Automotive Forums

Sensors

 
Through the years vehicle security sensors have gone through a rapid evolution in technology. In the good old days they were simple mechanical devices. Today they are nothing less than miniature computers.

Pendulums, Mercury, Contact Switches et al
In the neophyte days of vehicle security when a chain draped over a car's hood kept in place by a pad lock deterred the bad guys from getting at a car's battery, sensors were relatively primitive. Perhaps the most widely used sensors at the time were motion sensors. These devices were meant to sense when the vehicle moved. There were commonly two types of these sensors on the market -- the mercury and the pendulum. The mercury included a glob of mercury -- the same stuff you see in the thermometer you use to take your temperature. The sensor would detect the movement of the mercury and ascertain that the vehicle was in motion. The pendulum style motion sensor detects the movement of a pendulum.

Shock sensors were another one of the original methods vehicle security used to detect an attack on a vehicle in the early days. These sensors featured a finely threaded pointed screw. The tip of the screw had a sharp point and a metal cup on a spring was placed at the end of it. When the vehicle was attacked the vibration would cause the screw to contact the cup and the sensor would trip.

Glass breaking sensors were also devised to sense the sound of breaking glass. These devices included a microphone usually placed under the dash.

Ultrasonic sensors were a rage in the 1980s. These sensors included a transmitter and a receiver and would emit a high frequency sound. It would analyze the sound being received by the receiver to determine air movement in the vehicle.

Then came the radar or microwave sensors. These devices bathe the interior of a vehicle with radio frequencies and analyze the bubble created to detect movement.

False Alarms
These early sensors did their jobs, but because the technology wasn't too advanced they were susceptible to false alarms. In the case of the motion sensors, if a vehicle was parked on a hill, the mercury or pendulum sensors would sense movement and false. In the case of the shock sensors, low frequency sounds such as tires squealing in a closed garage, the sound of an airplane taking off or landing, a motor cycle, or moving traffic could cause the sensor to go off. Glass breaking sensors would false when the dashboard creaked as it expanded in the heat of the sun. Ultrasonic sensors were susceptible to temperature changes.

As a result of these problems the research and development departments of the companies that manufacture vehicle security products went to work and developed sensors that were more discriminating in its analysis of information to minimize the possibilities of false alarms.

More Analysis Means Fewer Falses
Sensors began to look at a certain spectrum of frequencies. Audio sensors, like the glass breaking sensors, for example, listen for audible frequencies roughly between 20 Hz. and 20kHz. Moreover there are a lot of other factors that the sensors determine as they do their work. They sense, for example, the intensity and  timing of the sound they detect, as well as specific frequency of the sound.

Many glass breaking and shock sensors look for a low frequency rush of the air at about 0 to 5 Hz. When the frequency is sensed, a spike is created which is transmitted to an amplifier and then on to the alarm brain where it is analyzed to ascertain if the spike is over a programmed threshold. If so, the alarm is tripped.

Ultrasonic sensors use frequencies that are higher than 20kHz.

Infrasonic sensors have been developed to detect frequencies below 20 Hz. These sensors use two microphones -- one mounted outside the vehicle and one inside. If there is an impact on the window of a vehicle, the window pushes in and causes a little change in the air volume inside the vehicle. The microphone inside the vehicle measures the change in air pressure and compares it to the air pressure outside the vehicle.

Radar or microwave field disturbance sensors analyze the high end of the frequency spectrum -- above 1gHz. The sensor is designed to detect the mass density or physical mass that comes into the field it produces. The mass is first detected and monitored to see if it moves and how it moves.

Shock sensors detect frequencies in the 30 to 40 Hz. range. There are many types of shock sensors. For example, there are piezo shock sensors. These devices feature metal materials that are adhered to each other. They detect frequencies and feed an electronic circuit and the electronic circuit processes the information. These sensors are commonly found on entry level security products. They do a pretty good job, but their sensitivity varies a lot depending on how they are mounted. If the piezo shock sensors are mounted properly, they should be okay.

Other shock sensors use light emitted from an LED which shines off a reflective surface. These sensors are called laser or reflective shocks sensors. As the reflective surface moves around the signal is analyzed. There are shock sensors that use infrared technology. They feature an infrared beam that the sensor tracks. If the beam is disturbed or broken, the sensor will either command the alarm to issue a warning chirp or a full blown alarm condition.

Moreover, there are electro-magnetic shock sensors. These sensors feature a suspended magnet that moves in front of a coil when there is an impact on the vehicle and the resulting signal is processed and analyzed.

Electro-magnetic shock sensors were very susceptible to electro-magnetic induction. High powered CB radios can put out radio frequencies that generate signals that the wiring in a vehicle can pick up. An electro-magnetic field can be created which can trip the sensor. Many companies in the field of vehicle security still offer electro-magnetic shock sensor, but these sensors are low electro-magnetic interference shock sensors which lessen the possibility of false alarm.  

Some companies offer air pressure sensors. These sensors work like a barometer and senses the drop in air pressure.

In short, sensors have become miniature computers. Impact sensors use digital analysis to discriminate vibrations that it senses. There are even glass tampering sensors that use digital analysis to provide better discrimination.

Tilt/Motion sensors that are designed to sense when the car is being jacked up or towed, are also available. This sensor relies on electronics rather than a pendulum or mercury glob on which earlier motion sensors relied. The sensor calibrates itself and uses its own microprocessor to do the evaluations and calculations.

Choosing the Proper Sensor
Most alarm systems come with either a glass breaking sensor or a form of a shock sensor. However, some sensors may prove to provide better protection than other sensors based on the way you use your car, where it is parked, etc.

For example, glass breaking or air pressure sensors are not as effective if you tend to leave your windows open slightly or if your vehicle features a convertible or T-top that you like to leave open.  

In addition, if you tend to park your car near a construction site, glass breaking sensors may be susceptible to the noises generated by the construction.

A shock sensor, on the other hand, may be better for your situation. The sensor can cover an entire perimeter of a vehicle and it gives good cover whether the vehicle is open or closed. Still, some types of shock sensors may be susceptible to false alarm when the vehicle is parked in a structured garage or near an airport. Furthermore, if you have a cellular phone that you leave on, it can cause electro-magnetic shock sensors to false. If the cellular phone is left on often there is a check signal between the cellular site and the phone. Some times that is enough energy to trigger the shock sensor.

Radar or microwave sensors also referred to as field disturbance sensors are ideal for convertible or T-top vehicles and some styles of jeeps. Many of these vehicles have side walls that are easy to penetrate. A radar will help protect the interior of the vehicle if a thief tries to cut through a side wall. Moreover, some of these sensors come with dual zones which allows the security system to monitor not just inside the vehicle but also a limited area outside the vehicle as well. This permits the use of a warn away feature. If an intruder penetrates the perimeter of the area monitored outside the vehicle, the alarm will emit a warning chirp from the siren informing the prospective thief that the car is protected by an alarm.

It should be mentioned that radar or microwave sensors can also false. For example, radar can be affected by very strong electro-magnetic pulses that can deform the radar bubble. Such things as microwave emitters used on news vans or CB radios can cause these sensors to false. In addition, electro-magnetic noise caused by high wattage motors found in such items as garage door openers and hair dryers can cause radars to false.

Some styles of shock sensors also have a warn-away feature. A light tap on the exterior of the vehicle will cause the alarm to chirp a warning while a harder strike will cause the siren to wail in a full blown alarm condition.

Other considerations need to come into play when you select a sensor. Do you, for example, like to leave valuables on the seats? If so, you may want to consider a glass breaking sensor or a shock sensor or even a radar sensor. The glass breaking and shock sensors will deter the smash and grab type of theft and the radar can be used to give a prospective thief a warning before he even attempts to break a window of your vehicle.

As mentioned before, if you have a convertible or a T-top car a radar will be the best alternative because it provides good coverage for an open vehicle.

You will also want to select a sensor that you can turn off via the remote. This way, when you find yourself in a situation that may cause a false alarm you can turn off the sensor and prevent the false while the alarm continues to protect the vehicle.

Being able to adjust the sensitivity of the sensor may be something you want to consider as well. Although most false alarms are due to improper adjustment of the sensitivity of the sensor, you may want to control that sensitivity at will because you will often find yourself in conditions that may call for more or less sensitivity. Some alarms allow you to adjust the sensitivity of sensors via the remote. Many manufacturers, however, believe that the adjustments should be made by a trained technician and should not be accessible to the consumer. In either case, it may prove beneficial for you to be present when the installer adjusts the sensitivity of the sensor on your alarm so you get what you want.

There are also alarm systems that will shut off a sensor if it trips more than a certain number of times within a specific time frame.

Finally, you need to realize that all sensors are not created equal. Some are more sophisticated than others. Since today's sensors are miniature computers which can analyze data, there are several levels of sophistication. In other words, some sensors will analyze the data more discriminatory than other sensors and can therefore avoid some false alarms. It is best, therefore, that you ask the salesperson to demonstrate more than one sensor in a given category so that you can see for yourself the difference in sophistication.

How Does R&D Do It
On many occasions, the research and development departments of the manufacturers of sensors start with requests from their dealers and, believe it or not, you, the consumer. Concerns about present sensor performance is past on to the manufacturers. Engineers, tech personnel, and even representatives from the marketing department determine if changes are possible and what those changes can be. A prototype is constructed and lab tested, then tweaked until the project supervisors are satisfied and then samples are made and past on to the manufacturers' top dealers for more field testing. The dealers mount the prototypes in their staffs' cars and evaluate their performance and pass that information back to the manufacturers.

Development of new sensors from concept to delivery to consumers can take from several weeks to a year.

And how can you be sure about the quality of the new sensors being offered? Many companies are involved with international quality certification programs known as ISO-9001. This is an outside group that reviews the internal processes a manufacturer goes through in the research, development, and manufacturing of its products. The group requires external independent auditing a couple times a year. The audit reviews how things are manufactured, but does not necessarily test the products themselves. That is commonly done by the manufacturer.

Don't Stop Thinking About Tomorrow
As for the future, look for manufacturers to continue with the digital revolution. More and more sensors will become miniature computers in their own right with their own processors and software. Instead of relying on the alarm system's main brain or module to do the calculations and evaluations, this process will be done by the sensor and the result -- whether to trip the alarm, issue a warning chirp, or no response will be passed on to the module for the rest of the alarm to react or not to react accordingly.