Do Photoelectric Sensors Really Measure Up?


I recently spent the week at the IMTS show in Chicago. If you have never been to this show I highly recommend you go in 2012. The show occupied over 1.1 million square feet and there were over 1700 manufacturers and or vendors. If you do attend this show I think, you really need take at least two days to go through it. No matter how comfortable you think your shoes are I guarantee that your feet will hurt after going through the four buildings of exhibits.

While I was working at our booth, I had several customers ask me about using photoelectric sensors in measuring applications. The questions ranged from the basic how they work, what are some of the application concerns to the infamous what is the accuracy of the sensor.

Here are some of the highlights to summarize our conversations:

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Analog Signals: 0 to 10V vs. 4 to 20 mA

In the world of linear position sensors, analog reigns supreme. Sure there are all kinds of other sensor interface types available: digital start/stop, synchronous serial interface, various flavors of fieldbus, and so on. But linear position sensors with analog outputs still account for probably two-thirds of all linear position sensors sold.

When choosing an analog-output position sensor, your choice generally comes down to analog voltage (e.g., 0…10 V), or analog current (e.g., 4…20 mA). So which should you choose?

0…10V versus 4…20 mA

When it comes to sensor interface signals, 0…10V is like vanilla ice creamr. It’s nothing fancy, but it gets the job done.  It’s common, it’s straightforward, it’s easy to troubleshoot, and nearly every industrial controller on the planet will accept a 0…10V sensor signal. However, there are some downsides. All analog signals are susceptible to electrical interference, and a 0…10V signal is certainly no exception. Devices such as motors, relays, and “noisy” power supplies can induce voltages onto signal lines that can degrade the 0…10V sensor signal.  Also, a 0…10V signal is susceptible to voltage drops caused by wire resistance, especially over long cable runs.

A 4…20 mA signal, on the other hand, offers increased immunity to both electrical interference and signal loss over long cable runs. And most newer industrial controllers will accept current signals. As an added bonus, a 4…20 mA signal provides inherent error condition detection since the signal, even at its lowest value, is still active. Even at the extreme low end, or “zero” position, the sensor is still providing a 4 mA signal. If the value ever goes to 0 mA, something is wrong.  The same can not be said for a 0…10V sensor.  Zero volts could mean zero position, or it could mean that your sensor has ceased to function.

In some cases, 4…20 mA sensors can be slightly more costly compared to 0…10V sensors. But the cost difference is becoming smaller as more sensor types incorporate current-output capability.

For more information on linear position sensors, click here.

Linear Position Sensor Output Types – Which do I choose?

Linear position sensors are available with a variety of different output signal types to suit various application requirements and control architectures.  Let’s take a look at three of the most common output signal types for linear position sensors; 1) analog, 2) time-based digital, and 3) serial digital, and discuss some of the pros and cons of each.

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