7 Behavior Fusion

This project's purpose for acquiring human target locations is ultimately to point the pan/tilt/zoom camera at them. The task of moving the camera while sensing falls under a growing area of research referred to as active vision [121]. Active vision may be defined as the automatic modification of camera orientation or image processing strategy in response to sensor information. This directs the allocation of resources, be they camera pixels or processing cycles, to the analysis of a subset of the viewable world. For the application described here, camera motion may be used to accomplish two goals: (a) to better acquire information about the state of targets in the world, and (b) to improve a human user's view of activity in the scene as required by the surveillance or videoconferencing application. In some cases these goals may be coincidental; in others, they may conflict. It is the job of the behavioral control system to accomplish these goals and resolve the conflicts in a satisfactory way.

 

7.1 Multi-Agent Reactive Control

Autonomous robots, such as the automatic camera or the mobile robot MARGE [80], must react to dynamic events in unstructured environments. The challenge of integrating noisy and incomplete sensor information into control actions in real-time has led to a variety of reactive "behavior-based" control schemes for mobile robots. Much of this work was inspired by the layered control system and subsumption architecture developed by Brooks [73,74]. Brooks' robots utilize multiple task-achieving behaviors which closely couple sensors to actuators. Connell [75] used distributed subsumption-based behaviors to enable a mobile robot to search through unmapped rooms collecting empty soda cans. In recent competitions between autonomous robots [81], the winning entries made extensive use of low-level reactive perception and control.

Reactive control involves mapping sensor inputs to control signals quickly, usually involving little or no intermediate representation. Most feedback control systems, such as servomotor controllers, involve only one or two inputs. Linear gain parameters are often sufficient for determining the actuator output for such simple applications. But for more complex reactive systems, the size of the input space demands a more sophisticated control algorithm. This task can be made more manageable by breaking down the input space for analysis by multiple agents, each of which responds to specific types of situations, and then integrating the recommendations of these agents.

Agents can be designed independently, to exhibit behaviors tuned for different objectives. This is an important level of abstraction for system integration, and can serve as a guide for the division of computation among distributed processors. Multiple behaviors can also provide robust behavior and degrade gracefully. The combined effect of multiple behaviors is referred to as emergent behavior, and ideally will share the characteristics of the individual behaviors. Unfortunately, the development and integration of agent recommendations usually involves ad hoc algorithms that are application-specific. Brooks' subsumption architecture uses finite state machines and hierarchical arbitration to select between multiple behaviors. This allows fail-safe behaviors like obstacle avoidance to subsume goal seeking when necessary. Arkin [76] fuses independent behaviors called "schemas" with artificial potential-field techniques. In this chapter a modular method for developing reactive control behaviors using fuzzy control rules is described. Fuzzy control allows smooth generalization between multiple modes of operation. The function is typically a continuous surface, as opposed to many behavior-switching schemes, and follows the philosophy that similar situations result in similar outputs. This system allows the integration of many more sensor inputs than do previous real-time fuzzy systems, without special hardware.

 

7.2 Fuzzy Control Behaviors

The success of fuzzy control is owed in large part to the technology's ability to convert qualitative linguistic descriptions into nonlinear mathematical functions. Bridging the gap between human expert knowledge and the world of digital systems has led to its use in many consumer products. Most of these fuzzy control implementations, however, feature a single layer of inferencing between only a handful of inputs and outputs. This may lead one to suspect that fuzzy control is only useful in simple systems such as these. On the contrary, this chapter presents a multi-layered fuzzy control network for reactive control of an autonomous robot. Given the complexity of the system, linguistic rules provide an essential tool for implementing the robot's control, for which no mathematical model needs to be developed.

A fuzzy control function may be defined by using fuzzy sets as adjectives in a qualitative rule base. The effect of each rule inference is then proportional to the degree of truth of the fuzzy sets associated with it. When programming a fuzzy system with fuzzy rules, the system designer provides his or her own qualitative understanding of the problem.

 

7.2.1 Evaluation of Fuzzy Rules

A fuzzy rule performs an inference with a certainty, or weight, dependent on the set operations in its antecedent. A fuzzy rule for a servo controller may take a form such as: "If error is SMALL and error is ZERO then output is SMALL" This rule assigns the value of SMALL to the variable output with a weight determined by the intersection (minimum value of two membership function evaluations) of the sets describing error and error. The application of multiple fuzzy rules results in multiple output recommendations. In many fuzzy expert systems, antecedent weights are intersected with the output sets to describe the control output as a fuzzy variable. The output value has its own membership function, which is useful if it is to be used by other rules in a chain of fuzzy inferences. For control applications, however, defuzzification is necessary to obtain a singleton value that can be passed on to actuators. This is popularly achieved by taking the centroid the output membership function. Unfortunately, the manipulation of fuzzy sets and calculation of the membership centroid is computationally expensive compared to ordinary linear control algorithms. Real-time systems that define outputs as fuzzy sets in this manner often rely on special hardware to perform rule evaluations [77].

For most control applications, outputs do not need to take the form of fuzzy numbers. A much faster method can be used to evaluate rules by calculating the centroid from singleton recommendations. If each rule i prescribes an output value of oi with an antecedent certainty of wi, then the output of a controller with N rules is calculated as

 

Figure 56: Example Membership Functions

To compare these two techniques, consider a very simple mapping with one input and one output. Figure 56 defines the sets to be used in this example. The first function in this example uses two rules with fuzzy output sets:

 

Function A:

1. If input is ZERO then output is ZERO.

2. If input is SMALL then output is SMALL.

 

Suppose one wishes to calculate the output value of Function A for an input of value of 70. The first rule will have a weight of 0.3 and the second will have a weight of 0.7. Figure 57 shows the result of the weighting operations on the respective fuzzy output sets. The output of Function A is the centroid of the two shaded regions, which is calculated to be 64.

 Figure 57: Centroid for Fuzzy Output

Now consider the same function with output recommendations defined as singleton values instead:

 

Function B:

1. If input is ZERO then output is 0.0.

2. If input is SMALL then output is 100.0.

 

Figure 58: Centroid for Singleton Output

 For Function B, the crisp centroid is much easier to calculate, as illustrated in Figure 58. For an input value of 70, its output is ((0.3)(0.0) + (0.7)(100.0))/(0.3 + 0.7) = 70. The singleton centroid scheme of Function B allows fuzzy rules to perform real-time control functions on ordinary processor hardware. This method of rule evaluation was used for the mobile robot MARGE as well as the camera control system described here.

Figure 59: Rule Components

Figure 60: Combination of Rules

Figure 59 shows the block diagram of a fuzzy rule for the system. A rule performs a sum-of-products (intersection before union) operation on the fuzzy set comparisons in its antecedent using max-min or arithmetic sum and product operations. The resultant weight is assigned to the source value for the centroid calculation. Note that the source may be a fixed value, as is the case with most fuzzy control systems, or it may be a value passed from another operation. This allows a set of rules to act as a fuzzy multiplexer, by blending recommendations from other agents according to qualitative terms. Figure 60 shows the configuration of multiple rules for the evaluation of a fuzzy node. Notice that black arrowheads denote set inputs, while gray arrowheads denote sources for output recommendations.

 

7.2.2 When Centroid Defuzzification Fails

 Centroid defuzzification works well for simple control systems that involve outputs with convex membership functions, e.g., servo controllers. It allows the system to compromise among multiple objectives, as shown in Figure 61. However, when the control system must choose between two exclusive output values, and a weighted-average compromise is unacceptable, centroid defuzzification will produce an undesirable control output. Figure 62 illustrates this problem faced by a mobile robot and a camera controller. For the mobile robot, the fuzzy control rules for steering recommend steering left or right as acceptable outputs, but not straight. For the camera controller, Figure 63(a) shows the output of the steer controller using fuzzy output sets, and Figure 63(b) shows the same for outputs defined with singleton values. Note that in either case, centroid defuzzification results in no net steering effect. The robot will therefore either crash into the object, or, if it is able to stop in time, may remain stuck in front of the object in a state of what could be termed "symmetric indecision."

Figure 61: Acceptable Compromises

Figure 62: Mutually Exclusive Options

Figure 63: Unacceptable Centroids

There are a few different ways for a system to handle the erroneous centroid problem:

 Human beings who suddenly meet one another at a doorway or hallway often find themselves in a symmetric indecision dilemma. It is not always easy to quickly choose one of two equally attractive paths, and so people often hesitate during the decision process. When reactive decision making breaks down, slower high-level processing may be required. Thus cognizant failure may be an acceptable option if it can be permitted safely, and recovered from by other processes.

 

7.2.3 Effects of a Large Input Space

Most fuzzy control applications involve only a small number of inputs. This allows them to perform the entire inferencing mapping in one step. All of the fuzzy rules typically look at the same inputs and affect the same output. Suppose we wish to design a controller for a system with N inputs, and each input i is to be described by Mi fuzzy sets. A different rule may be written for every intersection of set descriptions that describes the N inputs. This exhaustive method yields a rule set of the following size:

 

 Unfortunately, the number of fuzzy set evaluations in a rule base increases exponentially as more inputs are added to the controller. This results an impractical computational load for systems with high input dimensionality. It also makes it difficult for the programmer to manually define rules that span the input space. In order to keep the rule base manageable, other autonomous robot implementations have reduced the input space by throwing away what might otherwise be useful sensor data or by first matching it to a symbolic world model and extracting state variables.

Rather than reducing the input space by non-fuzzy means, a system was developed to process a larger data space with multiple fuzzy agents. Consider a symmetric N-dimensional input space where each input dimension is to be spanned by the same number of fuzzy sets, M. If the fuzzy inference mapping is done in one step, the number of set evaluations that must be performed is N(M^N). If the input space dimensionality is broken down for processing by local agents, fewer set evaluations must be performed. Suppose one employs M^n local agents, where each will be assigned the same N - n inputs. These agents then may be fused by a fuzzy multiplexer that uses the n remaining inputs. The multiplexer will perform nM^n set evaluations, while each agent performs (N - n)(M^(N-n)). The total number set evaluations for this scheme is then N(M^N) n( M^N - M^n ) < N(M^N). A large input space can be broken down in several steps for considerable savings in computation. Figure 64 shows a 3-dimensional input space processed by local experts and fused by a multiplexer. Note that there is no restriction to using agents of uniform size or architecture; rather, the flexibility of using specialized agents of different types is an important part of this scheme. Each of the local agents, for instance, may be composed of additional agents in a hierarchical network.

Figure 64: Fusion of Local Agents by a Fuzzy Multiplexer

 Fuzzy agents can also pre-process data for other nodes to use as inputs rather than sources. This allows the input space to be transformed into another form that may be more useful for agents to react to. In most control applications, raw sensor data is not directly mapped to actuators without transforming it first, for purposes such as filtering or to calculate error signals and derivative terms. This reduces the computation required for some applications and makes the system easier to specify manually for by providing a higher level of abstraction than the direct input space. This system can be easily configured to accomplish such pre-processing functions with fuzzy rules.

 

7.3 Arbitration Mechanisms

In any multi-agent behavior-based control system, multiple behaviors eventually must be fused to result in the robot's motor control. Many different schemes have been used for this in the literature, such as hierarchical switching and weighted averaging. The approach used in the control scheme for MARGE and the robot camera combines these techniques by using the flexibility of fuzzy rules to perform the fusion operation. Fusion rules may use sensor data, motivational state, or the values of the behavior outputs themselves to determine the appropriate weight for each behavior. The following are a few possible ways to blend or arbitrate between recommendations from multiple sources.

 

 

 

 

 

Fuzzy behaviors provide a convenient abstraction for the development of sensor-based control systems for unstructured environments. A large input space can be mapped to actuation by dividing the problem into simpler domains and developing individual agents that compete and cooperate to perform the task. Fuzzy control rules make the job of defining a control surface easier by providing a linguistic interface to the programmer. This system was demonstrated successfully on MARGE at the 1993 AAAI robotics competition, and was also applied to camera control as described in the following chapters.