Improving Doppler-Based Sensing SystemsSeptember 1, 2007 By: Priyabrata Sinha, Microchip Technology Inc. Sensors
By incorporating high-performance digital signal controllers (DSCs), with their integrated architectures and small footprints, designers can simplify Doppler-based sensing systems, enhance their performance, and reduce development cycles.
There have been many advances in Doppler-effect sensing systems. One of these is the use of computationally intensive algorithms to respond to multiple sensors. This approach is implemented by incorporating digital signal controllers (DSCs) in the sensing system to perform the added functions. Featuring sensor-oriented on-chip peripherals, such as analog-to-digital converters (ADCs), comparators, and voltage references, DSCs reduce component counts and system costs. This enables designers to build more economical and reliable noninvasive systems that are more accurate than those using other approaches.
The Doppler effect measures the change in a propagating wave's frequency, as felt by an observer moving relative to the source of the waves. To be effective, the measurement system must accurately control the frequency and amplitude of the sound waves. A DSC can accomplish this by generating pulse-width modulation (PWM) waveforms of varying periods and/or duty cycles (high/low times). Moreover, the measurement system should be able to focus the generated signal in a certain direction (known as the steering angle). In real-life applications, however, you may not be able to physically rotate the transducer. Instead, to achieve the desired directionality, you may have to use a phased array (also known as beamforming).
A phased array (Figure 1) comprises a group of sensors in which the relative phases of the signals feeding each sensor can be altered to enhance the effective radiation pattern of the array in a desired direction. The array achieves this by multiplying each individual antenna signal by a fixed or adaptive delay. This operation is computationally intensive and requires the efficient implementation of operations such as multiplication and—in some advanced algorithms—matrix inversion. Depending on the application, you can use a DSC to program the fixed or adaptive delay. By doing so, you're able to combine the inputs from various sensors and arrive at the focus point of interest without physically moving the sensors.
Figure 1. A phased array using a DSC-based programmable delay
A fast and accurate ADC—preferably an on-chip device to minimize system cost and size—must convert the signals received by the Doppler measurement system (Figure 2) to digital form for further processing. The resolution of the ADC is key to ensuring high-fidelity measurements. You can use a fast Fourier transform to analyze the frequency content of the signal to estimate the position and velocity of the objects of interest. DSC-enabled Doppler effect-based sensors support efficient implementation of several types of IIR and FIR digital filters, which can be used to eliminate noise or implement a phased array and similar topologies.
Figure 2. A DSC-based Doppler measurement system
Because of their inherent nonlinear characteristics, most of these sensor systems must store large amounts of calibration data and filter coefficients in nonvolatile memory. You can meet this design objective by using a DSC featuring on-chip flash memory that can be modified during run time. This approach effectively addresses concerns regarding space constraints and memory access speed.
In many closed-loop systems, the Doppler sensor is part of the feedback loop and can therefore benefit from a DSC's efficient execution of control algorithms. In fact, DSCs make it relatively straightforward to implement control-loop algorithms, such as PID controllers. Depending on the Doppler-effect sensing application, you can use the on-chip ADCs and multiple PWMs to run multiple control loops in the application, for fast and accurate sensing performance.
Industrial applications. You can use the Doppler effect to make high-precision, high-frequency nonintrusive flow measurements. Both laser Doppler velocimeters (LDVs) and acoustic Doppler velocimeters (ADVs) measure fluid-flow velocities. They accomplish this by emitting a light or acoustic beam and then measuring the resultant Doppler shift in wavelengths of the reflections from particles moving with the flow. The instruments compute the actual flow as a function of the fluid velocity and pressure.
These advanced flowmeters combine digital Doppler radar velocity-sensing technology with ultrasonic pulse echo-level sensing to remotely measure open-channel flow. The instrument transmits a digital Doppler radar beam that interacts with the fluid and reflects signals at a different frequency than the original. The flowmeters compare the reflected signals with the transmitted frequency to measure the velocity and direction in which the fluid is moving. Based on this calculation, the instruments detect the fluid level and calculate the flow by multiplying the average fluid velocity by the area.
Automotive applications. Vehicle collision-detection systems can provide visual and audible warnings that indicate object distances with accuracies up to a few inches. During adaptive cruise control, the system warns the driver when the vehicle is in reverse gear or when another vehicle gets too close. Automakers can also implement parking-assistance systems using the same design principle—sending out acoustic signals and detecting an obstacle through the regrouping of the transmitted signals.
These systems use an array of three or more sensors to provide complete coverage of the rear of the vehicle. When the vehicle is put in reverse gear, the collision-detection system's sensors transmit ultrasonic signals. The sensors detect objects behind the vehicle and send the information to a DSC, which processes the signals from all sensors simultaneously to estimate the distance and the closing speed between the vehicle and the obstacles in the vehicle's path. The DSC continuously works on the sensor data and transmits the results to the vehicle's dashboard, where visual or audible warnings are provided.
Medical applications. Designers have also used the Doppler-measurement technique for velocity measurements in medical-imaging applications. Keep in mind, though, that most applications must measure the phase shift rather than the frequency shift (Doppler shift) of the signal being measured. Vascular problems, such as stenosis, can be diagnosed by measuring the velocity of blood flow in arteries and veins, based on the Doppler effect.
Using the Doppler effect, an echocardiogram can produce accurate assessments of the direction and velocity of blood flow through the cardiac tissue at any predetermined point. For this measurement, the ultrasound beam must be as parallel to the blood flow as possible. The velocity measurements assist the evaluation of cardiac-valve areas and their functions.
High-performance DSCs, such as the dsPIC33FJ12GP family from Microchip Technology, simplify development of Doppler-sensing implementations (Figure 3). The compact and integrated architectures of these devices enable signal-processing techniques to be implemented in a cost-effective and flexible manner, thereby reducing the system cost. Additionally, space-constrained sensing applications stand to benefit by using DSCs in small packages (6 mm by 6 mm QFN to 12 mm by 12 mm TQFP), with 18-pin to 100-pin options.
Figure 3. DSC-based Doppler-effect sensing applications
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