the general level for any specific type of energy source, the energy will flow into the robot and will be delivered to a structure that converts the energy into pressurized fluid. The device that makes this energy conversion is referred to as the prime mover for the robot. This prime mover uses the energy so that the prime mover can accept a flow of low pressure fluid and convert it into a flow of pressurized fluid. This fluid is piped to the various pistons via tubing and the flow to each piston is controlled via a set of valves present for each piston.
The sequence of the energy loop for this robot is that initially fluid under low or zero pressure flows to the prime mover, and the prime mover pressurizes the fluid. The pressurized fluid is available to be sent to the various pistons under the control of the sets of valves that are present for all of the pistons. Once a flow of fluid under pressure goes to a piston, it flows into the piston as the high pressure source fluid for the piston. In each case of piston motion, there will be fluid of high pressure flowing into the piston, and fluid under low pressure flowing out of the piston. The fluid under low pressure is also controlled by sets of valves for each piston. This low pressure fluid is eventually directed via tubing back to the prime mover, and is available as a source of low pressure fluid for the prime mover as this prime mover creates pressurized fluid. As one can see, this process (at least with respect to the fluid) is a closed loop, in that there is no loss or gain of the total amount of fluid present in the robot as the robot moves its various pistons and bones.
The pistons of the robot are of standard type. They consist of a cylinder and a rod. The rod has a portion of itself inside the cylinder. On the end of the rod inside the cylinder is a cap or rod cap and this cap fits snugly inside the inner diameter of the cylinder. Pressurized fluid that flows into the base of the cylinder will fill the cylinder in the space between the cylinder base and the rod cap. Providing that fluid is allowed to flow out of the upper portion of the cylinder, and there is enough fluid pressure on the rod cap to overcome any resistance to motion that the rod may have, then as fluid flows into the cylinder base, then the rod cap will move away from the cylinder base and the rod will extend out of the cylinder from the other end of the cylinder. We can call this end of the cylinder the rod end or the head of the cylinder.
Conversely if fluid is allowed to flow into the head end of the cylinder and also fluid is allowed to flow out of the base end of the cylinder, then the fluid flowing into the head of the cylinder pushes the rod cap toward the base of the cylinder, and the rod retracts itself into the cylinder. We see that there can be two motions of the rod, extension (the rod is coming out of the cylinder) or retraction (the rod is going into the cylinder). We also note that it is common to agree that extension or retraction of the rod have really the same meaning as extension or retraction of the cylinder.
We should note that in a real world viewpoint, exactly what part of the piston is seen to move depends or which part of the piston is held so it cannot move. That is, if the cylinder is held so it cannot move, then the rod will be seen as the moving part, and if the rod is held, then the cylinder will be seen as the moving part. If both can move easily or equally, then it may not make sense to try to describe what of the piston is moving, and instead it would probably be clearer to simply state that the piston retracted or extended.
The defining characteristic of the robot is that it moves. The motion of the robot is in three dimensional space and speaking about space and motions in three dimensions absolutely requires some agreement on the terminology for the discussion. It is very traditional to divide three dimensional space into three separate axes of motion. The advantage of this is that any of the potentially complex motions that involve changes of all three axes can be subdivided into separate motions one at a time along each of the axes. This simplifies visualizing the motion and simplifies describing it. A significant problem is setting up a frame of reference because the three axes will simply make no sense if everybody involved has these three axes pointing in different directions.
The most common manner to establish the frame of reference, an least with human or living shapes, is to agree that the frame of reference will always be set as being with respect to the human form. This would mean for example, that unless otherwise stated, up will always be in the direction of where the head is, front will be where the front of the body is, right is the right side of the body, left is the left side, etc. Once we have this agreed upon frame of reference, then the three axes of motion can be specified. These axes are: up-down, forward-backward, right-left.
We will also establish another specified frame of reference using the coordinates of the compass. Again, we choose the compass because its setup is immediately familiar and comprehensible to almost everyone. The advantage of the compass system is that it has an enhanced precision of reference because points on the compass or points of direction can be specified down to the level of degrees or fractions of degrees.
In order to be comprehensible to all, the combining of the robot frame of reference and the compass form of reference also needs a stipulated agreement. The stipulation will be that the so called “standard” position of the robot is that the robot is standing in a stable and balanced position so that if left alone it would stand even if no power were sent to it. It will be standing so that each foot is located under each hip, the feet are facing forward, the middle of the back of the foot is aligned with the middle of the front of the foot, the knee is basically directly above the ankle, and the hip is basically directly above the knee. The pelvis is level in both the front to back axis and the left to right axis. The compass is associated with the robot so that when the robot is in its “standard” position, the robot is stipulated as facing directly due north. This would mean that the left side of the robot is facing due west and the right side is facing due east.
This compass connection allows more specified discussions of various types of positions of the robot. Instead of stating the robot has turned “some” and its now facing “sort of back and to the right”, we can be more clear and state that the robot turned so that the front of the pelvis is now pointed to the northeast, or to north-northeast, or to some specified degree angle of the compass. And very importantly we can make statements such as: “….the robot twisted itself from its standard position so that the pelvis is now facing north-east, but the right and left feet did not move and they are both still pointing to due north….”
There is a descriptive term in discussing humans that is related to the concept of maintaining the human as the central point of reference, independent of what has happened to the human with respect to the human’s orientation to the physical world. The effect of this is that there are certain aspects of the human’s orientation that never change. This form of control of reference has many names such as: body centric, or person oriented. The advantage of this form of reference is that there is no need to continually keep stating and updating the condition of the human with reference to how the human has changed it’s orientation with respect to the physical environment outside of the human.
Using this form of reference for positioning parts of the robot, then we can set up an axis of reference that is related to the robot. In general, these axes are related to dividing the structure in question based on elements of the symmetry of the structure. For example, if one contemplated a solid sphere, then one could divide it with a plane in almost any orientation (as long as the plane passed through the center of the sphere) and the resulting parts would be equal and symmetrical. With other types of structures, however, the logical planes for setting up axes of reference become more limited. Consider a football. One could set up a plane of reference with a plane dividing the football in half, with the plane perpendicular to the short axis of the oval that describes the football. The other logical division would be with a plane parallel to the long axis of the oval that describes the football. Dividing the football any other way does not seem reasonable and does not create halves that are equal.
Consider a more complex shape, for example, a chair. One of the normal types of chairs with four legs, that support a seating area, and with a back of the chair arising up from one of the sides of the seating area of the chair. There is really only one logical division plane for this chair, and that would be a plane parallel to the up and down axis of the chair, dividing the chair so that the seating area was divided in half, as was the back of the chair, and the legs. Of interest, one can notice that a chair divided in this manner does not really create 2 halves that are identical. It creates what most people would “see” as a right side of the chair and a left side of the chair. These two “halves” are not identical, but are instead mirror images of each other.
We note that so much of the natural world of living beings and animals has recurrent axes that divide mirror images of the being. The mirror image concept is very natural and not difficult to image or work with.
In a similar manner, we will set as a dividing plane for the robot the plane that divides it on its long axis (that is the up vs down axis) so that a “right” side and a “left” side are created. It is noted that these two sides are very identical, except that they are mirror images of each other.
This plane of division can then be used as a reference plane that is specific to the robot, and does not change its relationship to the robot, even as the robot moves. This plane can easily be thought of as the middle of the robot, because it divides the robot neatly in half, and divides the pelvis into two parts that are exactly the same, except that they are mirror images of each other. We can bring in the words medial or lateral and describe their relation to this plane of division. The relation is very simple, since we consider this plane of division to be the “center” or “middle” of the robot, the motion toward the center or middle is a medial motion, whereas motion away from the center or middle would be motion toward the side. The word lateral has the connotation of side. So that motion away from the middle or center is stated as being a lateral motion.
The self related nature of medial and lateral are demonstrated as follows: if the robot is facing due north, and the right foot is moved toward the west, then this motion is considered to be a medial motion of this foot, or that this foot has been moved medially. If we rotate the robot so that now it is facing due south, then if we move the right foot in exactly the opposite direction, that is to say toward the east, then we are still going to say that we moved this foot medially. As another example, if the robot was lying down on the ground with its right side down, then if we moved the right foot straight up, then this actual motion of the foot of straight up would still be considered as moving the foot medially. So we have three totally different directions of motion of the exact same foot, and yet they are all the same motion (with respect to medial vs lateral) because we have altered the starting position condition of the robot.
Lateral and medial are robot centric or self related. They have a powerful convenience in the special situations where all that matters is the relationship of the motion of an item with respect to the robot, and the relationship of the motion to the actual three dimensional space is not important.
Now we have all our terminological parameters set out at least with respect to motions of the robot and its time to discuss some movements. And as we start to do this……
Let us go back to the standard condition of the robot. It is standing in a balanced state with its feet pointing due north.