Moving thousands of tons of freight on trains more than 2,000 meters long is a tricky business. Derailments can be caused by unbalanced weight distribution between the front and rear of the train. If heavier cars are positioned at the rear and braking is too sudden or forceful, compression forces can derail the lighter cars in front. For almost as long as there have been freight trains, the problem of weight distribution has received a lot of attention.
To further complicate the challenge, freight trains are constantly being reconfigured at depots along the route. Cars are removed and replaced by other cars of different weights. In short—over the long haul—it’s a major challenge to know how the train is loaded. The traditional solution has been to weigh each car individually on a static scale, which is time consuming, inefficient and costly.
Sensors the size of a postage stamp are now being used to solve the load distribution problem more efficiently. Piezoelectric strain gauge sensors welded to the side of a rail are a critical component of weigh-in-motion (WIM) technology. The strain gauges sense minute changes in the rail’s electrical properties when a train car’s wheel passes over it, and the railcar’s weight can be accurately derived from this data.
The aggregated information for the entire train gives the railroad operator two corrective options:
1) Adjust the brake settings to accommodate and control how much pressure is applied; and/or,
2) Reconfigure a precariously loaded train.
It all sounds simple enough but engineering the solution involves a number of technologies including:
• Sensors that may be based on several technologies. Piezoelectric sensors using quartz crystals are common and produce an electric charge proportional to the force. Since the signal has very high impedance, it is not sensitive to electrical interference.
• A MOSFET-based charge amplifier is used to convert the signal to a voltage output.
• Inductive loops to identify and timestamp the rail car’s entry and exit from the WIM station.
• The WIM software, which has to take into account the railcar’s weight, speed and other variables. It is not uncommon to include a GPS-synchronized time stamp as a means of tracking the data.
• Wireless communications, which is often Wi-Fi although cellular technology or modems can also be used.
Sensors that use quartz have a sensitivity of about 4.3 pC/N (i.e., the force of one Newton applied to the sensor produces a charge of 4.3 Picocoulombs). Quartz is being replaced with gallium phosphate (GaPO4) as the piezoelectric crystal in many applications such as WIM. GaPO4 can achieve twice the sensitivity because twice as much charge is produced by the same force.
The sensitivity of the sensor remains the same regardless of the nominal (rated) force and very large sensors can also be used to measure a very small force. Another advantage is that charges can be physically set to zero. A short circuit can be used to produce a charge of zero pC on the input when the sensor is loaded by a force such as a pre-stress.