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.

Analog
The overwhelming majority of linear position sensors used in industrial automation are analog output.  Typically, analog-output sensors provide an output in the form of either a DC voltage or a DC current.  Common voltage output ranges include 0 to 10 Vdc and -10 to +10 Vdc.  Common current output ranges include 0 to 20 mA and 4 to 20 mA.

Pros:

• Ease of implementation – Analog inputs on industrial controllers are increasingly common and are now less expensive
• Ease of troubleshooting – Analog signals can be tested and verified using simple and readily available multimeters.  There is no need for specialized test equipment.

Cons:

• Susceptibility to electrical interference (noise) – The nature of analog position signals makes them susceptible to interference from noise-generating devices such as motors, drives, solenoids, etc.  Even with proper grounding and shielding, electrical noise can sometimes be an issue.  Ultimately, electrical interference can limit the effective resolution available from an analog position sensor.

Time-based Digital
Primarily implemented into linear position sensors based on the magnetostrictive principle, sensors with time-based digital outputs generate digital pulses, separated by time.  The time between pulses is directly proportional to the linear position being measured.  Common output formats include Start/Stop, Pulse-Width Modulated (PWM), and recirculated PWM.

Pros:

• Immunity from electrical interference – The use of differential line drivers to send and receive the digital pulses means that the signals are mostly immune from electrical interference.  Differential line drivers also allow for long (>1500 ft.) cable runs between the sensor and controller.
•  Lower cost – Compared to analog sensors of the same type, sensors with time-based digital outputs are often lower priced.  This is because the time-based digital signal is the native signal at the heart of these sensors,  so there is no need for additional processing electronics inside the sensor itself.

Cons:

• Somewhat specialized interface – Time-based digital sensors generally require a dedicated, purpose-built interface module in a PLC or motion controller.
• More difficult to troubleshoot – In some cases, specialized test equipment (such as an oscilloscope) may be required to test these types of sensor signals.

Serial Digital
Linear position sensors with serial digital outputs represent a fairly small segment of the industrial sensor market.  But the segment is expanding.  Serial digital protocols, such as Synchronous Serial Interface (SSI), DeviceNet, Ethernet, and Profibus, to name a few, bring a new level of sophistication and advanced capabilities to industrial linear position sensors.

Pros:

• Built-in diagnostics – Many serial data protocols allow for real-time status monitoring of the sensor.  In many cases, a dedicated bit (or bits) can be used to indicate an operational failure of the sensor.
• More information – In addition to position information, many serial digital sensors can also provide linear velocity feedback.  This added functionality can free up processor overhead by not requiring the controller to calculate velocity.

Cons:

• Expense – Because of the additional internal processing required, serial digital sensors are generally more costly than their analog or time-based digital counterparts.

So, whichoutput signal type is best?  There really isn’t an answer to this question.  Which output signal format you choose depends on what is most important in your application.  If simplicity and ease of troubleshooting top your list, then an analog sensor probably makes sense.  If you need sensor-level diagnostic capabilities, a serial digital sensor is probably the best choice.  If you need a low-cost sensor that will perform well in an electrically noisy environment, you may want to consider a time-based digital output.

But, at the end of the day, it’s nice to have choices.  For more information on linear position transducers, click here.