Sonic position measurement system

A sonic position sensing system for measuring the position of an object, such as an elevator, includes transmitting a sonic signal along a conductor by a transmitter; receiving said sonic signal at a first receiver, located a first distance along said conductor from said transmitter; receiving said sonic signal at a second receiver located a second distance from said transmitter and on an opposite side of said transmitter from said first receiver; calculating a first distance between said transmitter and said first receiver; calculating a second distance between said transmitter and said second receiver; and calculating a total measured distance between said first receiver and said second receiver as the sum of said first distance and said second distance as the sum of said first distances and said second distance. The first or second distances may be scaled by the total measured distance to compensate for temperature effects. The position may also be corrected for velocity and/or acceleration of the object.

TECHNICAL FIELD 
This invention relates to a position measurement system and more 
particularly to a sonic position measurement system. 
BACKGROUND OF THE INVENTION 
It is known in the art of sonic position measurement systems to use an 
ultrasonic transmitter connected to the object whose position is to be 
detected. It is also known to have the sonic transmitter transmit a sonic 
pulse along a sound signal conductor which extends along the path which 
the object moves, the conductor having a predetermined sound propagation 
speed. The sonic pulse is sent in both directions along the conductor and 
is received by receivers at each end of the conductor, between which the 
object moves. Each receiver provides an electrical signal (indicative of 
when the pulse arrives at the respective receiver) to an evaluation 
circuit, which measures the time difference between the arrival of the 
pulse at the two receivers. From this time difference and the known speed 
of sound in the conductor, the position of the object between the 
receivers is determined. Such a system is described in European patent 
application EP 0,694,792 A1, published Jan. 31, 1996 filed Jul. 21, 1995, 
applicant K. A. Schmersal GmbH and Co. of Germany. 
In an elevator application, the operation of an elevator depends on 
accurate and reliable position and speed information. This information is 
used to control the motion of the elevator as well as certain safety roles 
and operations, such as opening of the doors. Modern elevators use a 
high-resolution position information from a source such as an incremental 
encoder on a motor to control the speed profile. However, for 
safety-relevant functions they rely on additional discrete switches in the 
hoistway. 
One way to implement a high-resolution measurement of the elevator car's 
position in the hoistway is to use the sonic position sensor discussed 
hereinbefore. In that case, the measurement ultrasonic pulse is launched 
along a wire from the car to the receivers at the bottom and top of the 
hoistway. From the difference of the reception times, the position of the 
car is calculated. 
However, such a system has numerous disadvantages. In particular, if the 
length of the signal conductor changes due to temperature effects it 
induces inaccuracies into the measurements of the location. Furthermore, 
using the difference between the reception times provides only one channel 
of position information. Thus, an accidental displacement of a receiver or 
any other change in length of the signal path results in erroneous 
measurements without the ability of being detected from the signal 
information. Further, inaccuracies induced by temperature-dependent 
variations in the signal speed are fully transferred to the position 
measurements. Accordingly, it is desirable to overcome these drawbacks for 
an elevator application. 
DISCLOSURE OF THE INVENTION 
Objects of the invention include provision of a sonic position measurement 
system which is fault-tolerant and accurate over temperature. 
According to the present invention, a method for measuring the position of 
an object, includes: (a) transmitting a sonic signal along a conductor by 
a transmitter connected to the object; (b) receiving said sonic signal at 
a first receiver, located a first distance along said conductor from said 
transmitter; (c) receiving said sonic signal at a second receiver located 
a second distance from said transmitter and on an opposite side of said 
transmitter from said first receiver; (d) calculating a first distance 
between said transmitter and said first receiver; (e) calculating a second 
distance between said transmitter and said second receiver; and (f) 
calculating a total measured distance between said first receiver and said 
second receiver as the sum of said first distance and said second 
distance. 
The invention represents a significant improvement over the prior art sonic 
position measurement systems by adding an additional signal to indicate 
when the sonic pulse is transmitted, thereby allowing the system to detect 
a damaged or altered signal path (e.g., a mechanical displacement of a 
receiver, a loose sonic wire, or a change in the length of the sonic 
wire), or by irregular pulses coupled into the signal path by an outside 
source. Also, the present invention provides a reference condition of the 
distance between the receivers, which is used by the system to compensate 
for the expansion and contraction of a building or other inaccuracies 
caused by temperature variations. The invention may be used as a position 
measurement (or reference) system for an elevator in a hoistway or for 
detecting the position of any object that moves along a predetermined path 
.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIGS. 1 and 3, a prior art position sensing system comprises 
an elevator car 12 having a sonic transmitter 14 (or signal coupler) 
connected thereto. The transmitter 14 generates a sound signal (or pulse) 
16 at a predetermined rate and couples the sonic pulse to a sound signal 
conductor 18 (or wire) which is held or clamped at both ends by a clamping 
device (not shown). The sonic pulse 16 propagates away from the 
transmitter 14 in both directions along the wire 18 and is received at 
sonic receivers 20, 22 located at opposite sides of the transmitter 14 
along the wire 18. When the receivers 20, 22 receive the sonic pulse 16, 
they each transmit an electrical pulse R1, R2 on lines 24, 26, 
respectively, to an evaluation circuit (or logic) 30. The logic 30 
comprises the necessary hardware (e.g., a microprocessor, analogue or 
digital circuits) and/or software necessary to perform the functions 
described herein. 
The logic 30 measures the time difference Td between the arrival of the two 
pulses 16 at the receivers 20, 22. The logic 30 computes the position of 
the elevator based on the time difference Td and the known speed of sound 
in the wire 18, and provides a position signal on a line 32 to an elevator 
controller 34. For example, Td may be converted to a distance (by the 
speed of sound) which is indicative of the distance from the midpoint 
between the receivers, the direction from the midpoint is based on the 
sign of Td. If the time Td is zero, the transmitter is located mid-way 
between the two receivers 20, 22. Such a system is similar to that 
described in European patent application EP 0,694,792 A1 published Jan. 
31, 1996 filed Jul. 21, 1995. 
Referring to FIGS. 2 and 4, the position reference system of the present 
invention is similar to the system of FIG. 1, but adds a signal TS from 
the transmitter 14, which is provided on a line 36 to an evaluation 
circuit (or logic) 40 (which replaces the evaluation logic 30 of FIG. 1). 
The logic 40 comprises the hardware and/or software necessary to perform 
the functions described herein. The transmitter 14 provides the electrical 
signal (or pulse) TS on the line 36 when the sonic pulse 16 is launched 
(or sent) into the wire 18. The signal TS allows the measurement of the 
absolute distance from the transmitter 14 to each of the receivers 20, 22 
via the measured flight times and the known speed of the sonic pulse 16, 
thereby providing two independent measurements of the position of the car 
in the hoistway. 
Referring to FIGS. 4 and 5, the evaluation logic 40 enters at a step 100 
which calculates the time T1 between the pulses TS and R1, and the time T2 
between the pulses TS and R2. Next a series of steps 102 calculate D1 and 
D2 based on T1 and T2 and the known speed of sound Vsound in the wire 18. 
Then, a step 104 calculates a Total Distance (TD) equal to the sum of DI 
and D2. Next, a step 106 determines whether the system is in 
initialization or calibration mode. If in calibration or initialization, 
TD is saved as a Total Reference Distance (TRD). If not in calibration or 
initialization, TD is saved as a Total Measured Distance (TMD). When the 
system is in normal measurement operation, the value of TMD is updated at 
a predetermined rate, e.g., each time position is calculated. All 
predefined positions in the hoistway, e.g. floor levels, etc., are related 
to TMD and/or TRD. 
Next, a step 112 compares TMD to TRD, and if the difference between TMD and 
TRD is greater than a predetermined fault threshold, e.g., 8 mm, a step 
114, sets a Fault flag to one which is provided to the control 34 on a 
line 42. Other fault thresholds may be used. Then, a step 116 checks 
whether the change in TMD is greater than a predetermined amount (x%), 
e.g., 50%, from the previous value of TMD. Other percent changes may be 
used if desired. If it is, the step 114 sets the Fault flag to one which 
is provided to the control 34 on the line 42. 
Then, a step 118 compensates for temperature variations by calculating a 
scaled value of D1 as D1.sub.SCALED by multiplying D1 by a scale factor 
TRD/TMD. D1.sub.SCALED is equal to the absolute position of the elevator 
car. 
The TRD and TMD parameters are used in the steps 112,116 to detect 
anomalies such as a damaged or altered signal path. Such anomalies can be 
caused by a mechanical displacement of a receiver, a disruption or 
mechanical displacement in the signal path (e.g., sonic wire pulled loose 
or changed length) or by irregular pulses coupled into the signal path by 
an outside source, etc. 
Also, TRD and TMD are used in the step 118 to compensate for temperature 
variations with the scale factor. In particular, when TMD deviates 
gradually from the TRD and the deviations do not exceed the fault 
threshold, temperature variations are deemed to be the cause of the 
deviation and the calculations of D1 and/or D2 are scaled. Instead of or 
in addition to calculating D1.sub.SCALED, the logic may calculate 
D2.sub.SCALED as D2.times.(TRD/TMD). The scaled distances are then 
indicative of the true position of the car in the hoistway. This scaling 
also compensates for the expansion of the building caused by temperature 
changes provided the temperature distribution in the hoistway is 
homogeneous. 
Next, a step 120 calculates a velocity of the car V.sub.CAR using the 
equation: 
##EQU1## 
where Position(n-2) and Position (n-1) are the Position values calculated 
in the previous two calculations of Position and .DELTA.T is the update 
rate of the position calculation. The velocity V.sub.CAR provides an 
adjustment to the value of D1.sub.SCALED to compensate for changes in the 
car position during the delay time (Td) from when the sonic signal 16 is 
launched into the conductor 18 to when the position signal is provided to 
the control 34. The delay time Td comprises two main components, a sound 
propagation delay of the sonic signal along the conductor 18 and a 
computation time/update rate for the logic 30 to provide the position 
signal. 
The propagation delay component of the time delay Td is determined based on 
the longer of the two propagation times T1,T2 (for the current position 
calculation) if the logic waits to receive both T1 and T2 before 
calculating position (i.e., TMD is calculated at the same update rate as 
position). Alternatively, the shorter of the two times T1 ,T2 may be used 
if the calculation updates the total distance TMD at a slower update rate 
than the calculation of position. The computation component of Td is 
predetermined based on the average update rate and computation time of the 
logic 30. Other techniques may be used to calculate Td. 
In addition to velocity correction, the logic 30 may also provide 
acceleration correction of position, i.e., change in velocity over the 
delay time Td. In that case, a step 122 calculates the car acceleration 
A.sub.CAR. The car acceleration A.sub.CAR may be calculated directly from 
position, by taking the second derivative of position. 
Next, a step 124 calculates a velocity and/or acceleration-corrected 
position signal which is provided on the line 32 to the control 34 by the 
equation: 
EQU Position=D1.sub.SCALED +P.sub.OFFSET 
where P.sub.OFFSET is a velocity and/or acceleration correction term. For a 
velocity correction, P.sub.OFFSET may be (V.sub.CAR .times.Td). For a 
velocity and acceleration correction P.sub.OFFSET may be (V.sub.CAR 
.times.Td)+(1/2) A.sub.CAR Td.sup.2. Alternatively, to correct for both 
velocity and acceleration, the value of V.sub.CAR in (V.sub.CAR .times.Td) 
may be adjusted based on an average change in velocity over the time delay 
Td. Other equations for compensating for velocity and/or acceleration may 
be used if desired. 
Also, the logic 30 may provide the velocity V.sub.CAR and/or acceleration 
A.sub.CAR to the control 34 on lines 38. The control 34 may use V.sub.CAR 
and/or A.sub.CAR for safety systems or for floor alignment systems, or for 
other uses. Instead of calculating V.sub.CAR and/or A.sub.CAR in the steps 
120,122, respectively, V.sub.CAR and/or A.sub.CAR may be provided to the 
logic 30 by a velocity signal from another device such as a velocity 
sensor or the control 34. 
Also, the value of Position may be filtered or averaged over a 
predetermined number of updates. For example, the calculation of 
D1.sub.SCALED may be performed at a first update rate, e.g., 1 millisecond 
(msec), and filtered or averaged over a predetermined number of updates, 
e.g., 3 to 10 updates. Then, the calculation of Position may be performed 
at a slower update rate, e.g., 10 msec, using the filtered value of 
D1.sub.SCALED. Other update rates may be used if desired. Also, in that 
case, the value of Td for correcting for velocity and/or acceleration may 
be an averaged or filtered value of Td. 
The velocity and/or acceleration correction of the position is not required 
for the present invention, but may be needed to provide the desired 
accuracy of position. If velocity and/or acceleration correction is not 
employed, the steps 120, 122, respectively, would not be performed and the 
step 124 would set Position equal to D1.sub.SCALED or D2.sub.SCALED, as 
appropriate. 
The sound signal conductor 18 may be a steel rail or a wire cable or other 
suitable waveguide for propagating an ultrasonic signal having a 
predetermined sound propagation speed. For example, if the sound signal 
conductor 18 is made of steel, the speed of sound Vsound in the conductor 
18 is about 5,300 meters/second. With a time resolution of 188 nanoseconds 
the local resolution of the measurement path is approximately 1 mm. Other 
materials with other sound propagation speeds may be used. 
The sonic transmitter 14 may be the same as or similar to a transmitter 
made by K. A. Schmersal GmbH & Co. of Germany; however, the transmitter 14 
has the additional output signal TS. The transmitter 14 may operate 
inductively to couple an electrical pulse received from a pulse generation 
circuit (not shown) into the sonic pulse on the wire 18. 
The sonic receivers 20, 22 may be the same as or similar to receivers made 
by K. A. Schmersal GmbH & Co. of Germany. The receivers 20,22 may be a 
piezoelectric signal output coupler, or be inductive or capacitive. Other 
types of transmitters and/or receivers may be used if desired to couple 
the sonic signal to and from the wire 18. 
Also, the signals R1,R2,TS on the lines 24,26,36 may be optical, 
electrical, microwave, or any other type of the signal which is read by 
the logic 40. Further, instead of being transmitted over the lines 
24,26,36, the signal R1,R2,TS may be transmitted through the air using 
known wireless technology, e.g., RF, microwave, optical, modulated, etc. 
Although the invention has been described and illustrated with respect to 
exemplary embodiments thereof, it should be understood by those skilled in 
the art that the foregoing and various other changes, omissions and 
additions may be made without departing from the spirit and scope of the 
present invention.