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Extreme sensors

October 11, 2010
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Sensors are essential to automatic carwashes. At the very least, they detect that a vehicle has entered or is leaving. But sensors do more — they help cut down on waste by determining the location and size of the vehicle as exactly as possible, so water, soap, and wax are applied where needed.

Although carwashes use a variety of different sensor types, for sizing up a vehicle in a specific location they use opposed-mode industrial photoelectric sensors, often called photo-eyes.

Opposed mode sensors have two pieces — an emitter and a receiver — on opposite sides of the area they are sensing. The emitter sends a beam of infrared light to the receiver.

When something, such as a vehicle, breaks the beam, the sensor detects the vehicle’s presence. It works a little like the photo-eyes used in garage doors openers, but these are far more heavy-duty.

A demanding environment
Thousands of opposed-mode photoelectric sensors on the market can handle the sensing jobs in a carwash — as long as you don’t turn the water on. But the spray, soap and dirt knock most sensors out of the running. To succeed in a carwash, a sensor has to meet some tough requirements:

  • Burn through the gunk. The mist in the air reduces the amount of light that goes between the two units — as does the dirt, soap and wax build-up on lenses. A weak beam of light can make a sensor think it detects the part of the vehicle it is looking for when, in fact, nothing is there.
  • Be leakproof. To keep moisture from seeping into the sensor and corroding the circuit board, the housing must be able to withstand water — especially hot, spraying water.
  • Be hard to fool. For a sensor, a carwash holds a lot of distractions. Sunlight, blowers, the electric noise of big motors, large droplets of liquid, light bouncing off wet surfaces — all these can trick a sensor into thinking it detects a vehicle that isn’t there, or make it miss a vehicle that is there.

What to look for
Whether you’re retrofitting an existing carwash with sensors or are evaluating a system for purchase, it’s important to know what to look for in a sensor.

Power
It takes a powerful photoelectric sensor to burn through the atmosphere in a carwash. That power is measured in terms of excess gain.

Excess gain is the amount of light that reaches the receiver above and beyond what is needed for the sensor to work at all. In essence, excess gain is the amount of overkill a sensor has.

In the case of a carwash, where so much can interfere with the light, more is definitely better.

It’s easy to figure out if a photoelectric sensor has enough power. Take the distance the emitter and receiver must span and multiply that number by 50. That’s the range the sensor must be able to cover.

So, for example, if the emitter and receiver will be 10 feet apart, the sensor must have a range of at least 500 feet. It takes that much power — that much overkill — to be effective for a carwash.

Accuracy
The beam of infrared light from a photoelectric sensor isn’t a straight narrow line from the emitter to the receiver. Instead, it spreads out like a cone. The less the light spreads out, the more accurate it is.

That accuracy is important in a carwash, because if the cone is wide enough to hit the wet floor or another reflective surface, light could bounce off and reach the receiver. The receiver misinterprets that bounced light as coming straight from the emitter, so it thinks nothing is between it and the emitter, when in fact something — like a vehicle — could actually be blocking the beam.

In short, this phenomenon, called an optical short circuit, prevents the sensor from seeing something that is there.

To find out how well a sensor focuses, check the specs for the beam pattern to learn how big the beam is at its widest point. The sensor has to be mounted further than that distance from the floor or other reflective surface.

Choosing a sensor with a small beam pattern gives you more flexibility in where you can place the sensor and still count on it to be reliable.

IP Rating
To get around the risk of leaks, look for a high IP rating, which tells you how well the housing protects the critical parts of the sensor from moisture.

A sensor with an IP69K rating is as water tight as they come. The IP69K rating means the sensor can withstand a hot, close-up, high-pressure jet stream (1160–1450 psi at 176ºF within 4–6 inches).

The next best choice is an IP67 rating, which means the sensor can withstand temporary immersion in water 3 feet deep for 30 minutes.

By using high-power, accurate photoelectric sensors with an IP69K rating, you can rely on the sensor to perform well and for a long time with just basic maintenance such as lens cleaning and alignment. You’ll economize on water and cleaning products and your customers will drive away happy.


Lee E. Kielblock is senior application engineer for Banner Engineering Corporation.