How Accelerometers Work
In our last blog post, we detailed how harsh accelerations are both costly and dangerous for fleets and their drivers. We also detailed how cutting-edge hardware solutions like Positioning Universal’s FT7500 and the new AI Fleet Camera Solution can help monitor harsh accelerations and other important driving behaviors so that fleet managers can know how their drivers are driving and help create a safe fleet. But how exactly do these solutions monitor harsh accelerations, braking, collision risks, and impacts?
The FT7500 gateway utilizes an accelerometer to measure changes in velocity and calculate these invaluable metrics. For example, it can calculate harsh accelerations based on sudden changes in acceleration. It can detect a harsh brake event based on a rapid deceleration. And from there, when coupled with the AI Camera, more layers can be added such as whether or not that harsh brake was because of a collision risk, stop sign, or red light. It can even spot traffic violations—such as running a red light or stop sign—by seeing the light/sign and then using the accelerometer to ensure that the vehicle came to a full stop before going. The accelerometer in the FT7500 unlocks vital capabilities that make the device as special as it is.
How Does an Accelerometer Work?
Initially when I think of acceleration, I think of speeding up/slowing down over a certain amount of time. For example, if a car can go from 0-60 mph in 6 seconds, it is accelerating at a rate of 10 mph per second. While that is a perfect line of thinking for understanding acceleration, this analysis using a time interval (in this case the 6 seconds) poses issues for real-time, instant acceleration readings from accelerometers. Having instant acceleration readings are vital for our advanced capabilities to work, causing this path for figuring out a car’s acceleration infeasible. So, we need to find an alternate method of calculating acceleration before we can figure how to make a device that does so.
Another way to calculate acceleration is by looking at the force created on an object. Thanks to the second law of motion, we can find that the force is equal to the mass of the object multiplied by its acceleration. For example, the more force you give into throwing a ball, the faster it is going to fly through the air. With this knowledge, if we can calculate the force applied to a know unit of mass, we can know the instantaneous acceleration!
By measuring the force on a tiny mass, accelerometers can calculate the acceleration of a vehicle. Generally, this force is measured by the mass that is suspended by a spring pressing up against an object of some sort, but different types of accelerometers measure the force in slightly different ways.
Types of Accelerometers
There are many different types of accelerometers, but for our purposes we will go over three: mechanical, piezoelectric, and Micro Electro Mechanical Systems (MEMS).
A mechanical accelerometer is the basic model of an accelerometer, not requiring any fancy technology, and finding its home normally in science classrooms. There is an outer casing that holds a mass which is suspended by a spring, and when a force occurs, the outer casing moves but the mass lags behind as the spring stretches. From there, the distance that the spring stretches is measured as it is proportional to the actual force, and then we have our acceleration. Interestingly, earthquakes are roughly measured in this way, where a pen is attached to the mass and draws a line as the spring snaps the mass back to its normal position.
Piezoelectric accelerometers also involve a mass and a spring, but instead of just having a pen that draws a line, the mass is pressed against a piezoelectric crystal while it rests. From there, when the force causes the outer casing to move and the spring to stretch, the crystal gets squeezed by the mass and this squeezing produces a tiny electric voltage that translates to the acceleration. Based on how much force is applied, that is how much current flows.
Piezoelectric accelerometers are extremely accurate and used in many industrial applications. However, the most common accelerometer—and the one used for the FT7500—is the MEMS accelerometer.
Micro Electro Mechanical Systems (MEMS) accelerometers are much different from the other two. Items like smartphones and wearable technologies need accelerometers for many different uses like screen rotation and advanced movement analytics, but they cannot hold accelerometers like the piezoelectric model as it is much too big to fit. Instead, the accelerometers inside these devices work in very tiny microchips that can be directly integrated into the device’s circuit board. All the components of the accelerometer are etched onto silicon instead of wrapped in an outer casing like with the prior two examples. This is where it gets its name from. It is essentially a micro-sized system that brings in mechanical processes (acceleration) with electrical ones.
At a high level, MEMS accelerometers work using the same guiding principles of mass and spring but use them to create differing capacitances instead of voltages or pen lines. Zooming in, the structure of the accelerometer will look like the diagram below. The mass is attached to springs on both sides that will move in the direction of the acceleration. Also, on the mass there are plates that are attached to it and move as it moves (blue) and fixed plates that are not attached to the mass (green).
The movement creates altering distances between the blue and green plates, which creates a different capacitance for each region (red regions). Based on the change in capacitance, the acceleration that caused that change can be calculated. These movements are microscopic, but because it is already integrated on the circuit board, that information can be directly applied for other device uses.
Accelerometers have a vast range of uses and therefore different types exist to suit each specific use. MEMS capacitors allow for mobile devices like smartphones, wearables, and even telematics hardware to accurately detect changes in acceleration while minimizing space and staying within the main circuit board. For telematics and fleet safety, MEMS accelerometers help us spot potentially dangerous driving situations so that fleet managers can address them swiftly.