1. The Field of the Invention
The present invention relates generally to a real-time, data acquisition system. More particularly, the present invention relates to a data acquisition system including a plurality of sensors attached to a relatively large, physical structure or object, and daisy-chained to a relatively long, single cable.
2. The Background Art
It is often desirable to measure certain physical characteristics, such as vibration, of large structures, such as bridges or aircraft. Such measurements often require making measurements at a large number of select data points about the structure. Thus, a large number of sensors are often required. For example, referring to FIG. 5, a bridge 514 is shown with a plurality of sensors 518 disposed about the bridge at desired locations. A plurality of cables 522 are used to connect the sensors 518 to a data acquisition system or instrumentation rack 526, so that each sensor 518 supplies data through its own cable 522. Thus, it will be appreciated that a large number of long cables are typically required.
One disadvantage with such a system is the large number of cables and long length of the cabling. Numerous cables are required because of the numerous data points which are typically monitored. The cables must be lengthy because the structures being studied are relatively large or long. It will be appreciated that long cables are expensive, and that the large number of cables multiplies the cost. In addition, the set-up process for a test is long and expensive because of the number of cables which must be disposed about the structure, and the length of the structure over which the cables must be disposed. Furthermore, use of a large number of cables with variable, long lengths can alter or otherwise affect the results of the testing.
It is known to utilize fewer cables, such as in a daisy chain configuration, for other types of data transmission schemes. Such schemes typically utilize frequency division multiple access (FDMA) or time division multiple access (TDMA) techniques. FDMA divides the available spectrum into smaller sections or channels, with a single transmission assigned to each channel. One disadvantage with FDMA is that it is limited by the available spectrum, and the bandwidth requirements for each channel. TDMA divides the available spectrum into time slots, with each transmission assigned to a single time slot. One disadvantage of TDMA is that it does not deal effectively with synchronous sampling or phase coherence of large channels. Another disadvantage with both frequency domain or time domain multiplexing in the analog domain is that it does not provide adequate signal isolation at low cost. In addition, complex filtering, echo resolution, etc. also make it cost prohibitive.
Therefore, it would be advantageous to develop a data acquisition system which would reduce the cost of cabling, set-up, and instrumentation. It would also be advantageous to develop such a system capable of high data bandwidth, high dynamic range, and which is capable of dealing with crosstalk. It would also be advantageous to develop such a system with instrumentation which is inexpensive and simple. It would also be advantageous to develop such a system with low electro-magnetic interference (EMI) emissions and susceptibility.
It is therefore an object of the present invention to provide a real-time, or synchronous, data acquisition system to acquire data from relatively large structures, such as bridges, aircraft, etc.
It is another object of the present invention to provide such a system which reduces the cost of cabling and set-up time for the cabling, while maintaining the number of data points.
It is another object of the present invention to provide such a system which has a high data bandwidth, a high dynamic range, and which reduces crosstalk.
It is yet another object of the present invention to provide such a system with simple and inexpensive instrumentation.
The above objects and others not specifically recited are realized in a specific illustrative embodiment of a real-time, or synchronous, sensor data acquisition system to sense properties or physical characteristics associated with a structure or physical system. The data acquisition system includes a plurality of sensors disposed about the structure and connected in series to a single cable. The sensors may sense vibration, noise, temperature, acceleration, pressure, strain, force, etc. The sensors have sensor circuitry configured to transmit data signals representing the physical characteristics along the cable. A controller or receiver is connected to the cable to receive the data signals from the sensors. A display may be connected to the controller for displaying the properties or physical characteristics associated with the structure or physical system.
The sensors may transmit the data signals at greater than 100 dB dynamic range, with a bandwidth between approximately 5 to 20 KHz, and in a single bit wide data stream.
In accordance with one aspect of the present invention, the sensor and receiver utilize code division multiple access techniques to respectively send and receive the data signals. Thus, the sensors are configured to code the data signals, and the control circuitry is configured to decode the data signals.
Each sensor is configured to produce a sensor signal, and each sensor includes a modulator configured to modulate the sensor signal with a distinct code to produce the data signal. The sensor includes sensor circuitry, such as an analog-to-digital converter to convert the sensor signal to a digital bit stream, and a modulator configured to encode the digital bit stream with an m-sequence modulation. A chip generator may generate one of a plurality of distinct chip codes. The sensors transmit the data signals with distinct delays to match a delay of the furthest sensor.
The controller includes a demodulator to demodulate the data signals. In addition, the controller is configured to correct for delay of the data signals to within 25 nanoseconds. The control includes a delay control counter and clock phase selector which match arrival times of the data signals transmitted from different location distances to one-half a period of a chip clock frequency.
In addition, the controller includes circuitry to correct intersymbol interference by summing the data signals to produce a sum, multiplying the sum by a corrective ratio to produce a correction signal, summing the correction signal to the data signals.
In accordance with another aspect of the present invention, the cable may be over 100 feet long, and includes at least two wire pairs. A first signal and control pair carries data signals from the sensor circuitry and control signals from the controller. A second clock pair carries clock signals from the controller. Power may be provided on either pair, or on a separate third pair.
In accordance with another aspect of the present invention, the single cable is one of a plurality of cables. Each cable is connected to a plurality of sensors, such that each sensor is connected to a cable in series with another sensor.
A method of using the system of the present invention includes locating a plurality of sensors at desired sensor locations on the structure. A cable is located along the desired sensor locations. The sensors are connected in series to the cable. The sensors sense properties or physical characteristics associated with the structure or physical system. The sensors produce data signals representing the physical characteristics, and transmit the data signals along the cable with distinct delays. The data signals are synchronously received by a receiver, and the physical characteristics or properties associated with the structure or physical system are displayed on a display.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by the practice of the invention without undue experimentation. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.