GB2253926A - Robot navigation apparatus - Google Patents

Abstract

Robot navigation apparatus uses a resistive god having nodes, each representing an individual and discrete position within a navigable environment 1, and internodal connections, all represented on a logic chip. Open circuit internodal connection(s) result in failure of the robot 3 to move further along the navigable path, and can lead to collision with obstacles 5, 6, 7 within the navigable environment. Detection of open circuit internodal connection(s) and prevention of movement of the robot along a path represented by the open circuit internodal connection(s) can eliminate problems associated with navigation along open circuit internodal connection paths. Selection criteria for robot movement direction include lowest node voltage, highest node voltage and calculated current magnitude comparison. Typical resistive grid topologies include hexagonal and rectilinear. <IMAGE>

Description

2253926 1 ROBOT NAVIGATION APPARATUS.

1 11 This invention relates to apparatus for robot navigation.

Robot navigation in a known indoor environment subject to a constraint of obstacle avoidance is a known problem in the field of robot path planning. There are various methods of overcoming this problem, and these include apparatus such as eg those which search all available paths and select the most appropriate path before the robot moves from its start position, and also apparatus which implement an Artificial Potential Field method. Where the Artificial Potential Field method is used the obstacles are represented in a software simulation on a logic chip associated with the robot as repelling forces and the goal as an attracting force. A logic chip is normally an electronic chip which contains all logic for navigation of the robot, where the logic is normally implemented as electronic binary components such as AND gates, OR gates, NAND gates etc. Use of the Artificial Potential Field method creates a potential field over the representation of the environment to be navigated. A path is selected by performing gradient descent of the potential field from the robot's start position towards the goal. Use of this solution suffers from the problem that the algorithm is liable with certain configurations of obstacles, especially where the obstacles are concave, to find a local potential field minimum and terminate there. rather than reaching the goal.

Alternatively, robot navigation apparatus may implement a resistive 2 grid. The environment to be navigated is notionally divided into a grid having nodes at intersections of the notional grid divisions, and each node represents a discrete position of the navigable region. On the logic chip the resistive grid is represented such that each internodal connection is assigned a particular resistance (normally in the form of a resistor). The resistive grid normally has a hexagonal or a rectangular connection topology. Obstacles are represented on the logic chip as areas of high or infinite resistance on a constant low-resistance background. A voltage is then applied to the logic chip between the node representing the robot's present position or start position and the node representing goal. Current will flow, and by following the path of maximum current a near optimum path to the goal will be found. This method has the advantages that:

1. if at least one path exists between the robot's present position and the goal, then by following the direction of maximum current a path will be found (unlike the Artificial Potential Field method), and 2. it can be implemented in direct hardware capable of being run in real time, ie where the robot's position within the navigable region is continuously assessed and thus the robot's present position is continuously taken as the robot's start position.

Frequently, shorter paths can be found by reapplying the voltage at each node reached on the path, and deciding which direction to move just one node at a time. This variation also takes into account errors in the position of the robot if it drifts off the prescribed path.

3 In one implementation of the resistive grid method (see Parallel analogue computation for real-time path planning; Tarassenko, Marshall, Gomez- Castaneda and Murray, Proc. Oxford International workshop on VLSI for Artificial Intelligence-and Neural Networks, 1990). The internodal connections are made of single transistors, with a RAM cell (a Random Access Memory element) attached to a gate to program it to be either open (no connection ie obstacle present) or closed (low resistance connection ie part of the background).

In the above implementation each node is capable of connection by a pass transistor to a voltage supply rail. Application of a high voltage from the voltage supply rail to a particular node is interpreted by the logic chip and a robot movement control chip (also associated with the robot) as that node represents the robot's present position within the navigable region. Likewise. each node can be connected via pass transistors to ground, grounding the node to indicate goal. Thus, navigation of the robot then becomes a matter of:

1. programming the resistive grid with the periphery of the navigable region and the positions of the obstacles, 2. applying the voltage of the voltage supply rail to the node corresponding to the robot's present position, and 3. reading out the path of maximum current.

Read out of maximum current direction must be carried out without altering the equilibrium of the grid. Convenient use of the above implementation demands that this type of read out be performed on a chip, where this chip is preferably the logic chip. Normally this selection data is fed to a controlling chip which controls commands for 4 movements of the robot.

There are two methods which may be employed to perform the read out on the logic chip. The currents in the resistors surrounding the node representing the robot's present position can be read out and a selection made of the maximum current. The circuitry required to do this without altering the currents significantly is large and complicated, thus reducing the number of nodes which can be integrated on the logic chip. Alternatively. voltages of nodes surrounding the node representing the robot's present position are read out and the path of minimum voltage is selected. This second method is much simpler because a simple sourcefollower buffer can be used to isolate the read-out circuitry from the grid.

It is the aim of the invention to provide alternative apparatus for robot path planning.

According to this invention apparatus for robot path planning comprises means for implementation of a resistive grid Mving nodes and internodal connections, means for implementation of control of robot movements, means for implementation of selection of direction of robot movement, characterised in that the means for implementation of the selection of direction of robot movement comprises means for detection of open circuit internodal connections and means for avoidance of robot movement on a path associated with an open circuit internodal connection.

The invention provides apparatus suitable for overcoming a problem (ie open circuit internodal connections) which hitherto has not been recognized, with the associated implications not appreciated.

An internodal connection may be open circuit for a variety of reasons, eg it represents part of an obstacle or errors in manufacturing result in a faulty transistor which remains open circuit regardless of programming.

Typical apparatus which can be used to implement the invention includes that which designates an internodal connection open circuit between two nodes by assignment of appropriate node voltages as high relative to other node voltages, such that where selection of robot movement direction is made according to lowest node voltage criteria then a comparison of node voltages determines that an open circuit internodal connection does not represent a valid selection of direction of robot movement. Alternatively, where selection of robot movement is made according to highest node voltage criteria, then node voltage associated with an open circuit internodal connection can be assigned a low voltage relative to other node voltages. Both of the above examples of apparatus suitable for implementation of the invention use selection c riteria. which are based on comparison of voltages in order to select a voltage nearest to that associated with a goal node voltage. Other implementations include a remote assessment of node voltages of nodes surrounding a robot's position with subsequent selection of movement direction based upon calculated current magnitude. Such calculations can include interpolation schemes eg selection of direction of movement according to determination of a weighted sum of all calculated current 6 vectors.

The invention also provides apparatus suitable for overcoming the problem of oscillation of robot movement between two nodes. The oscillation may be overcome by detecting the start of an oscillation cycle between two nodes, and where this occurs to cause the internodal connection to become open circuit and thus break the oscillation loop.

In order that the invention may be more fully understood, it will now be further described by way of example only and with reference to the following figures in which Figure 1 is a schematic representation of a navigable environment.

Figure 2 is a schematic representation of a robot.

Figure 3 is an illustration of a part of a resistive grid where a node having an associated lowest voltage does not correspond to a path of maximum current.

Figure 4 is a flow diagram of apparatus used for robot path planning.

Figure 5 is a typical circuit for assigning a high voltage to a busline Figure 6 is a typical circuit for assigning a voltage to a busline.

Figure 7 is a typical circuit for assigning edges of a resistive grid 7 as open circuit connections.

Figure 8 is a schematic diagram of typical comparator circuitry used to select direction of robot movement.

Figure 1 is a schematic representation of a navigable environment 1, eg a room or a factory floor. which is notionally divided up into a rectangular topology network of nodes called an internodal network. Typically a room of approximately 9 m 2 would be divided up into an internodal network containing 100 nodes, although finer and coarser internodal networks could also be used. Each node of the internodal network represents an individual and discrete position within the navigable environment. Node 2 represents positioning of a (typically cylindrical) robot 3, whilst node 4 represents a target position for the robot to navigate to. called a goal position. Obstacles 5, 6 and 7 are also present in the navigable environment. Typically these obstacles could be items such as tables, doors etc where the navigable environment is a room, or equipment etc where the navigable environment is a factory floor. A typical robot navigation problem would be that of making the robot navigate a path from an initial start position, eg node 2, to a goal position, eg at node 4, in such a way that the robot avoids any obstacles contained within the navigable environment, eg obstacles 5, 6 and 7. Navigation by implementation of a method called the resistive grid method uses the nodes of the internodal network as a representation of discrete positions within the navigable environment, and assigns resistances to each connection between each respective node and the respective node's neighbouring nodes. The resistances are assigned to each internodal connection according to standard resistive 8 grid techniques such that obstacles are represented as regions of high or infinitely high resistance against a background of constant low resistance. Typical standard resistive grid techniques have internodal connections which lie completely outside an obstacle and are assigned a low resistance, and internodal connections representing positions which lie partially or wholly within an obstacle are assigned a high resistance. Representation of each obstacle is increased in size to take into account the robot's finite size (eg where the robot is cylindrical, then representation of the robot is increased by half the diameter of the robot), and internodal connections falling partially or wholly within this representation of an enlarged region are assigned a resistance which can be either high or medium when compared to a low resistance background.

Figure 2 schematically shows the robot 3 suitable for navigation in a navigable environment. The robot has sensors 10 which are used to detect positioning of the robot within a navigable environment. The sensors are typically electromagnetic detectors, eg infra-red detectors, or vibrational, eg multiwave or sonar. Information provided by the sensors is used as input to an interpretation chip 11, which interprets information received from the sensors in such a way as to provide information to logic chip 12 and control module 13 in the form of eg coordinates of the robot's current start position in relation to the internodal network and its associated resistive grid. The logic chip contains inter alia logic for navigation of the robot from its initial start position to the goal position. The control module performs functions which include movement of the robot 3 by way of eg motor unit 14, steering mechanism 15 and wheels 16. Other functions carried out by 9 the control module can include transmission of signals to the logic chip as to the robot's present position, obstacle detection and communication of positioning of new or changed obstacles to the logic chip.

Figure 3 shows a simple resistive grid 20 having an open circuit internodal connection, possibly due to a faulty transistor or representation of part of an obstacle. The resistive grid is based on an internodal network of hexagonal topology, although this resistive grid can also be viewed as part of a larger typical resistive grid. In the resistive grid 20 node 21 is designated as the robot's start position by a control module (not shown), and so according to conventional resistive grid methods this node is connected to a high voltage, in this case a voltage taken from a voltage supply rail (also not shown). The voltage supply rail can be at any selected voltage, but conventionally is kept as near to 5 V as possible. Node 27 is designated to represent a goal position to be reached by the robot, and thus is grounded according to conventional resistive grid methods ie set at a voltage of 0 Volts. Node 21's neighbours are nodes 22, 23 and 24, of which node 23 is assigned a lower voltage by the logic chip than each of the other neighbours since it is closest to the node representing the goal position, node 27. However, selection of a movement path between nodes 21 and 27 by use of conventional resistive grid logic will cause an invalid robot movement as internodal connection 28 between node 21 and 23 is open circuit. Where a robot moves along a path represented by an open circuit internodal connection, then this stops any further movement of the robot. The problem presented by open circuit internodal connections is rare in the use of resistive grids for robot navigation, but nevertheless does have a catastrophic result on movement of the robot when it does occur since movement of the robot in the direction represented by an open circuit internodal connection could result in collision with an obstacle and subsequent damage to the robot and/or obstacle.

Typical steps which may be performed by apparatus for robot path 25 navigation within a navigable environment will now be described. The navigable environment is notionally divided up into an internodal network. Nodes of the internodal network are discrete positions within the navigable environment, and are represented on a logic chip as nodes of a resistive grid. The navigable environment is surveyed by the robot's sensors, such that the initial start position, and obstacle position(s) are detected. The sensors send a signal to an interpretation chip, which in turn sends signals to the logic chip containing information of these positions. The logic chip combines this information with that of the goal position (as supplied by the command module) to provide appropriate resistances to connections between each node of a resistive grid. Each resistive grid internodal connection is given a resistance according to its position in relation to the obstacle position(s). The start position, which is the initial start position at the beginning of navigation of the robot. is stored in a shift register. The logic chip then reads voltages of each node surrounding the node representing the initial start position. It is able to do this because it has a bus incorporated within its structure, where a bus is a means of transferring a number of items of information on parallel from one part of logic to another. A bus consists of a number of buslines, typically wires or connections, each capable of transferring one item of information. Within logic chip 12 the buslines are distributed 11 throughout the grid such that for each particular node there are sufficient buslines available to transfer information about each available direction of movement from that particular node. Thus, for a hexagonal resistive grid topology the bus would have 6 lines capable of transferring 6 pieces of information. Although each node has access to providing its voltage as information to an appropriate bus line, only voltages of those nodes which are immediate neighbouring nodes of the node representing the initial start position are allowed to be put on the bus. Having defined the appropriate connections for supply of node voltage information to the bus, each appropriate internodal connection is assessed as to whether it is an open circuit internodal connection. Where any internodal connection between the node representing the initial start position and an immediate neighbouring node is open circuit, then a high voltage is applied to the appropriate busline for the immediate neighbouring node. Typically this high voltage is the same as the voltage applied to the node representing the start position eg 5 Volts from a voltage supply rail. Alternatively, the high voltage can be arranged to be different and slightly less than the supply rail voltage. Where an internodal connection is not open circuit, then the voltage associated with that particular immediate neighbouring node is transferred by the appropriate busline to the logic chip 12. When information from the bus is downloaded, a selection of the minimum voltage is made. The logic chip provides this information to the command module 13, in the form of a selected node which has the minimum voltage associated with it. The command module then instructs the motor unit 14, steering mechanism 15 and wheels 16 to move the robot to the position which is represented by a node with a minimum voltage between it and the node representing the initial start position. The robot's 12 new position now becomes its current start position, with all operations applicable to the initial start now being applied in the same flow loop to the current start position.

The current start position changes during movement of the robot as each movement creates a new robot current start position, whilst goal and obstacle positions may also change during navigation of the navigable environment. The internodal connection resistances are subject to possible change after each movement of the robot navigating within the navigable environment, in order to have a renewed and correct assessment of the obstacle position(s). In order to accommodate any possible changes in obstacle position(s), then operational flow of the apparatus incorporates a logic step of assessing this possibility and program these changes into the resistive grid in order that internodal connection resistances can be changed to reflect the new navigable environment. Operational flow of the apparatus as described above is summarised in a flow diagram given in Figure 4.

Where the robot has moved at least twice from it's initial start position, then an extra logic step can be included in the operational flow of the apparatus. This additional logic step can also be seen in the flow chart as seen in Figure 4. This logic step assesses whether the robot's current start position, as stored by the shift register, is the same as it's start position two movements prior to the assessment. If the robot is at the same position as two movements ago, then this is a consequence of oscillation. In order to break the oscillation sequence a high resistance is assigned to the internodal connection which is selected as being in the direction corresponding to the maximum 13 voltage drop from the start position from the current start position (ie the same internodal connection to a node assessed as the robot's current start position one movement prior to the assessment), thus preventing the logic chip selecting this particular internodal connection again as a minimum voltage internodal connection.

A typical algorithm which can be implemented by the logic chip 12 for assignment of a high voltage to an open circuit internodal connection is now given. This logic is performed for each internodal connection.

for each connection if {ground bus cycle} then ground bus to discharge it; else if {node X connected to source} then if {connection to node Y closed} then put voltage Y on busline; else pull busline high; else if {node Y connected to source} then if {connection to node X closed} then put voltage X on busline; else pull busline high; else 14 leave busline floating.

Thus, for each appropriate internodal connection (here generalised as being between nodes X and Y), the appropriate busline for that particular internodal connection is grounded ie checked to make sure that it is empty and thus prepared to receive the voltage of the node. After this operation the algorithm checks as to whether node X has been assigned a high voltage (ie checks whether the robot's current start position is that represented by node X) and whether the internodal connection between nodes X and Y is closed ie not open circuit. If node X is the node representing the robot's current start position and the internodal connection between nodes X and Y (internodal connection XY) is closed, then the voltage of node Y is put onto the appropriate busline for that internodal connection. If the internodal connection between X and Y is open circuit, then the appropriate busline for that internodal connection is pulled high (ie assigned a high voltage). However, if node X is not connected to source ie is not the node representing the robot's current start position, then the algorithm goes on to investigate whether node Y has been assigned a high voltage (ie whether the robot's current start position is that represented by node Y) and whether the internodal connection between nodes Y and X (internodal connection XY) is closed. If both the above conditions as applied to node Y are met, then the voltage of node X is put onto the appropriate busline for that internodal connection. Where the internodal connection XY is open circuit, then the appropriate busline for that internodal connection is pulled high (ie it is assigned a high voltage). If node Y does not represent the robot's current start position, then the busline appropriate for internodal connection is left floating, ie does not accept voltage information from the nodes connected to internodal connection XY as neither X nor Y represent the robot's current start position.

Figure 5 shows logic circuitry 29, as implemented by C-MOS logic circuitry, for pulling a busline high if internodal connection XY is open circuit. The circuit 29 is constructed between a voltage supplied by a voltage supply rail 30 at substantially 5 Volts and 0 Volts. The bus is grounded by transistors 35, 44 and 45. If node X represents the robot's curr(ent start position, then a circuit path exists through transistors 31, 32, 38 and 40. If node Y represents the robot's current start position, then a circuit path exists through transistors 33, 39, and 41. If either node X or node Y represents the robot's current start position, then a circuit path through transistor 36 and gate 37 occurs when internodal connection XY is open circuit. If neither X nor Y represent the robot's current start position, then a circuit path exists through transistors 31, 32, 33 and 34, and gates 38, 39, 40 and 41, such that no voltage is put onto the appropriate busline. Transistor 42 and gate 43 are used to invert signals such that signals which reach transistor 45 are inverted. If signals reaching transistor 45 are due to the robot's current start position being represented by X (or Y) and internodal connection XY is closed, then internodal connection XY voltage is allowed through to the busline through the circuit described below with reference to figure 6. However, if the robot's current start position is represented by node X (or Y) but internodal connection XY is open circuit, then switch 46 is kept closed and the busline is assigned a voltage of about 5 Volts from the voltage supply rail.

1 16 Figure 6 gives logic circuitry 50 used to put internodal connection XY voltage onto the appropriate busline. This circuit is operated when logic circuitry 29 has designated that node X (or Y) represents the robot's current start position and that internodal connection XY is closed. Where it is decided to share some of logic circuitry 50 with logic circuitry 29. then transistors 51 and 52 can be shared such as to be transistors 31 and 33 (or 32 and 34 where node Y represents the robot's current start position) of logic circuitry 29. The busline is grounded by transistor 53 and gate 55. Where node X (or Y) has been designated as the node representing initial start position or current start position. a circuit path exists through transistors 51, 52, 56 and 57. gates 54 and 58 are open if internodal connection XY is closed. Thus, a high voltage exists at circuit connection 60 and gate 61 allows a buffered node voltage onto the appropriate busline.

Where a robot's current start position is at the edge of the navigable 1 environment 1, then edge effects are dealt with by causing a voltage taken from the voltage supply rail 30 to be placed on the appropriate busline if a non-existent connection is addressed. This is implemented by the use of logic circuitry placed at each of the nodes representing a position at the edge of the navigable environment where a dummy connection is incorporated in the logic circuitry.

Figure 7 shows logic circuitry for implementation of a dummy connection. It can be seen that this logic circuitry is substantially the same as logic circuitry 29, with the exception that transistors 33 and 34, and gates 39 and 41 are missing from the components used to define nodes 17 representing possible current start positioning of the robot. Gates 36 and 37 are also not present, leading the logic circuitry allow switch 46 to stop signals reaching transistor 45 and thus for the dummy internodal connection to place a high voltage on the busline from the voltage supply rail 30.

Once each of the six buslines has been assigned a voltage, it is necessary to pick a minimum voltage from the six downloaded by the bus. One possible method of doing this is to pass each of the voltages through an analogue-to-digital converter and to digitally select the minimum voltage. Alternatively an analogue circuit 70 such as that seen in figure 8 can'be used. Comparators 71. 72, 73, 74, 75, and 76 are configured from operational amplifiers. The six buslines of the bus are downloaded such that each individual busline is downloaded into a particular comparator as an input. Each comparator also has an input from a reference voltage 78. The reference voltage is gradually increased from 0 Volts to the voltage supply rail voltage. The first comparator to flip state ie detected by encoder logic 79 to be outputting a substantially lower voltage than the other comparators, corresponds to the comparator busline downloaded from the lowest voltage. Once the flip state has been achieved, then a feedback 80 from the encoder logic stops the reference voltage from increasing further. The encoder logic then outputs a digital number representing the busline which provided the lowest voltage as an input the logic chip, which in turn is able to send this information to the command module in the form of the direction in which the robot should move. The command module then instructs the motor unit 14, steering mechanism 15 and wheels 16 to move the robot in the direction determined by the logic chip and for a 18 distance predetermined as representing one internodal distance of the internodal network. The robot thus moves "node by node" until it reaches the goal position.

The embodiments given above are described with most, or all, of the direction selection logic located on the logic chip 12. However, it is possible to allow most (or all) of the selection logic to be carried outoff-chip on an external processor, allowing the logic chip circuitry to be simplified. Typically, any of the logic for determination of eg node voltages on buslines, grounding of buslines or assignment of high voltage to a particular node can be carried out by an external processor, which then provides direction selection information to the appropriate portion of robot navigation apparatus.

Darticular Implementation of logic (both on- and off-chip) can be dictated by a number of criteria - eg power consumption of the various chips, speed of response required by navigation apparatus, ease of communication between off-chip and on-chip logic chips, number of buslines available, topology of internodal networks etc. Typical implementation of logic that is able to reduce power consumption of direction selection logic can include use of one load transistor per busline for use as a node readout buffer, as opposed to one load transistor per node. Typically, buslines can be limited in number by use of storage of appropriate node readout voltages and then comparison of voltage magnitudes, as opposed to comparison of simultaneously provided node readout voltages.

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Claims (1)

1. Claims

   1. Apparatus for robot path planning comprising: means for implementation of a resistive grid, having nodes and internodal connections, means for implementation of control of robot movements, means for implementation of selection of direction of robot movement, characterised in that the means for implementation of the selection of direction of robot movement comprises means for detection of open circuit internodal connections and means for avoidance of robot movement on a path associated with an open circuit internodal connection.

   2. Apparatus for robot path planning according to claim 1 where the means for implementation of selection of direction of robot movement comprises means for comparison of node assigned voltages.

   3. Apparatus for robot path planning according to claim 2 where the means for comparison of node assigned voltages comprises means for selection of lowest node assigned voltage.

   4. Apparatus for robot path planning according to claim 3 where means for avoidance of robot movement on a path associated with an open circuit internodal connection comprises means for assignment of a high voltage to a node having an open circuit internodal connection.

   Apparatus for robot path planning according to claim 2 where the means for comparison of node assigned voltages c9mprises means for selection of highest node assigned voltage.

   Apparatus for robot path planning according to claim 5 where means for avoidance of robot movement on a path associated with an open circuit internodal connection comprises means for assignment of a low voltage to a node having an open circuit internodal connection.

   7. Apparatus for robot path planning according to claim 1 where the means for implementation of selection of direction of robot movement comprises means for remte assessment of node assigned voltages of nodes representing individual and discrete positions surrounding robot position, and means for selection of direction based on calculated current magnitude.

   Apparatus for robot path planning according to claim 1 where the resistive grid has a hexagonal topology.

   Apparatus for robot path planning according to claim 1 where the resistive grid has a rectilinear topology.