Solar-powered sensor controls traffic
A traffic-sensor post measures vehicle location and speed at an intersection, providing an accurate, effective, inexpensive approach to monitoring traffic flow.
Larry K Baxter, Capsense, Lexington, MA; Edited by Martin Rowe and Fran Granville -- EDN, November 26, 2009
Have you ever sat in your car waiting for the light to turn green when nobody’s using the cross street? This wait is due to the fact that the sensors controlling these traffic signals—in one large-suitcase-sized box per intersection—are classically dumb, with relays, cams, and switches, although they now may include software that accepts data from local sensors, automobile-sized inductive loops buried in the asphalt. Modern controllers have gained some intelligence. For example, they may share data with nearby intersections, respond to radio requests from emergency vehicles, and sometimes take commands from a traffic-control center. This Design Idea describes the TSP (traffic-sensor post), a more accurate, effective, inexpensive, and easy-to-install approach to monitoring traffic flow. These sensors measure vehicle location and speed in four or more streets at an intersection or at a distance from the intersection for early warning. A second application of this technology, the WIM (weight-in-motion) sensor, weighs moving trucks.
The circuit comprises a wireless, solar-powered sensor array that handles all the data collection at an intersection (Figure 1). Cities can install these sensors at each of the four corners of an intersection for full coverage. The sensors send data to the single controller box over IEEE 802.15.4 in a star network. The approach combines four sensors in an inexpensive, low-maintenance, 6-in.-diameter, 6-foot-tall post. You can build the circuit into the post that holds the traffic lights, or you can use it stand-alone. Not all TSPs require all four sensors; you can select those that your application needs based on usage. The TSP is the first wireless approach to this problem, and one of the sensors, the Cap Pad, provides a huge advantage over current expensive and inaccurate WIM sensors (Figure 2).
The TSP uses a PIR (passive-infrared) sensor that looks 10 microns into the deep-IR band for moving IR sources. This technology finds use in inexpensive motion-detecting lamp controls and senses vehicles from 30 feet away. The detection range is good, the parts are cheap, and the beam can see through a layer of dirt. It can’t measure speed, distance, or direction.
The TSP also uses conventional pneumatic tubes. Rubber tubes are stapled to the asphalt and feed two pressure sensors. This approach accurately measures speed, but permanent installations cannot use it because it gets damaged easily. Municipalities often deploy pneumatic tubes to measure traffic volume in road construction.
The Cap Pad comprises a 10-in.×12-foot sandwich of three 0.05-in.-thick stainless-steel sheets separated by two 0.05-in.-diameter closed-cell urethane-foam layers (Figure 3). You capacitively measure the 0.025-in. deflection of the pad under a truck’s tire to weigh the axle. One Cap Pad can handle the WIM requirements, and using two can add speed and direction information. You use multiple pads to handle multilane roads. The Cap Pad can be fastened to the asphalt with adhesive or pavement tape or buried under as much as an inch of asphalt for protection. Its materials cost is only a couple hundred dollars, a huge saving over the piezoelectric WIM sensors currently in use.
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The TSP also uses a near-IR transmitter/receiver using a pulsed LED for transmission and a PIN (positive-intrinsic-negative) photodiode for reception. Both need cylindrical lenses to focus the beam to a 2°-wide, 5°-high ellipse that covers a remote retroreflective screen, as in highway signs, or to the IR sensors on another TSP. A multilayer optical bandpass filter that removes visible light further improves the range.
Precision capacitive sensors can measure an air gap between adjacent metal plates to subnanometer accuracy. Unfortunately, accuracy in the WIM application requires flat and parallel surfaces, and the Cap Pad has neither. Capacitive sensors can also accurately measure a force on adjacent flat plates with a restoring spring, but flatness and parallelism are still requirements. Maintaining parallelism over a 10-in. pad would be difficult, and roads are seldom flat.
If compression of the air pockets in closed-cell foam provides the restoring force, however, the resulting spring constant changes from the conventional F=K×x of springs or cantilevered beams to F=P0×H/(H–x), where F is force, P0 is atmospheric pressure, H is the starting gap, and x is the displacement. The result of this equation is that the capacitance of the pad varies linearly with applied force, and the surfaces of the Cap Pad no longer need to be parallel or flat. It accurately measures a force regardless of its size.
Most of the circuit amplifies outputs from the four sensors, digitizes them with the MSP430’s 12 bit-ADC, does some preprocessing, and messages the controller. The 6V solar panel, 40 IXYS solar cells in series, charges a 19-Ahr, 3V, lithium-polymer battery through IC1. Low-dropout regulator/switch IC2 regulates battery output at 3V. The battery generates more than 4V at full charge and 3.2V at the end of charge, and the low-dropout regulator at 42 mA generates only 50 mV. IC2 also switches active-mode 3V power.
The road-strip sensor senses the 0.1-to 1-psi pulse when a car drives over the pneumatic tubes. A 400Ω silicon bridge sensor differentially outputs approximately 50 mV. Instrumentation amplifiers IC3 and IC4 boost the output to a few volts. The pressure sensor, as well as the Cap Pad and the PIN sensor, has a quiescent level with no traffic. A timer detects the no-traffic state and stores this level in RAM, updating every second to follow slow offset drifts from environmental factors, so sensor offset accuracy is not critical. The pressure sensor’s scale accuracy—at approximately 30%—is relatively uncritical, but the Cap Pad’s scale accuracy should be a few percentage points or less. All sensors must have good resolution.
IC5 handles accurate temperature measurements, which are necessary for the Cap Pad, whose temperature dependence results from the elastic modulus change of polyurethane. The Cap Pad has a nominal capacitance of about 24 nF at rest, with a change of approximately 7% full-scale when a truck passes. The Cap Drive pulse discharges this capacitance at a 700-Hz rate, and a 100-kΩ resistor charges it to 3V with a 240-µsec time constant. A timer times the number of pulses it takes to cross the internal VDD/2 reference using the internal comparator, and, because you can clock the timer at 12 MHz, the resolution is 1%. You can get increased resolution by timing out the nominal quiescent pulse width and capturing the pulse’s level at that point with the 12-bit ADC.
The Cap Pad’s sandwich construction shields the active element from electromagnetic interference, but a 3W zener diode cleans up any remnant lightning strokes. The IR LED drive is a 20-to-1 current mirror to handle LED voltage variation. A DAC handles the PIN photodetector’s offset because the extreme night-to-day dynamic range would overrange the 12-bit DAC. The PIR sensor turns moving deep-IR targets into bipolar millivolt voltage pulses with its special segmented lens and dual-element pyroelectric detector. A PGA (programmable-gain amplifier) selects and variably amplifies the PIR sensor’s signal and the PIN signal. The timer uses standard connections.
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