Apparatus for guiding a trackless vehicle

Apparatus for guiding a trackless vehicle along an A.C. conductor marking a roadway, having at least two sensors to detect a magnetic field emanating from the current conductor the output signals of which are routed to interpretive electronics. The interpretive electronics include a phase-locked loop (PPL) the input signal to which is the output signal of one of the sensors, and the output signal of which is used for demodulation, by mixers, of the sensor signals.

This invention relates to apparatus for guiding a trackless vehicle along 
an A.C. conductor marking the roadway, the apparatus having at least two 
sensors, to detect the magnetic field emanating from the current 
conductor, and interpretive electronics processing the sensor output 
signals to form steering signals. 
Devices of this description are known to exist in various versions. One 
time-tested system makes use of two sensors to scan a magnetic field 
generated by a guide cable, one sensor responding to the horizontal, and 
the other to the vertical, component of the magnetic field. In this 
arrangement, the quotient of the signals, or voltages, output of the two 
sensors, U.sub.v, U.sub.h, is, owing to the relationships 
##EQU1## 
proportional to the horizontal deviation "a" of the sensors from the line 
conductor. 
F.sub.v and F.sub.h are respectively, the vertical and horizontal field 
components, and "d" is the vertical distance between the sensor and the 
guide cable. 
A device utilizing this response is known from German Pat. No. 21 37 631, 
where the interpretive electronics contain two selective amplifiers, each 
being connected to a sensor, which by level control indirectly generate an 
amplified signal proportional to the quotient U.sub.h /U.sub.v. The 
amplified signals are finally routed to a phase-dependent rectifier to 
produce a steering signal. Interpretive electronics of this description, 
however, are capable of interpreting signals only if of a predetermined 
frequency. The sensors, however, accept a mix of various frequencies 
derived from diverse information, redundant guide cables, shunts, etc. If 
for detouring the vehicle, e.g., it is intended to change to another 
frequency, the known device would require additional components. 
In a broad aspect of the present invention, apparatus of this general 
description is provided to ensure change-frequency, safe control of the 
vehicle while being of simple construction which is easy to manufacture 
from a small number of components. 
It is a particular object of the present invention to provide an 
arrangement including a phase-locked loop (PLL), the input to the PLL 
being the output of one of the sensors, and the output from the PLL being 
used to demodulate the outputs from the sensors. 
Using a sensor signal driven PLL makes possible, in a simple way, 
generation of a demodulation signal phase-coupled to a frequency contained 
in the sensor signals, the demodulation signal serving to simultaneously 
interpret several sensor signals of the same frequency. This reduces the 
amount of circuitry, especially where several antenna signals are used for 
controlling the vehicle. 
The interpretive electronics of the present invention afford another 
advantage in that unlike the known device, they eliminate the need for 
selective filters and can be operated at various frequencies simply by 
changing the free-running frequency of the PLL. 
A further advantage provided by the present interpretive electronics of the 
present invention is the ability of phase-locked loops to produce a 
no-noise output signal even at low effective signal level and concurrently 
high interference level, thus adding to the working range of the 
interpretive electronics and to their reliability. 
For demodulation of the sensor signals, use can be made of simple mixers or 
demodulators in lieu of high-grade analog multipliers. The signals 
demodulated in this manner can be processed, using a known method, to 
produce control signals, such as forming the quotient. 
In a further aspect of the present invention, the PLL is fitted with a 
phase-sensitive detector followed by a control unit to transform the 
output signal of the phase-sensistive detector into a suitable control 
voltage for a voltage-controlled oscillator (VCO) of the PLL. The output 
signal of the oscillator is routed, as a PLL signal, to the 
phase-sensitive detector, with the PLL signal being phase-shifted through 
90.degree. relative to the sensor signal. As a result, the output of the 
phase-sensitive detector will be a voltage, the mean valve of which is 
proportional to the frequency or phase difference between the PLL 
frequency and the sensor signal, which makes it suitable for the 
adjustment or synchronization of the PLL frequency. 
The bandwidth of the PLL can be adjusted within wide ranges via the 
bandwith of the control unit characteristic, especially using extremely 
narrow-band filters, to reduce the requisite amount of frequency spacing 
if, e.g., use is made of guide cables which are fed several frequencies. 
A demodulation signal shifted through 90.degree. relative to the PLL signal 
can be generated in a simple manner if the voltage-controlled oscillator 
generates a rectangular output signal of 2.sup.n times the PLL frequency, 
with a frequency divider generating the phase-shifted demodulation signal. 
In a further aspect of the present invention, the VCO signal is routed to a 
frequency counter used to monitor the PLL frequency. For this purpose, use 
can also be made of the PLL signal instead of the VCO signal. 
Use of a control unit having an integral portion provides an advantage in 
that minor differences between the free-running frequency and the sensor 
signal frequency will be completely eliminated. 
For monitoring noninterpreted frequencies contained in the sensor signal, a 
bandpass filter of fixed or changeable midfrequency plus a rectifier with 
a threshold detector driven by the output signal of the phase-sensitive 
detector can be provided in a simple design according to the present 
invention. This advantageously enables the occurrence of redundant guide 
cable frequencies to be recognized without unncessarily adding to the 
complexity of design. 
The electronics design of the present invention is suitable especially for 
antenna systems where the sensors each scan at least one horizontal and 
one vertical component of the magnetic field. This enables processing by 
forming quotients of the sensor signals.

With reference now to FIG. 1, the numeral 10 indicates a roadway in which 
an A.C. guide cable 11 is arranged to produce a magnetic field 12, the 
field force F of which can be resolved into orthogonal field components 
F.sub.h and F.sub.v. The magnetic field 12 is scanned by crossed antennas 
13 consisting of a first sensor 14, in the form of a vertically arranged 
coil, and a second sensor 15, in the form of a horizontally arranged coil, 
each coil serving to scan its respective field component. 
The voltage signal U.sub.h of sensor 15, for the horizontal component 
F.sub.h, is routed to a phase-locked loop (PLL) for generating a 
demodulation signal 16, and is also used for generating control signals. 
The resultant demodulation signal 16 can be used to demodulate any number 
of sensor signals U.sub.h, U.sub.v, U.sub.v ' by means of mixers of 
demodulation circuitry to be used to process the demodulated signals 20 to 
22 to form corresponding control signals. 
When using a crossed-antennas system shown in FIG. 1, it will be an 
advantage to use the signal U.sub.h of the horizontal component for 
generating the demodulation signal 16 to maintain independence of antenna 
13 positioning, since the horizontal field component F.sub.h will 
invariably be a finite value above the roadway 10, whereas the vertical 
component F.sub.v will become zero on the V axis. In the example here 
described, sensors 14 and 15 are provided to scan orthogonal field 
components F.sub.h and F.sub.v ; the method here described for 
demodulating sensor signals will nevertheless be applicable also to other 
antenna systems. 
FIG. 2 is a block diagram illustrating a more detailed version of the PLL, 
and an embodiment of a processing circuit 30 for the demodulated sensor 
signals 20 and 21. The PLL loop within the dash-dotted line has at its 
input end a phase-sensitive detector PSD to multiply the sensor signal 
U.sub.h by a PLL signal 31, which in the PLL is fed back and phase-shifted 
by 90.degree. relative to U.sub.h, in order to generate a phase-difference 
signal 32. Contained in the low-frequency range of the phase-difference 
signal 32 are signal portions that are proportional to the frequency or 
phase difference between the PLL signal 31 and U.sub.h. 
A control unit arranged behind the PSD interprets the phase difference of 
the signal 32 in the low-frequency range and generates a control voltage 
34 for a voltage-controlled oscillator VCO. The control unit 33 reduces 
the phase differences of the control voltage 34 until the phase difference 
or control unit difference is reduced to zero. This process is generally 
termed synchronization. In this condition, the PLL signal 31 is shifted 
through 90.degree. relative to the input signal, U.sub.h, of the PSD. 
The control unit is characterized by its amplification and frequency 
response. The harmonic content of the phase detector output signal makes a 
low-pass filter necessary. This is achieved by using a control unit with 
integral behavior (I control unit); if desired, a PI control unit or a 
PT.sub.1 control unit may be employed. The setting parameters are dictated 
by the requirements of band width (response time). 
For demodulating the sensor signals U.sub.h, U.sub.v or U.sub.v ', phase 
coincidence is required between the demodulation signal 16 and the sensor 
signals, for which purpose the PLL signal must be shifted through 
90.degree.. To this end the VCO is designed to have rectangular vibrations 
of at least twice the PLL frequency. The PLL generates besides the PLL 
signal, a demodulation signal 16 derived from the output signal 38 of the 
VCO, the PLL signal, and the two frequency dividers 39 and 40. From the 
output signal 38 of the VCO, coupled frequency dividers 39 and 40 are then 
used to generate the respective PLL signal 31 and the demodulation signal 
16 that are phase-shifted 90.degree. relative to one another. The coupled 
frequency dividers used to generate signal 16 may be two J-K master-slave 
flip-flops 39 and 40 connected in series. The first of the flip-flops 
divides the output signal 38 from the VCO into a signal 31 having half the 
frequency of signal 38 and shifted through 90.degree.. The second 
flip-flop 40 then shifts the output signal 31 of the first flip-flop 
through 90.degree. generating the demodulation signal 16. 
The demodulation signal 16 generated in the manner just described is 
invariably synchronous in phase and frequency with the sensor signals and 
can be changed to various frequencies, provided that the sensors 15, 14, 
14' receive and generate, respectively, mixed signals of various 
frequencies. The PLL circuit serves a function comparable to that of a 
bandpass filter, the mid-frequency of which is given by the free-running 
frequency of the VCO, and its characteristic by that of the control unit 
33 in the low-frequency range. In this manner, using a frequency changer 
42, the free-running frequency of the VCO can be changed and the 
mid-frequency of the bandpass characteristic be shifted accordingly, via 
an offset voltage 43 fed by the frequency changer, such that various 
carrier frequencies can be selected from among the sensor signals. This is 
an important consideration especially when along its route, the vehicle 
must negotiate shunts or change routes. In order to achieve 
synchronization when changing the frequency of the PLL, an amplification 
changer 45 can be used to change the amplification of the control unit 33 
that the lock-in range or the bandwidth of the PLL is widened. The 
amplification changer 45 is driven automatically by the output signal 46 
of a threshold detector 47, which monitors the demodulated and 
lowpass-filtered signal 50 of the sensor 15 for the horizontal field 
component. If the demodulation signal 16 is not synchronous with the 
sensor signal, the signal 50 will drop below the accordingly designed 
level of the threshold detector 47 to trigger a signal 46 causing the 
amplification change circuit 45 to decrease control unit amplification. 
When synchronization has been achieved, the signal 50 will exceed the 
given level, causing the amplification changer to be reversed to a high 
position. 
The output signal 32 of the PSD can be utilized for monitoring the PLL 
circuit, redundant guide cable frequencies, and shunt frequencies. 
Provided for this purpose is bandpass filter 52, the frequency of which 
can be changed using a drive signal 53 and the output of which is 
successively routed to a rectifier 55 and a threshold detector 56. This is 
to monitor the presence of parallel frequencies, the approach of 
junctions, etc. 
For interpretation of the sensor signals 20 to 22, circuitry 30 adapted to 
the crossed-antenna arrangement is provided to generate the control 
signals 60 and 61 by dividing signals derived from various field 
components. For this purpose, dividing circuits 62 and 63 are provided, 
the input signals to which are D.C. signals 64 to 66, which are 
proportional to the sensor voltages U.sub.h, U.sub.v, U.sub.v ' and which 
are generated by lowpass-filtering the demodulated signals 20 to 22. Each 
sensor signal is assigned a lowpass filter 67, 68 or 69. The deviation 
signals 60 and 61 are thus formed independently of one another, making it 
possible, by using several vertical component scans, to provide a 
partially redundant system with no more than one horizontal scan. The 
deviation signals 60 and 61 are routed to a steering system 70 to guide 
the vehicle. Provided also are display and control units 71 to 74 to 
monitor, indicate, or control synchronization, frequency, and parallel 
frequencies. The PLL frequency at any moment can be displayed using a 
frequency counter 75. 
The circuitry of FIG. 2 is also suitable for receiving information 
transmitted through the guide cable, if these are contained in the sensor 
signal U.sub.h, following amplitude, frequency, or phase modulation. For 
this purpose, the output signal 32 (FIG. 3) of the PSD can be routed, 
through a lowpass filter 80, and the output signal of the control unit 33 
can be applied directly, to an information decoder 81 constructed of 
simple logic components. 
This invention has been shown and described in preferred form only, and by 
way of example, and many variations may be made in the invention which 
will still be comprised within its spirit. It is understood, therefore, 
that the invention is not limited to any specific form or embodiment 
except insofar as such limitation are included in the appended claims.