Source: http://www.google.com/patents/US4811229?dq=5,758,352
Timestamp: 2013-12-11 19:21:03
Document Index: 178940342

Matched Legal Cases: ['art 1', 'art 1', 'arts 2', 'art 1', 'art 2', 'art 6', 'arts 3', 'art 1', 'art 1', 'art 2', 'art 1', 'art 6', 'art 1', 'art 2', 'art 1', 'art 1', 'art 1', 'arts 3', 'art 1']

Patent US4811229 - Control system for automatic guided vehicles - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Advanced Patent Search | Sign inAdvanced Patent SearchPatentsA control system for guiding a vehicle over a floor. The system includes a narrow length of retroflective tape applied to the floor to define a path, an array of LED-phototransistor sensors to generate light toward the tape and generate sensor output signals proportional to the portion of the sensor...http://www.google.com/patents/US4811229?utm_source=gb-gplus-sharePatent US4811229 - Control system for automatic guided vehiclesPublication numberUS4811229 APublication typeGrantApplication numberUS 07/000,500Publication dateMar 7, 1989Filing dateJan 5, 1987Priority dateJan 5, 1987Fee statusLapsedPublication number000500, 07000500, US 4811229 A, US 4811229A, US-A-4811229, US4811229 A, US4811229AInventorsRichard A. WilsonOriginal AssigneeHewlett-Packard CompanyExport CitationBiBTeX, EndNote, RefManPatent Citations (9), Referenced by (28), Classifications (7), Legal Events (6) External Links: USPTO, USPTO Assignment, EspacenetControl system for automatic guided vehiclesUS 4811229 AAbstract A control system for guiding a vehicle over a floor. The system includes a narrow length of retroflective tape applied to the floor to define a path, an array of LED-phototransistor sensors to generate light toward the tape and generate sensor output signals proportional to the portion of the sensor fields of view occupied by the retroreflective tape. The output signal of each of the sensors is compared to a stair-step reference signal during a 10 millisecond cyclic time period and a binary output signal for each sensor is generated indicating whether the amplitude of the sensor output signal is above or below the changing threshold reference signal. The binary value of each of the sensor output signals is sampled once each millisecond during the time period and the binary values are combined to generate a byte for each sampling occasion. Each of the bytes corresponds to a location in a lookup table which includes interim vehicle lateral directional and displacement commands. The commands corresponding to each of the table location values generated during the 10 millisecond period are mathematically summed to generate a final vehicle command. Two lookup tables are provided for selectively following the left or right edge of the tape.
I claim: 1. A control system for guiding a vehicle over a floor along a long, narrow stripe of reflective material applied to the floor to define a desired path for the vehicle to travel, comprising:means for generating light and directing said light toward the reflective material; a generally linear array of light sensors mountable on the vehicle generally transverse to the stripe of reflective material, each sensor having a field of view and generating an output signal generally proportional to the portion of the sensor's field of view occupied by the reflective material; reference signal means for generating a reference signal with an amplitude generally unidirectionally and progressively changing over a preselected time period; means for comparing each of said sensor output signals to said reference signal during said preselected time period, and generating a binary output signal having binary states for each of said sensors, said binary output signal in one binary state indicating if the amplitude of said sensor output signal is above said reference signal and in the other binary state indicating if the amplitude of said sensor output signal is below said reference signal; means for sampling the value of each of said sensor binary output signals on a plurality of sampling occasions during said preselected time period and combining said sampled values of said sensor binary output signals for each sampling occasion in a predetermined order to generate a byte; value determining means for providing for each said byte a corresponding position indicating value indicative of the lateral position of said sensor array relative to the reflective material at the time of said sampling occasion based upon the changing threshold provided by said reference signal; and summing means for mathematically summing all said position indicating values for said one preselected time period and generating a vehicle command comprising a vehicle lateral directional command to control the direction of lateral movement of the vehicle as it travels along the path and a vehicle lateral displacement command to control the amount of lateral movement of the vehicle as it travels along the path. 2. The control system of claim 1 wherein each of said bytes generated during said preselected time period comprises a corresponding table location value, and said value determining means includes:a look-up table storing a plurality of said position indicating values, each position indicating value being stored at a table location corresponding to one of said table location values, each said position indicating value comprising a vehicle lateral directional command component indicative of the amount of lateral movement of the vehicle required to travel along the path, and a vehicle lateral displacement command component indicative of the amount of lateral movement of the vehicle required to travel along the path, said vehicle directional and displacement command components being based upon the lateral position of said sensor array relative to said reflective material at the time of said sampling occasion corresponding to each of said bytes as measured using the changing threshold provided by said reference signal; and look-up means for looking up ones of said position indicating values stored in said look-up table at said table locations corresponding to said table location values. 3. The control system of claim 2 wherein said look-up table includes a first table storing a first plurality of said position indicating values, each comprising one of said vehicle directional and displacement command components directing the vehicle to follow the left edge of the reflective material, and a second table storing a second plurality of said position indicating values, each comprising one of each of said vehicle directional and displacement command components directing the vehicle to follow the right edge of the reflective material.
TECHNICAL FIELD The present invention relates to a control system for guiding a wheeled vehicle over a floor along a predetermined path.
BACKGROUND OF THE INVENTION In certain situations, it is desirable to have a robotic vehicle which can autonomously travel a selected path, for example, to distribute and collect mail within an office building, or to distribute to or transfer parts between work stations in a factory. Such automatic guided vehicles (AGV's) generally use a guide wire buried in the floor which carries an AC signal to be sensed by the AGV. This wire guided approach is undesirable in situations where the AC signal could interfere with adjacent equipment, such as in factories manufacturing or testing delicate electronics equipment, and in situations where it is not easy to retrofit an existing floor with a buried wire.
DISCLOSURE OF THE INVENTION The present invention resides in a control system for guiding a vehicle over a floor. The control system includes a narrow length of reflective material applied to the floor to define a desired path for the vehicle to travel along, means for generating light and directing the light toward the reflective material, and a generally linear array of light sensors mountable on the vehicle generally transverse to the length of reflective material. Each sensor has a field of view and generates an output signal generally proportional to the portion of the sensors' field of view occupied by the reflective material.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of a vehicle embodying the control system of the present invention shown in position over a path made of retroreflective tape applied to a floor.
BEST MODE FOR CARRYING OUT THE INVENTION As shown in the drawings for purposes of illustration, the invention is embodied in a control system indicated generally by the reference numeral 10, for guiding a wheeled vehicle 12, which includes a steering mechanism and a self contained propulsion system (not shown), over a substantially planar floor 14, such as a concrete or tile floor in an office or factory (see FIG. 1). The control system 10 utilizes a narrow length of reflective material, such as a length of retroreflective one-sided adhesive tape 16, applied to the floor 14 to define a desired path for the vehicle 12 to travel along. The direction of forward motion for the vehicle 12 is shown by the arrow 13.
A functional block diagram illustrating the overall operation of the control system 10 of the present invention is shown in FIG. 7. A reflective sensor means "A", which includes the linear array of reflective sensors 20a-e, simultaneously provides five analog output signals proportional to the percentage of the field of view of the sensors that is occupied by the reflective tape 16. The signals are provided to a comparator means "B". The comparator means compares each of the output signals to a stair-step reference voltage signal V.sub.ss generated by a stair-step reference signal means "C". The reference signal has a 10 millisecond period, and a comparison is made once each millisecond of each of the five sensor output signals with the reference signal. If at a sampling occasion a sensor's output signal is above the reference signal, the output of the comparator means for that output signal will be a binary "0". Should the progressively increasing stair-step reference signal increase to a level above the sensor's output signal at the next sampling occasion, the output of the comparator means for that output signal will change to a binary "1".
The counter 30 and the operational amplifier 38 in combination generate the stair-step voltage reference signal V.sub.ss having a 10 millisecond period on the output of the operational amplifier 38, as shown in waveform Chart 1 of FIG. 3. The stair-step signal V.sub.ss on the output of the operational amplifier 38 produced at test point TP1 of FIG. 4 has 10 steps at progressively decreasing voltage levels with each step having a duration of 1 millisecond. It is noted that while the stair-step signal V.sub.ss is shown and described herein as being progressively increasing for ease of understanding, the embodiment of the circuit disclosed in FIG. 4 actually utilizes existing utilize a progressively decreasing stair-step signal. As described above, the circuitry of FIG. 4 has the four parallel outputs of the counter 30 each connected in parallel through one of the four binary weighted resistors 36 to the negative input of the operational amplifier 38, thus producing a progressively decreasing reference signal which is the inverse of the progressively increasing stair-step signal V.sub.ss shown and described as such for ease of understanding. Either may be used and the invention is intended to cover the use of both an increasing and decreasing reference signal.
The stair-step signal V.sub.ss is used as a reference voltage for each of five comparator circuits, each comparator circuit being indicated in FIG. 4 by reference numerals 46a, 46b, 46c, 46d, and 46e which correspond, respectively, to the reflective sensors 20a-e. One comparator circuit is used with each reflective sensor. For brevity, only the comparator circuit 46a and the reflective sensor 20a are shown in detail in FIG. 4 and will be described herein, but all five comparator circuits and reflective sensors are identical in design and function.
The stair-step signal V.sub.ss provided on the output of operational amplifier 38 is connected through an input resistor 48 to the positive input of an operational amplifier 50. A feedback resistor 51 is provided between the positive input and the output of the operational amplifier 50 to supply hysteresis. The emitter of the phototransistor 26 of the reflective sensor 20a is connected to ground through a variable resistor 52, and the wiper arm of the variable resistor 52 is connected to the negative input of the operational amplifier 50. The cathode of the LED 24 of the reflective sensor 20a is connected to ground through a resistor 54. The anode of the LED 24 and the collector of the phototransistor 26 are connected to +5 VDC.
The operational amplifier 50 serves as a comparator and compares the voltage level of the stair-step signal V.sub.ss to the analog voltage level of the voltage on the wiper arm of resistor 52, which is directly dependent upon the output current signal produced by the phototransistor 26 of the reflective sensor 20a. As previously mentioned, the current produced by the phototransistor 26 is approximately proportional to the percentage of the field of view of the sensor that is occupied by the retroreflective tape 16. By use of a variable resistor for the resistor 52, a means is provided for calibrating the analog voltage level provided on the negative input of the operational amplifier 50 by the reflective sensor 20a.
The output signal of the operational amplifier 50 is a binary signal with a positive pulse duration dependent upon the level of the analog voltage on the wiper arm of the resistor 52 and applied to the negative input of the operational amplifier. This analog voltage level is itself dependent upon the amount of light detected by the phototransistor 26 of the reflective sensor 20a, and hence upon the percentage of the field of view of the sensor occupied by the retroreflective tape 16. By way of example, a number of possible analog voltage levels V.sub.A, V.sub.B, V.sub.C, V.sub.D and, V.sub.E produced by the reflective sensor 20a on the negative input of the operational amplifier 50 are shown in Chart 1 of FIG. 3. Analog voltage level V.sub.A corresponds to the retroreflective tape 16 being totally out of the field of view of the reflective sensor 16, and levels V.sub.B -V.sub.E correspond to the tape being increasingly within the field of view of the sensor, with level V.sub.E being produced when the tape is fully within the sensor's field of view. Waveform Charts 2-6 illustrate the respective output signals V.sub.AA -V.sub.EE of the operational amplifier 50 which are produced by the analog voltage levels V.sub.A -V.sub.E of Chart 1.
When adjusted properly and as shown in Chart 2 of FIG. 3, the output signal V.sub.AA of the operational amplifier 50 will be a continuous high (binary "1") for the entire 10 millisecond period of the stair-step signal V.sub.SS when the retroreflective tape 16 is not within the field of view of the reflective sensor 20a, with only a nonretroreflective material such as a floor within the field of view. This is because the analog voltage level V.sub.A on the negative input of the operational amplifier 50 of the comparator circuit 46a is at all times less than the stair-step signal V.sub.SS.
At the opposite extreme, as shown in Chart 6, the output signal V.sub.EE of the operational amplifier 50 will be a continuous low (binary "0") for the entire 10 millisecond period of the stair-step signal V.sub.ss when the retroreflective tape 16 is directly below the reflective sensor 20a and fully within the field of view of the sensor. This is because the analog voltage level V.sub.E on the negative input of the operational amplifier 50 of the comparator circuit 46a is at all times greater than the stair-step signal V.sub.ss.
When the retroreflective tape 16 is partially within the field of view of the reflective sensor 20a, such is the case when one edge of the tape is beneath the sensor, the output signal of the operational amplifier during the 10 millisecond period will be initially low (binary "0"), and then go high (binary "1") for the remainder of the 10 milliseconds, with the change in state occurring at the time the stair-step signal V.sub.ss crosses over the analog voltage level on the negative input of the operational amplifier 50. This is illustrated in Charts 3-5 of FIG. 3 for the analog voltage levels V.sub.B -V.sub.D shown in Chart 1. As can be readily seen, the duration of the low and high pulse portions of the output signal of the operational amplifier 50 is dependent upon the percentage of the field of view of the reflective sensor 20a occupied by the reflective tape 16, with the more the retroreflective tape 16 is within the field of view of the sensor, the greater the duration of the low pulse portion (binary "0") on the output of the operational amplifier 50. With the present invention, the reflective sensors 20a-e are used not only to indicate the presence or absence of the reflective tape 16 in their respective fields of view 28a-e, but also, in conjunction with the changing reference provided by the stair-step signal V.sub.ss and the comparator circuits 46a-e, to detect how much of the tape is within the field of view of each sensor.
The microprocessor 62 operates on a 10 millisecond, 10 byte cycle synchronized with the 10 millisecond period of the stair-step signal V.sub.ss. Once each millisecond during the period the five output signals of the comparator circuits 46a-e are sampled in order to generate 10 separate bytes. The mathematical value of the byte generated on each sampling occasion is dependent upon the changing voltage threshold level of the stair-step signal V.sub.ss, as will be demonstrated using FIGS. 5 and 6, and represents a unique value which corresponds to the lateral position of the sensor array 18 relative to the reflective tape 16 for the state of the changing reference provided by the stair-step signal V.sub.ss. This unique value corresponds to a unique lateral control command for the vehicle 12, which as will be described below is stored in one of two lookup tables. Each of the 10 bytes generated during one 10 millisecond period is referred to as a table location value. For ease of understanding, these table location values will be represented as decimal system base number-10 values, although the microprocessor 62 operates in binary and no such conversion actually occurs.
As shown in the corresponding data table of FIG. 6a, which presents the 10 bytes produced during the 10 millisecond example period in table form, the outputs of every comparator circuit 46a-e corresponding to the reflective sensors 20a-e is binary "1" for the first sampling occasion, as well as for each of the 9 additional sampling occasions during the 10 millisecond period. The byte generated for each sampling occasion is thus "11111". The mathematical value of the byte for the first sampling occasion, converted into an equivalent base number-10 table location value for ease of understanding, is the number "31". The table location value for each of the other 9 additional sampling occasions is also "31". Referring back to FIG. 3, since the retroreflective tape 16 in this example is not within the field of view of the reflective sensors 20a-e, the analog voltage level applied to the negative input of each of the operational amplifiers 50 for the comparator circuits 46a-e is the voltage level V.sub.A shown in Chart 1 of FIG. 3, and the comparator circuit output signal for each of the comparator circuits is a continuous high (binary "1") for the entire 10 millisecond period, as shown by the output signal V.sub.AA in Chart 2 of FIG. 3.
After the 10 bytes are generated by the microprocessor 62 for a particular 10 millisecond period, the corresponding 10 table location values are used to read the 10 corresponding vehicle indicating values from the desired left or right edge lookup Table 7a or 7b. It is noted that while each table location value corresponds to an indicating value which includes an interim command to control the direction of lateral movement and amount of lateral movement of the vehicle 12, a single interim command cannot be used by itself to control the movement of the vehicle 12 if it is desired to realize the increased resolution of the control system 10 of the present invention. Rather the vehicle indicating values corresponding to each of the 10 table location values for the 10 millisecond period must be mathematically summed together to generate a final vehicle command which can be used to accurately control the direction and amount of lateral movement of the vehicle. This is because the bytes which were converted to indicate which of the individual interim commands to read from the table are a result of comparing the analog voltage levels produced by the reflective sensors 20a-e to a changing voltage threshold supplied by the stair-step signal V.sub.ss. The summation of the 10 vehicle indicating values produces the interpolation effect which provides the increased resolution of the present invention.
For purposes of illustration, five additional examples are shown in FIGS. 5.b-5.f with the sensor array 18 at a variety of lateral positions relative to the longitudinal center line of the retroreflective tape 16. The bytes produced as a result of the 10 sampling occasions during each of the 10 milliseconds example period are shown in corresponding data tables in FIGS. 6.b-6.f, respectively. For brevity, only the example in FIG. 5.c will be described in detail. In the example of FIG. 5.c, the sensor array 18 is shown with the reflective sensor 20b positioned slightly to the left of the center of the retroreflective tape 16. In this position, the field of view 28b of the reflective sensor 20b is fully occupied by the retroreflective tape 16, and the output voltage signal for the sensor 20b is as shown by analog voltage level V.sub.E in Chart 1 of FIG. 3, and hence the output signal of the comparator circuit 46b is a continuous low for the entire 10 millisecond period, as shown by output signal V.sub.EE in Chart 6 of FIG. 3. As shown in the corresponding data table of FIG. 6.c, the second most significant bit of the byte generated based on the sensor 20b will be a binary "0" during each of the 10 sampling occasions during the 10 millisecond period.
This is to be compared with sensors 20d and 20e, which are completely off to the right of the retroreflective tape 16 and the fields of view 28d and 28e of the sensors 20d and 20e are not occupied at all by the retroreflective tape. As with the first described example of FIG. 5.a where all sensors were off the tape, the output voltage signal will be at the analog voltage level V.sub.A in Chart 1 of FIG. 3, and the output signal of the corresponding comparator circuits 46d and 46e will be a continuous high for the entire 10 millisecond period, as shown by output signal V.sub.AA in Chart 2 of FIG. 3. As shown in the data table of FIG. 6.c, the two right most bits of the byte generated based on the sensors 20d and 20e will be a binary "1" during each of the 10 sampling occasions during the 10 millisecond period.
The situation is significantly different for sensors 20a and 20c since the left and right edges of the retroreflective tape 16 are within their respective fields of view 28a and 28c, with the tape edge being positioned between the sensor and the next adjacent sensor. As such, the output voltage signals of the sensors 20a and 20c will be somewhere between the extremes of the analog voltage levels V.sub.A and V.sub.E in Chart 1 of FIG. 3. Since reflective sensor 20a has less than one-half of its field of view 28a occupied by the retroreflective tape 16, the output voltage signal will be at about the analog voltage level V.sub.B in Chart 1 of FIG. 3. Since reflective sensor 20c has more than one-half of its field of view 28c occupied by the tape 16, the output voltage signal will be at about the analog voltage level V.sub.D shown in Chart 1 of FIG. 3. As such, the output signal of the corresponding comparator circuits 46a and 46c will be as shown by the output signals V.sub.BB and V.sub.DD, respectively, in Charts 3 and 5 of FIG. 3. Each has an initial low signal portion (binary "0") and a high signal portion (binary "1") for the remaining time of the 10 millisecond period.
Since the reflective sensor 20a has the retroreflective tape 16 occupying less of its field of view than does sensor 20c, the sensor output voltage V.sub.B produced is lower and the stair-step signal V.sub.ss cross-over occurs sooner, causing the comparator circuit output signal V.sub.BB for comparator circuit 46a to go high sooner than the comparator output signal V.sub.DD for comparator circuit 46c. As can be best seen in the data table of FIG. 6.c corresponding to the example of FIG. 5.c, the bit of the byte generated based on the sensor 20a will be a binary "0" during the first 3 sampling occasions of the 10 millisecond example period and a binary "1" during the remaining 7 sampling occasions. In comparison, the bit of the byte generated based upon the sensor 20c will be a binary "0" during the first 7 sampling occasions of the 10 millisecond period and a binary " 1" only during the remaining 3 sampling occasions.
It is noted that had the sensor been directly over the edge of the retroreflective tape 16, such as is the situation for sensors 20b and 20d in the example of FIG. 5.e, the bit would be a binary "0" for the first 5 sampling occasions, and a binary "1" for the second 5 sampling occasions (see FIG. 6.e). This is because the output voltage signals of the sensors 20b and 20d would be at the analog voltage level of V.sub.c in Chart 1 of FIG. 3 where during the first one-half of the 10 millisecond period, the stair-step signal V.sub.ss is below the sensor voltage signal, and during the second one-half of the period the stair-step signal V.sub.ss is above the sensor voltage signal.
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