76 CHAPTER 5. EXAMPLES
Figure 5-13: A hydraulic piston
(behavior
(qspace minf 0 c f inf) (input push force ()
((0 inf) f const))
(output cap-displacement position ((fixed 0)) ((0 inf) c const)))
Figure 5-14: Zeroth-order behavior of the piston
but no worse than any other for the purposes of obtaining a qualitative model.
ef-5.2. THE HYDRAULIC PISTON 77
Figure 5-15: Piston with interpreted I/O variables
fort source (connected to a 0-junction), and the displacement of the cap is interpreted as the integral of a ow (the cap's velocity), indicated by a 1-junction.
5.2.1 Connecting Bond Graph Fragments
To analyze the model of Figure 5-15, MM must construct a connected bond graph from the two fragments. In earlier discussions I have glossed over this step, but it is not a mere detail. Here, I explain MM's connection algorithm in depth.
MM's structure recognition rules look for regions of interesting behavior in a model.
Once a set of interesting regions has been found, the rest of the structure is, by implication, uninteresting. But this does not mean it is irrelevant. Though there may be no dynamic behavior associated with uninteresting regions of the structure, some of these regions might still be involved in the energy ow of the model: they may serve as conduits of power that link the dynamically interesting parts of the model. That is essentially what we are saying when we place a bond through a region of space: we are asserting that power ows through the region.
The model connection problem, then, can be stated as follows: given two regions in the structure|the interesting regions|determine the paths between them (if any) through which power ows. The model can be completely connected by successively connecting pairs of interesting regions.
My general approach to nding thesepower paths uses the idea of ow. The direc-tion of ow|whether it be mass ow, current ow, heat ow, etc.|is (by convendirec-tion) the direction in which power ows. SinceMM deals only with translational and uid systems, it deals only with mass ow.
The method requires that the system structure be divided into convex polygons of uniform composition. This step is a by-product of MM's division of the structure into segments, since segments are just groups of adjacent convex polygons of the same composition.
The approach proceeds by determining, for each polygon, in which directions the
78 CHAPTER 5. EXAMPLES polygon (or more precisely, the substance contained in it) can move. Here are the rules MM uses to perform this step (recall that there are three compositions: rigid, exible and uid).
1. If the polygon is rigid and xed, it does not move.
2. If the polygon is solid (either rigid or exible) and is unxed, it can move in the direction(s) of any applied forces.
3. If the polygon is either exible, or rigid and unxed, and an adjacent polygon (of any composition) moves into it, then it moves in the same direction. This applies to xed exible polygons, where the movement is deformation.
4. If the polygon is exible and xed, and moves in some direction, it also moves in the opposite direction. This captures the elastic property of exible objects.
5. If the polygon is uid, ow can occur through any edge that does not border a xed, rigid object. Note that ow can occur through an edge bordering exible material, even if it is xed; this corresponds to a deformation of the exible substance.
These rules embody a rather coarse and in some cases inaccurate view of ow. For instance, without quantitative information rule 2 has no way to resolve the impinging forces to determine a single, unique direction of motion. This rule, therefore, only gives accurate results in the case of a single impinging force. The rules completely ignore kinematic constraints; in general,MM does not handle kinematics. They also ignore rotation.
The algorithm does perform acceptably in the domain for which MM is special-ized: mechanical-hydraulic systems with trivial or no kinematics, where the parts are in contact. Rule 5 accurately predicts possible uid ow directions, and the other rules handle interactions of solid parts that correspond to one-dimensional translation without rotation.
Once the movement or ow directions of each polygon have been determined, a graph is constructed. The nodes of the graph are polygons, and an directed arc joins two polygons if the rst would move into the second. To determine whether one polygon would move into another, MMtranslates the rst polygon by a small amount in its allowed movement directions and tests for overlap with the second. It is this step of the algorithm that imposes the assumption that objects be touching, or nearly touching.
When the graph is complete, determination of power paths has been reduced to graph search: the power paths between two polygons are the paths in the graph between the polygons. The graph for the piston is shown in Figure 5-16. The arrows in the picture are the arcs of the graph.
This graph is derived as follows. The initial, external force (not shown) acts against the piston shaft. Since the shaft is not xed, it can potentially move in the
5.2. THE HYDRAULIC PISTON 79
shaft
plug
top wall*
bottom wall*
cap*
Figure 5-16: Flow graph for the piston. Starred polygons are xed in place.
force's direction (rule 2). The movement of the shaft impinges on the plug, so the plug also can move in the same direction (rule 2). The uid polygon can potentially move either to the left, into the unxed plug, or to the right, deforming the exible cap (rule 5), which moves left into the uid (rule 4). The piston walls are xed, so the uid cannot move in those directions. The xed polygons cannot move at all (rule 1).Once it has constructed the ow graph, MM uses it to compute power paths between polygons that contain bond graph pieces. If no path is found, MM does not connect the pieces. When there is more than one path, MM rst eliminates paths with cycles, then connects the pieces along all remaining paths.
Avoiding cycles is a good idea, since they are generally artifacts resulting from the overgeneral treatment of motion. For instance, in the piston ow graph of Figure 5-16, there is a cycle between the plug and uid polygon which should rightly be ignored.
Sometimes, however, meaningful power paths will contain cycles. Consider a device in which an object's horizontal movement results, through a series of interactions, in that same object being moved vertically, perhaps interacting with some other object.
That series of motions appears as a cycle in the ow graph, but it is a useful one because the object's directions of motion dier. The ow graph is not rened enough to capture this distinction.
Once a power path has been found, it is not sucient to simply draw a bond along the path that connects the polygons. If the power path crosses energy domains, transformer or gyrator bond graph elements must be inserted at these junctions to mark the fact. Currently,MMassumes that the transformer is the correct element to insert. This assumption is often correct when dealing with the change from mechanical to uid domains in which the uid is contained, for then a change in force on the mechanical side will tend to cause a corresponding pressure change in the contained
80 CHAPTER 5. EXAMPLES
Figure 5-17: Interpreted piston model, connected
uid, and this is a transformer relationship. However, in other cases, even mechanical-uid cases, a gyrator is the correct element (e.g. a turbine). MM currently does not have sucient geometrical or physical knowledge to distinguish these cases. This is an area for future work.
Note that a transformer has no qualitative eect on the model, since it says only that one eort is proportional to another, and likewise for ows. However,MMwould be remiss if it did not include them in the model, for they do describe a crucial piece of the model, namely a change in energy domain.
Figure 5-17 shows the connected version of the model in Figure 5-15, with trans-formers. This model is zeroth-order, but is rejected because MM observes that there is no relationship between the input eort and the output ow. The mere presence of a bond does not guarantee such a relationship. Recall that a bond represents two variables, an eort and ow, but does not place any constraint between them. The bond graph of Figure 5-17 contains no elements which relate the eort and ow, and
MMchecks for this before performing a qualitative simulation. If it did not, the qual-itative simulation would say that the model agrees with the given behavior; because the model's output is unconstrained, it will agree with any behavior.
5.2.2 The Zeroth-order Model
One of next models that MM constructs, shown in Figure 5-18, is a correct one. MM constructs it using the following structure recognition rule:
capacitor-force/trans
(Propose a capacitor in a exible region that can move in opposition to a force)IF there is a force F
AND there is a rectangle R containing exible material such that the material can expand or contract in the direction of F,
THEN add a capacitor to the model and set its region equal to the exible portion of R.