We have been researching backup sensors and cameras to learn about what is out there, how well they work, where the better models are, and what their shortcomings are. There appear to be dozens of companies offering units ranging from bumper installable to stick-on to license plate installable models, and most of these products come out of China and Taiwan. We have bought and evaluated some of these units and have put together a short list of products that we like.Following is a summary of that which came out of our research.
Backup Sensors use ultrasonic proximity measurement by issuing an ultrasonic ‘ping’ and then measuring the time it takes for echoes, bouncing off of objects in the path of the ‘ping’, to return. In a few cases an electromagnetic antenna is used to issue a radio wave ‘ping’ but this appears to be limited in objects that it can sense. Once an object is detected within five or six feet of the sensor, an audible alarm is sounded, the ‘urgency’ of the alarm being greater for closer objects (in most cases a ‘beep-beep-beep’ sound is issued and the ‘beep’ rate increases as the sensor gets closer to an object). There is also a visual indicator either in the form of an LED array that indicates the approximate location of the detected object and, with the use of green, yellow and red colored LED’s, proximity is indicated. Some models use an LCD screen to display proximity and location information. The sensors themselves must be kept fairly clean in order to be able to properly send and receive ultrasonic waveforms.
Strengths: The backup sensor’s biggest strength is that it is an active obstacle detector and does not require the user to be observing a monitor to communicate to the user that an obstacle is in the path.
Weaknesses: Ultrasonic sensor systems have blind spots (such as at the extreme ends of the bumper or otherwise outside the cone of the emanating sound wave. Certain surfaces may also reflect the sound wave away from instead of back to the sensors, and if a surface is does not have enough density or ‘hardness’ it will absorb much of the sound wave instead of reflecting it.
Backup Cameras use a CCD or CMOS imaging sensor like those used in a camera to capture an image of the area behind the car. Units typically have a wide angle lens system that can range from 120 degrees of capture angle to as many as 170 degrees. The wider the angle, the more the image will be distorted unless there is more cost added to correct the distortion, either through electronic image processing or through optics. Some units come with a ‘night vision’ sensor and an array of infra-red LED’s that illuminate the area behind the car when it is dark.
A typical backup camera system will consist of a camera module that mounts in the back of the car somewhere, and a small video monitor that mounts on the dashboard or somewhere that the driver can see. Some make use of a rear-view mirror assembly that attaches over the existing rear-view mirror and which has built into the mirror an LCD display that is invisible until the car is put in reverse which turns power on to the camera and display. Our experience with this style of monitor is that during the day the light from the back window, which is what you are trying to see out of with the rear view mirror, swamps out the LCD image to where you can hardly see the image.
Strengths: The backup camera allows the user to see what is behind the car and make informed judgments. Some objects with surfaces which don’t reflect an ultrasonic sound wave back to the sensor well will still show up on the monitor if in the field of view of the camera.
Weakness: In order to avoid an accident, the user must observe the image in the monitor and recognize danger. Also, similar to the ultrasonic devices, there may be regions outside of the field of view of camera which are still within the backup profile of the vehicle.
Combination Units have both ultrasonic sensors and a camera module. This solution takes advantage of the strengths of both approaches, covering more of the blind areas and providing a more robust solution.
There are a few ‘Do It Yourself’ or DIY units that have the sensors and/or camera module mounted in a frame that attaches over the license plate to the license plate mounting holes. The advantage of these is that it is not necessary to drill holes in the bumper – a step that is a significant barrier for many potential customers. The sensors and/or camera module are positioned to radiate outward from the license plate area and cover much of the region behind the car. It is still necessary to route wiring from the license plate mounted unit to the inside of the car where the control box is and from there to the dashboard unit.
For a more detailed description of the technology behind ultrasonic backup sensing, please see ‘how ultrasonic backup sensors work’.
Features and Terminology
Installation Scheme: Backup sensor and/or camera ‘kits’ provide a user the hardware and tools to install the technology into cars that did not come equipped with it. Less expensive models typically come with sensors and/or camera modules that mount directly into the car’s bumper. The user is required to measure and drill a hole in the bumper where each sensor is to be mounted.Wiring from the sensors is then brought to a control box that is stationed in the trunk or back of the car. This control box has a wire that connects to the reverse light wiring of the car so that it is off unless the car reverse lights are on which would be the case when the user intends to back up. In the case of an ultrasonic sensor system, the control box determines proximity of objects using the sensors and then sends a signal to the display unit where an audible alarm and corresponding display are generated. Some display units are wired to the control box or camera, requiring the routing of wire from the back of the car to the display. In some cases the display and control box communicate wirelessly which avoids this routings step. For a camera system, the camera often connects directly via a long wire or wirelessly to the display unit. The display unit is typically mounted on the dashboard or otherwise where the driver can easily see it. Installation is not trivial, even if you don’t have to drill holes in your bumper, and some vendors solve this problem by offering installation services.
Viewing Angle: In camera systems, field of view is a cone that is of miniscule width at the camera and which widens outward in the direction away from the camera. This determines how well objects that are close to the car and to the side of the camera can be seen. If the viewing angle of a camera module is 120 degrees, then at one foot back from the camera the field of view will be 3.5 feet wide. This means that the imaging system is blind to objects that are close to the car but outside of the field of view. In the case of a camera module with a 170 degree viewing angle the field of view will be 23 feet wide at one foot back from the camera. The penalty for a wide field of view is image distortion, which can be dealt with by simply making a mental adjustment or by paying more for electronic or optical correction.
Sensitivity: In sensor systems, the sensitivity is usually indicated in the number of feet behind the vehicle where the sensor can still detect an object that has a good reflecting surface. This distance is usually on the order of 5 or 6 feet, which, at a modest back up rate, is plenty of space to respond and stop for a detected obstacle.
Sensor Count: The ultrasonic sensor is one of the more expensive components in a backup sensor system so you will see some low end units offer a system with 2 sensors. You will get better coverage and fewer blind spots with 4 sensors, although in the DIY systems where the sensors radiate outward there have been claims that 2 sensors works well enough to see most obstacles. A system with more than four sensors is intended for front and back detection.
How they work
The ultrasonic parking sensor device contains between 2 and 8 ultrasonic transducers which are weather resistant modules that act both as a loudspeaker and a microphone. By design they are tuned to operate at a single frequency (usually 40kHz) which makes them naturally reject noise and disturbances at all other frequencies such that they can detect proximity of an object off which an echo will bounce with a fair degree of reliability.
Because the sensors are built to be weather resistant, some of their capability is sacrificed through measures such as enclosing the emitting mechanics in a protective covering.
In order to generate an ultrasonic sound wave, or ‘ping’ to borrow from submarine parlance, a relatively high voltage must be applied across the terminals of the transducer. Since is usually done using a step up transformer. Applying a 12 volt signal across a 1:10 step up transformer will create a 120 volt signal to apply across the sensor which will give plenty of excitation to generate a sound wave. Once a sound wave has been generated, it propagates outward in an expanding cone of sound. Any hard object that has a surface that is roughly normal to the direction of propagation of the sound wave will cause a reflection back in the direction the sound wave came from. A human body typically has enough surface suitability to reflect a detectable echo.
Once the excitation sound wave has been issued, the transducer switches to microphone mode and listens for an echo. Preamplifier circuitry in the sensor device will amplify any sound waves picked up by the transducer. As reflections from the outgoing sound wave are detected and amplified, a timing circuit such as a microprocessor measures the amplitude of these reflections to determine whether they are large enough to declare detection of an echo or not. If so, the microprocessor calculates the time that has elapsed since the sound wave excitation was issued and from this time and the known velocity of propagation of sound waves, the distance the wave traveled and thus the distance to the object off which the sound wave reflected can be calculated.
The microprocessor then must communicate this to the driver somehow. Most units have both a display of distance and an audible warning device. The microprocessor lights up the appropriate bars of the display device, and generates periodic signals to drive the warning device to produce beeps. By changing the delay between beeps the microprocessor can communicate distance from the detected object.
Some models have a wireless communication path with the indoor unit that avoids the necessity of routing a wire from the sensor box that is typically installed in the trunk to the display and audio warning unit that is mounted near the driver.
The backup camera system has an exterior camera module that mounts in the middle of the back of the car facing backward. It is powered by the car’s 12 volt power system, typically through connection to the reverse light wiring so that it will only turn on when the car is in reverse.Inside the camera module is a lens assembly that captures light and focuses it on an imaging array much like those used in digital cameras. The imaging array sends the image in the same signal format as that used for analog television transmission (NTSC or PAL) and either through a wire or wirelessly to the monitor unit that is mounted near the driver. In the case of wireless transmission, the transmission frequency is usually 2.4GHz, which is the same frequency that Bluetooth, WiFi, and other electronic devices occupy. As a result of the number of devices that use this frequency, the image on the monitor may from time to time exhibit fuzziness or other artifacts in the image. This can sometimes be improved by relocating the monitor so that the RF path of the camera module to the monitor is different.
The interior monitor unit typically attaches via double-sticky tape to the dashboard. It receives power from a wired connection to the accessory receptacle (cigarette lighter). In a combined system where camera and ultrasonic sensing both reside, the monitor will also house a small speaker for audible warning and will display information about the proximity of obstacles that have been detected.
Technologies we’ve wondered about
How well would it work to use laser technology to perform distance measurement as is done with carpentry tools on the low end and other precision measurement devices on the high end. Issues likely would be getting a good reflection back off a variety of unpredictable surfaces.
The police seem to have a pretty good handle on determing how fast you are moving with a hand-held device. How hard would it be to use similar technology to sense position (how far away the car behind you is) instead of velocity (how fast the position of a car is changing)? The challenge would likely be in making the circuitry cost effective. The downside to electromagnetic sensing is that it may not sense non-metallic objects such as people as well so it would not be accomplishing as thorough a job as ultrasound.