A lot of people complain that driving across the United States is boring. Having done the coast-to-coast trip seven times now, I can’t agree. Sure, the stretches through the Corn Belt get a little monotonous, but for someone like me who wants to know how everything works, even endless agriculture is fascinating; I love me some center-pivot irrigation.

One thing that has always attracted my attention while on these long road trips is the weigh stations that pop up along the way, particularly when you transition from one state to another. Maybe it’s just getting a chance to look at something other than wheat, but weigh stations are interesting in their own right because of everything that’s going on in these massive roadside plazas. Gone are the days of a simple pull-off with a mechanical scale that was closed far more often than it was open. Today’s weigh stations are critical infrastructure installations that are bristling with sensors to provide a multi-modal insight into the state of the trucks — and drivers — plying our increasingly crowded highways.

All About the Axles

Before diving into the nuts and bolts of weigh stations, it might be helpful to discuss the rationale behind infrastructure whose main function, at least to the casual observer, seems to be making the truck driver’s job even more challenging, not to mention less profitable. We’ve all probably sped by long lines of semi trucks queued up for the scales alongside a highway, pitying the poor drivers and wondering if the whole endeavor is worth the diesel being wasted.

The answer to that question boils down to one word: axles. In the United States, the maximum legal gross vehicle weight (GVW) for a fully loaded semi truck is typically 40 tons, although permits are issued for overweight vehicles. The typical “18-wheeler” will distribute that load over five axles, which means each axle transmits 16,000 pounds of force into the pavement, assuming an even distribution of weight across the length of the vehicle. Studies conducted in the early 1960s revealed that heavier trucks caused more damage to roadways than lighter passenger vehicles, and that the increase in damage is proportional to the fourth power of axle weight. So, keeping a close eye on truck weights is critical to protecting the highways.

Just how much damage trucks can cause to pavement is pretty alarming. Each axle of a truck creates a compression wave as it rolls along the pavement, as much as a few millimeters deep, depending on road construction and loads. The relentless cycle of compression and expansion results in pavement fatigue and cracks, which let water into the interior of the roadway. In cold weather, freeze-thaw cycles exert tremendous forces on the pavement that can tear it apart in short order. The greater the load on the truck, the more stress it puts on the roadway and the faster it wears out.

The other, perhaps more obvious reason to monitor axles passing over a highway is that they’re critical to truck safety. A truck’s axles have to support huge loads in a dynamic environment, and every component mounted to each axle, including springs, brakes, and wheels, is subject to huge forces that can lead to wear and catastrophic failure. Complete failure of an axle isn’t uncommon, and a driver can be completely unaware that a wheel has detached from a trailer and become an unguided missile bouncing down the highway. Regular inspections of the running gear on trucks and trailers are critical to avoiding these potentially catastrophic occurrences.

Ways to Weigh

The first thing you’ll likely notice when driving past one of the approximately 700 official weigh stations lining the US Interstate highway system is how much space they take up. In contrast to the relatively modest weigh stations of the past, modern weigh stations take up a lot of real estate. Most weigh stations are optimized to get the greatest number of trucks processed as quickly as possible, which means constructing multiple lanes of approach to the scale house, along with lanes that can be used by exempt vehicles to bypass inspection, and turnout lanes and parking areas for closer inspection of select vehicles.

In addition to the physical footprint of the weigh station proper, supporting infrastructure can often be seen miles in advance. Fixed signs are usually the first indication that you’re getting near a weigh station, along with electronic signboards that can be changed remotely to indicate if the weigh station is open or closed. Signs give drivers time to figure out if they need to stop at the weigh station, and to begin the process of getting into the proper lane to negotiate the exit. Most weigh stations also have a net of sensors and cameras mounted to poles and overhead structures well before the weigh station exit. These are monitored by officers in the station to spot any trucks that are trying to avoid inspections.

Most weigh stations in the US are located off the right side of the highway, as left-hand exit ramps are generally more dangerous than right exits. Still, a single weigh station located in the median of the highway can serve traffic from both directions, so the extra risk of accidents from exiting the highway to the left is often outweighed by the savings of not having to build two separate facilities. Either way, the main feature of a weigh station is the scale house, a building with large windows that offer a commanding view of the entire plaza as well as an up-close look at the trucks passing over the scales embedded in the pavement directly adjacent to the structure.

Scales at a weigh station are generally of two types: static scales, and weigh-in-motion (WIM) systems. A static scale is a large platform, called a weighbridge, set into a pit in the inspection lane, with the surface flush with the roadway. The platform floats within the pit, supported by a set of cantilevers that transmit the force exerted by the truck to electronic load cells. The signal from the load cells is cleaned up by signal conditioners before going to analog-to-digital converters and being summed and dampened by a scale controller in the scale house.

The weighbridge on a static scale is usually long enough to accommodate an entire semi tractor and trailer, which accurately weighs the entire vehicle in one measurement. The disadvantage is that the entire truck has to come to a complete stop on the weighbridge to take a measurement. Add in the time it takes for the induced motion of the weighbridge to settle, along with the time needed for the driver to make a slow approach to the scale, and each measurement can add up to significant delays for truckers.

To avoid these issues, weigh-in-motion systems are often used. WIM systems use much the same equipment as the weighbridge on a static scale, although they tend to use piezoelectric sensors rather than traditional strain-gauge load cells, and usually have a platform that’s only big enough to have one axle bear on it at a time. A truck using a WIM scale remains in motion while the force exerted by each axle is measured, allowing the controller to come up with a final GVW as well as weights for each axle. While some WIM systems can measure the weight of a vehicle at highway speed, most weigh stations require trucks to keep their speed pretty slow, under five miles per hour. This is obviously for everyone’s safety, and even though the somewhat stately procession of trucks through a WIM can still plug traffic up, keeping trucks from having to come to a complete stop and set their brakes greatly increases weigh station throughput.

Another advantage of WIM systems is that the spacing between axles can be measured. The speed of the truck through the scale can be measured, usually using a pair of inductive loops embedded in the roadway around the WIM sensors. Knowing the vehicle’s speed through the scale allows the scale controller to calculate the distance between axles. Some states strictly regulate the distance between a trailer’s kingpin, which is where it attaches to the tractor, and the trailer’s first axle. Trailers that are not in compliance can be flagged and directed to a parking area to await a service truck to come by to adjust the spacing of the trailer bogie.

Keep It Moving, Buddy

Despite the increased throughput of WIM scales, there are often too many trucks trying to use a weigh station at peak times. To reduce congestion further, some states participate in automatic bypass systems. These systems, generically known as PrePass for the specific brand with the greatest market penetration, use in-cab transponders that are interrogated by transmitters mounted over the roadway well in advance of the weigh station. The transponder code is sent to PrePass for authentication, and if the truck ID comes back to a company that has gone through the PrePass certification process, a signal is sent to the transponder telling the driver to bypass the weigh station. The transponder lights a green LED in this case, which stays lit for about 15 minutes, just in case the driver gets stopped by an overzealous trooper who mistakes the truck for a scofflaw.

PrePass transponders are just one aspect of an entire suite of automatic vehicle identification (AVI) systems used in the typical modern weigh station. Most weigh stations are positively bristling with cameras, some of which are dedicated to automatic license plate recognition. These are integrated into the scale controller system and serve to associate WIM data with a specific truck, so violations can be flagged. They also help with the enforcement of traffic laws, as well as locating human traffickers, an increasingly common problem. Weigh stations also often have laser scanners mounted on bridges over the approach lanes to detect unpermitted oversized loads. Image analysis systems are also used to verify the presence and proper operation of required equipment, such a mirrors, lights, and mudflaps. Some weigh stations also have systems that can interrogate the electronic logging device inside the cab to verify that the driver isn’t in violation of hours of service laws, which dictate how long a driver can be on the road before taking breaks.

Sensors Galore

Another set of sensors often found in the outer reaches of the weigh station plaza is related to the mechanical status of the truck. Infrared cameras are often used to scan for excessive heat being emitted by an axle, often a sign of worn or damaged brakes. The status of a truck’s tires can also be monitored thanks to Tire Anomaly and Classification Systems (TACS), which use in-road sensors that can analyze the contact patch of each tire while the vehicle is in motion. TACS can detect flat tires, over- and under-inflated tires, tires that are completely missing from an axle, or even mismatched tires. Any of these anomalies can cause a tire to quickly wear out and potentially self-destruct at highway speeds, resulting in catastrophic damage to surrounding traffic.

Trucks with problems are diverted by overhead signboards and direction arrows to inspection lanes. There, trained truck inspectors will closely examine the flagged problem and verify the violation. If the problem is relatively minor, like a tire inflation problem, the driver might be able to fix the issue and get back on the road quickly. Trucks that can’t be made safe immediately might have to wait for mobile service units to come fix the problem, or possibly even be taken off the road completely. Only after the vehicle is rendered road-worthy again can you keep on trucking.

Featured image: “WeighStationSign” by [Wasted Time R]

Spending time as wee hackers perusing the family atlas taught us an appreciation for a good map, and [Billy Roberts], a cartographer at NREL, has served up a doozy with a map of the data center infrastructure in the United States. [via LinkedIn]

Fiber optic lines, electrical transmission capacity, and the data centers themselves are all here. Each data center is a dot with its size indicating how power hungry it is and its approximate location relative to nearby metropolitan areas. Color coding of these dots also helps us understand if the data center is already in operation (yellow), under construction (orange), or proposed (white).

Also of interest to renewable energy nerds would be the presence of some high voltage DC transmission lines on the map which may be the future of electrical transmission. As the exact location of fiber optic lines and other data making up the map are either proprietary, sensitive, or both, the map is only available as a static image.

If you’re itching to learn more about maps, how about exploring why they don’t quite match reality, how to bring OpenStreetMap data into Minecraft, or see how the live map in a 1960s airliner worked.

What happens when you build the largest machine in the world, but it’s still not big enough? That’s the situation the North American transmission system, the grid that connects power plants to substations and the distribution system, and which by some measures is the largest machine ever constructed, finds itself in right now. After more than a century of build-out, the towers and wires that stitch together a continent-sized grid aren’t up to the task they were designed for, and that’s a huge problem for a society with a seemingly insatiable need for more electricity.

There are plenty of reasons for this burgeoning demand, including the rapid growth of data centers to support AI and other cloud services and the move to wind and solar energy as the push to decarbonize the grid proceeds. The former introduces massive new loads to the grid with millions of hungry little GPUs, while the latter increases the supply side, as wind and solar plants are often located out of reach of existing transmission lines. Add in the anticipated expansion of the manufacturing base as industry seeks to re-home factories, and the scale of the potential problem only grows.

The bottom line to all this is that the grid needs to grow to support all this growth, and while there is often no other solution than building new transmission lines, that’s not always feasible. Even when it is, the process can take decades. What’s needed is a quick win, a way to increase the capacity of the existing infrastructure without having to build new lines from the ground up. That’s exactly what reconductoring promises, and the way it gets there presents some interesting engineering challenges and opportunities.

Bare Metal

Copper is probably the first material that comes to mind when thinking about electrical conductors. Copper is the best conductor of electricity after silver, it’s commonly available and relatively easy to extract, and it has all the physical characteristics, such as ductility and tensile strength, that make it easy to form into wire. Copper has become the go-to material for wiring residential and commercial structures, and even in industrial installations, copper wiring is a mainstay.

However, despite its advantages behind the meter, copper is rarely, if ever, used for overhead wiring in transmission and distribution systems. Instead, aluminum is favored for these systems, mainly due to its lower cost compared to the equivalent copper conductor. There’s also the factor of weight; copper is much denser than aluminum, so a transmission system built on copper wires would have to use much sturdier towers and poles to loft the wires. Copper is also much more subject to corrosion than aluminum, an important consideration for wires that will be exposed to the elements for decades.

Aluminum has its downsides, of course. Pure aluminum is only about 61% as conductive as copper, meaning that conductors need to have a larger circular area to carry the same amount of current as a copper cable. Aluminum also has only about half the tensile strength of copper, which would seem to be a problem for wires strung between poles or towers under a lot of tension. However, the greater diameter of aluminum conductors tends to make up for that lack of strength, as does the fact that most aluminum conductors in the transmission system are of composite construction.

The vast majority of the wires in the North American transmission system are composites of aluminum and steel known as ACSR, or aluminum conductor steel-reinforced. ACSR is made by wrapping high-purity aluminum wires around a core of galvanized steel wires. The core can be a single steel wire, but more commonly it’s made from seven strands, six wrapped around a single central wire; especially large ACSR might have a 19-wire core. The core wires are classified by their tensile strength and the thickness of their zinc coating, which determines how corrosion-resistant the core will be.

In standard ACSR, both the steel core and the aluminum outer strands are round in cross-section. Each layer of the cable is twisted in the opposite direction from the previous layer. Alternating the twist of each layer ensures that the finished cable doesn’t have a tendency to coil and kink during installation. In North America, all ACSR is constructed so that the outside layer has a right-hand lay.

ACSR is manufactured by machines called spinning or stranding machines, which have large cylindrical bodies that can carry up to 36 spools of aluminum wire. The wires are fed from the spools into circular spinning plates that collate the wires and spin them around the steel core fed through the center of the machine. The output of one spinning frame can be spooled up as finished ACSR or, if more layers are needed, can pass directly into another spinning frame for another layer of aluminum, in the opposite direction, of course.

Fiber to the Core

While ACSR is the backbone of the grid, it’s not the only show in town. There’s an entire beastiary of initialisms based on the materials and methods used to build composite cables. ACSS, or aluminum conductor steel-supported, is similar to ACSR but uses more steel in the core and is completely supported by the steel, as opposed to ACSR where the load is split between the steel and the aluminum. AAAC, or all-aluminum alloy conductor, has no steel in it at all, instead relying on high-strength aluminum alloys for the necessary tensile strength. AAAC has the advantage of being very lightweight as well as being much more resistant to core corrosion than ACSR.

Another approach to reducing core corrosion for aluminum-clad conductors is to switch to composite cores. These are known by various trade names, such as ACCC (aluminum conductor composite core) or ACCR (aluminum conductor composite reinforced). In general, these cables are known as HTLS, which stands for high-temperature, low-sag. They deliver on these twin promises by replacing the traditional steel core with a composite material such as carbon fiber, or in the case of ACCR, a fiber-reinforced metal matrix.

The point of composite cores is to provide the conductor with the necessary tensile strength and lower thermal expansion coefficient, so that heating due to loading and environmental conditions causes the cable to sag less. Controlling sag is critical to cable capacity; the less likely a cable is to sag when heated, the more load it can carry. Additionally, composite cores can have a smaller cross-sectional area than a steel core with the same tensile strength, leaving room for more aluminum in the outer layers while maintaining the same overall conductor diameter. And of course, more aluminum means these advanced conductors can carry more current.

Another way to increase the capacity in advanced conductors is by switching to trapezoidal wires. Traditional ACSR with round wires in the core and conductor layers has a significant amount of dielectric space trapped within the conductor, which contributes nothing to the cable’s current-carrying capacity. Filling those internal voids with aluminum is accomplished by wrapping round composite cores with aluminum wires that have a trapezoidal cross-section to pack tightly against each other. This greatly reduces the dielectric space trapped within a conductor, increasing its ampacity within the same overall diameter.

Unfortunately, trapezoidal aluminum conductors are much harder to manufacture than traditional round wires. While creating the trapezoids isn’t that much harder than drawing round aluminum wire — it really just requires switching to a different die — dealing with non-round wire is more of a challenge. Care must be taken not to twist the wire while it’s being rolled onto its spools, as well as when wrapping the wire onto the core. Also, the different layers of aluminum in the cable require different trapezoidal shapes, lest dielectric voids be introduced. The twist of the different layers of aluminum has to be controlled, too, just as with round wires. Trapezoidal wires can also complicate things for linemen in the field in terms of splicing and terminating cables, although most utilities and cable construction companies have invested in specialized tooling for advanced conductors.

Same Towers, Better Wires

The grid is what it is today in large part because of decisions made a hundred or more years ago, many of which had little to do with engineering. Power plants were located where it made sense to build them relative to the cities and towns they would serve and the availability of the fuel that would power them, while the transmission lines that move bulk power were built where it was possible to obtain rights-of-way. These decisions shaped the physical footprint of the grid, and except in cases where enough forethought was employed to secure rights-of-way generous enough to allow for expansion of the physical plant, that footprint is pretty much what engineers have to work with today.

Increasing the amount of power that can be moved within that limited footprint is what reconductoring is all about. Generally, reconductoring is pretty much what it sounds like: replacing the conductors on existing support structures with advanced conductors. There are certainly cases where reconductoring alone won’t do, such as when new solar or wind plants are built without existing transmission lines to connect them to the system. In those cases, little can be done except to build a new transmission line. And even where reconductoring can be done, it’s not cheap; it can cost 20% more per mile than building new towers on new rights-of-way. But reconductoring is much, much faster than building new lines. A typical reconductoring project can be completed in 18 to 36 months, as compared to the 5 to 15 years needed to build a new line, thanks to all the regulatory and legal challenges involved in obtaining the property to build the structures on. Reconductoring usually faces fewer of these challenges, since rights-of-way on existing lines were established long ago.

The exact methods of reconductoring depend on the specifics of the transmission line, but in general, reconductoring starts with a thorough engineering evaluation of the support structures. Since most advanced conductors are the same weight per unit length as the ACSR they’ll be replacing, loads on the towers should be about the same. But it’s prudent to make sure, and a field inspection of the towers on the line is needed to make sure they’re up to snuff. A careful analysis of the design capacity of the new line is also performed before the project goes through the permitting process. Reconductoring is generally performed on de-energized lines, which means loads have to be temporarily shifted to other lines, requiring careful coordination between utilities and transmission operators.

Once the preliminaries are in place, work begins. Despite how it may appear, most transmission lines are not one long cable per phase that spans dozens of towers across the countryside. Rather, most lines span just a few towers before dead-ending into insulators that use jumpers to carry current across to the next span of cable. This makes reconductoring largely a tower-by-tower affair, which somewhat simplifies the process, especially in terms of maintaining the tension on the towers while the conductors are swapped. Portable tensioning machines are used for that job, as well as for setting the proper tension in the new cable, which determines the sag for that span.

The tooling and methods used to connect advanced conductors to fixtures like midline splices or dead-end adapters are similar to those used for traditional ACSR construction, with allowances made for the switch to composite cores from steel. Hydraulic crimping tools do most of the work of forming a solid mechanical connection between the fixture and the core, and then to the outer aluminum conductors. A collet is also inserted over the core before it’s crimped, to provide additional mechanical strength against pullout.

Is all this extra work to manufacture and deploy advanced conductors worth it? In most cases, the answer is a resounding “Yes.” Advanced conductors can often carry twice the current as traditional ACSR or ACCC conductors of the same diameter. To take things even further, advanced AECC, or aluminum-encapsulated carbon core conductors, which use pretensioned carbon fiber cores covered by trapezoidal annealed aluminum conductors, can often triple the ampacity of equivalent-diameter ACSR.

Doubling or trebling the capacity of a line without the need to obtain new rights-of-way or build new structures is a huge win, even when the additional expense is factored in. And given that an estimated 98% of the existing transmission lines in North America are candidates for reconductoring, you can expect to see a lot of activity under your local power lines in the years to come.