No matter what type of EMS vehicle you drive, it's almost certain the vehicle design technology was derived from a production-based civilian model. In most cases, that isn't a bad thing because your vehicle has benefited from billions of dollars of research and millions of miles of testing.
Advances in automotive technologies have placed microcomputers in your rig that are more sophisticated than the spaceship technology of just a few decades ago. One of the newest areas of computer assistance in vehicles has been in dynamic vehicle-control technology. This technology was once limited to the highest of high-end vehicles, but it has trickled down into mainstream vehicles, including ambulance and specialty EMS rigs.
This article concisely explains newer technologies, how they work, what they feel like when deployed and what to do if your vehicle has them. It's crucial to understand these systems if your EMS vehicle is equipped with them so that you can maximize their potential if needed.
What It Is: Back in the 1960s and 1970s, you achieved traction control by either taking your foot off the gas pedal or spending a few extra bucks on a traction-aid differential, like a limited-slip. The limited-slip differential is currently present in Ford's E-Series Super Duty ambulance package and many SUVs. In four-wheel-drive (4WD) vehicles like the Expedition and Tahoe, it's usually present only in the rear axle. With this differential, a clutch pack inside the assembly constantly reallocates the amount of available power to the wheel with the most traction. This leads us to the next generation of electronically controlled traction-control devices found on the 2009 Ford E-350 series and Chevrolet vans, among others.
How It Works: Modern day traction control systems essentially use the speed sensors in anti-lock braking system (ABS) to measure an increase in acceleration of a particular wheel relative to the others. Once the traction-control system measures one wheel spinning more quickly than the others, it automatically pumps the brake to that wheel to reduce its speed and wheel slip. Many newer cars with smart transmissions and throttle-by-wire also incorporate the ability to reduce engine power when it senses traction loss.
What It Feels Like: Your first instinct will be that something has broken on your vehicle. The gas pedal will go to mush, and you will look in the rear view to see who threw out the anchor. Relax. This is normal. Once the system determines that you're indeed worthy to have full power again, the vehicle will leap back to normal. This change can surprise you if you aren't prepared.
If you're not sure how you feel about traction control, you're not alone. In some cases, it might help, but the bottom line is tires give youtraction, not power. If you have no traction in snow or ice because your tires simply won't grip, the most a traction-control system can do is moderate a bad situation because any application of the throttle will cause the system to either decrease power or obstruct it altogether. In bad weather, a two-wheel-drive vehicle with traction control is still no match for a regular all-wheel-drive or 4WD.
What To Do If You Have It:First, make sure you have the right tires for the season. Second, drive smoothly with the throttle so you never have to use the traction-control system. Third, make sure you have enough space when merging into traffic in inclement weather or accelerating rapidly from a stop or corner, because when the system activates, youwill slow down, making traffic approaching behind you arrive a lot sooner. Once activated, there isn't much you can do besides pull your foot off the throttle or wait until the system does its thing.
Anti-Lock Braking System
What It Is: A skidding tire is useless for control and stopping power. The theory behind ABS is simple: Keeping the wheels moving allows the tires to maintain rolling friction. This friction allows the driver to stop easier and safer, and steer the vehicle around the hazard rather than into it. ABS has been around for some time now, but many drivers still have no idea how it really works.
How It Works: ABS includes four basic components: speed sensors, the pump, valves and a computer controller. The speed sensor is attached to each brake wheel assembly and measures how fast the wheel is turning. The computer uses this information to determine the difference in speed of each wheel relative to the others. If one wheel is slowing down much faster than the rest, it's usually a sign the brake wheel is ready to lock up. On four-channel ABS, which most modern vehicles use, there's a valve in each brake line. When the system senses lock-up on a particular brake wheel, it uses the valve to regulate pressure. No matter how hard you press the brake pedal, the valve prevents any further pressure from making it to that particular brake wheel. The valve then releases pressure as needed. Once the system begins to stabilize, the ABS pump replaces the lost pressure the valve removed. This all happens in milliseconds.
What It Feels Like: When the ABS activates, the valve pump system repeatedly adjusts brake pressure to each brake wheel during the hard-braking conditions. The see-saw effect of the two results in a pulsation in the brake pedal that can be mild or severe depending on the cost of the vehicle and what generation system is installed. Newer ABS uses electronic brake-force metering instead of mechanical means so it can react more quickly and precisely.
What To Do If You Have It: First, do not pump the brake pedal thinking that the pulsations mean something's wrong—it's normal. If you aren't sure about the pulsation level and feeling provided by your EMS vehicle's ABS, take it out to an empty parking lot, run it up to 35 mph or so and stand on the brake pedal. As you come to a stop, feel the pulsation and steer left or right to get a feel of what the ABS is doing to help you maintain rolling friction. This is also a good time to find out if your rig doesn't have ABS installed.
It's important to know that ABS will generally notfunction if the brakes are so hot they can't produce enough stopping power to lock up a particular brake wheel, a problem created when the brake system's ability to convert kinetic energy to heat is overwhelmed. True, the basic hydraulic brake system works, but brake fade will delay implementation of ABS.
So, when managing your EMS vehicle in a Code-3 response, remember that if the brakes get too hot, you may be out of luck. No matter how good ABS is, a 10,000 lb. vehicle can stop only so fast, and steering hard around a hazard means a violent weight transfer in a matter of milliseconds. Plan ahead to avoid needing ABS in the first place.
Emergency Brake Assist
There's a newer braking technology married to ABS called "emergency brake assist." This system is available on the Mercedes-Benz Sprinter Ambulance, the Ford Expedition and numerous civilian vehicles. Emergency brake assist detects a situation where emergency braking is required by measuring the speed with which the brake pedal is depressed and relaying this information to the electronic control unit. If it registers an unusually high pedal speed, the system interprets an emergency braking situation and automatically increases pressure to the brakes in conjunction with the ABS. In some packages, emergency brake assist is also integrated into the electronic stability control system.
What It Is: Stability control systems use the same sensors as ABS and traction control, in addition to other components to address lateral (side-to-side) motions of the vehicle associated with chassis behavior. The system is called many names, such as ESP (Daimler AG), StabiliTrak (General Motors), Advance Trac (Ford), but all operate under essentially the same principles.
Why It's Important:EMS drivers must successfully negotiate roadways under the worst of conditions, so anything that can make our driving responsibilities easier is always welcome. Further, the hundreds of pounds of equipment we carry in our vehicles can worsen a bad-handling condition. Thankfully, stability control systems help.
A 2004 NHTSA study determined that stability control systems do appear to reduce accidents, especially in SUVs. According to NHTSA, 7.4% of the light-vehicle fleet in 2003 was sold with some form of stability control system. Of the vehicles studied, stability control systems reduced single-vehicle crashes in passenger cars by 35% when compared with the same models sold in prior years without the technology.
The effects on SUVs were even more impressive. The study showed that SUV single-vehicle crashes were reduced by 67% in models with stability control. Evaluating fatal crashes only, stability control was associated with a 30% reduction for passenger cars, and 63% for SUVs. So it appears a stability control system in your EMS rig can help you stay safer behind the wheel.
How It Works: Stability control adds steering-angle sensors and a yaw sensor. Yaw is movement of an object as it rotates on a vertical axis. In other words, when your vehicle starts to spin, and the lateral forces make the rear end want to touch the front end of your car, blame it on the yaw.
When that begins to happen, the yaw sensor determines that you're about to get in over your head and works with a lateral-force sensor as well as the steering-wheel sensor to evaluate whether you're about to go from hero to zero on that off-ramp. If the answer is yes, the system begins to apply braking force to one or several individual wheels to "steer" the vehicle back toward its intended path. In some cases, the system can also reduce power and torque to assist in traction and weight transfer.
The best part is that stability control often senses what's occurring well before you know what's about to happen, so it's already trying to resolve it well before you make the attempt. This means all you have to do is steer the vehicle. The system does the rest.
The system is more effective as the center of gravity increases because it can curb radical weight-transfer situations (which lead to rollovers), well before the driver can sense what's happening and react. In some cases, the stability-control system has multiple settings that allow drivers to adjust how much computer assistance they want.
What It Feels Like: Depending on the system's sophistication (read budget), the impact of stability control can be rather pronounced or mild. For example, in a system where braking force is utilized at one or more wheel(s) to induce weight transfer, the sensation will be similar to someone "stabbing" the brakes (i.e., there'll be an abrupt application of braking force in one or two corners of the vehicle that'll be noticeable from the driver's seat). It may feel like a "see-saw" effect as the system tries to shift weight over the proper end or side of the vehicle in an attempt to equalize the amount of traction of a particular tire and correct excessive yaw movement.
Another possible sensation is the loss of power. This happens in cases where excessive throttle has caused a handling problem; the computer attempts to reign in lateral motion by reducing wheel slippage. In both cases, the result is immediate and very apparent.
A Little Rain on the Parade: Of course, no mechanical system can violate the laws of physics, overcome sheer stupidity or neutralize bad judgment. After all, the only things connecting your vehicle to the roadway are the tires, and if you do something that vastly exceeds the capability of your tires to grip, no system will help you avoid disaster. Like ABS and traction control, stability-control systems can assist you with regaining control in emergency situations, but it can't replace the proper driving skills and decision-making processes that you need to employ on a day-to-day basis. That said, stability-control systems are one more option offered in modern EMS vehicles that can help you stay safe and on the road.
Traction control, ABS and stability control are just some of many new technologies finding their way into EMS vehicles. If you drive an ambulance or specialty vehicle, you owe it to yourself, your partner and your patients to know what your rig has and how it can be best used to help keep you safe on the roads. JEMS
This article originally appeared in January 2010 JEMS as "Shifting Gears: Advanced EMS vehicle technology."