Demystifying transducers in fluid power

Share

Transducer is a mystical word. Saying it emotes feelings of the word transmogrify, which itself describes the magical transformation of an object or person. The word transducer was certainly a mystery to me when I was a fluid power neophyte. I knew a transducer was some sort of electrical device, but I didn’t know how it worked, or why it was used in fluid power.

More than a decade into my hydraulic career, I now take for granted my knowledge of transducers. Put simply, a transducer converts one type of physical property into an electrical output signal. Hydraulic pressure is converted to a variable voltage or amperage output, for example. How this occurs is where transducers require demystifying.

How a transducer works will depend primarily on the type of measurement being observed—and even then, you can still have various methods to turn that physical property into a usable electrical output. There are transducers to measure pressure, temperature, flow, fluid level, position and even water or particle contamination.

Pressure transducers
Pressure transducers are the most common types used in hydraulic systems. Other properties are easily monitored via analogue gauges or indicators, but pressure is dynamic in range and rate of change, requiring the accurate and responsive measurement that only an electronic device can provide. As well, pressure transducers are at near commodity pricing, placing them in the purchasing range of a high quality analogue gauge.

A pressure transducer is often a strain gauge, which measures changes in resistance, inductance or capacitance. Here, strain refers to the deformation of the metallic membrane, as its shape changes while exposed to the force of pressure. Electrical resistance is the expression of how much the conducting material resists the flow of electrons.

Inductance and capacitance are more difficult to describe, but we’ll try to demystify them briefly. Inductance describes the inertia of electrical current. A big coil of wire, for example, will resist changes in current flow, either increasing or decreasing. Inductors are less practical in modern electronics, mostly because of their size.

Capacitors are much more common than coils, although they work differently. A capacitor stores excess incoming electrical energy, ready to release it when downstream energy drops again. If this description sounds familiar, you’re thinking about hydraulic accumulators. They absorb incoming hydraulic energy, ready to blast it out when downstream pressure drops below accumulator pressure. Capacitors, just like accumulators, are capable of extremely high bursts of energy, and should be carefully applied.

Regardless of the electrical change caused by strain force, an active pressure transducer will use its strain gauge to modify incoming electrical voltage into a usable variable analogue output. You see, unlike some other electrical sensing devices, like a thermocouple or microphone, an electronic pressure transducer uses various electrical components, such as a wheatstone bridge, to create an accurate variable output. The electronics onboard these transducers require a power source, which is typically 12, 24 or 120 V, although some sensors will take a range of input, such as 8-30 V.

The wheatstone bridge common to most modern transducers uses either bonded strain gauges, or piezoelectric types, which are less common in hydraulic applications. In its signature diamond shape, the wheatstone bridge is a combination of resistors and strain gauges that accurately measures strain force, and therefore pressure. However, because of the harsh environmental conditions of a hydraulic system, pressure transducers require other electronics to improve their accuracy, repeatability and reliability.

If you look at the electrical circuit below, Figure 1, you can easily spot the wheatstone bridge. However, all the other componentry is there for other purposes. Part of it is to ensure changes in temperature (which changes the electrical resistance) and don’t affect accuracy. If the resistance of the wheatstone bridge changes, so does the pressure signal. Other parts of the circuit improve hysteresis. Hysteresis is kind of like electrical lag—where a voltage, for example, takes time to reduce again after it rises.

When all is said and done, a pressure transducer will take input electrical power, read its load cell and then output something that a PLC or digital display can use to extrapolate pressure. In most hydraulic applications, output signal can be variable voltage or variable amperage. Typical voltages are 0-5 and 0-10, and variable amperage outputs are 4-20 mA. The latter starts at 4 mA to avoid the dead zone interpreted as a failure or broken wire. In applications with long runs of wires, variable amperage can have an advantage in that it avoids background interference that creates voltage spikes.

What a 0-5, 0-10 or 4-20 output means depends on the pressure range of the transducer it’s installed in. Pressure can be vacuum to 10,000 psi or more, so the output signal must provide a linear representation of the desired scale. For example, if you have a 15-psi transducer on your filter’s bypass indicator, and your output is 0-5 V, a 1 V signal equals 3 pounds of pressure. A 2.5 V signal would equal a 7.5-psi level, and so on.

With a variable amp output, any signal less than or equal to 4 mA is considered 0 psi of pressure. If we now use a 3,000 psi transducer as an example, we must now do some simple math to tell our PLC what the incoming signal means. There is 16 mA worth of usable resolution to spread amongst the 3,000 psi of pressure we want to measure. With 4 mA being zero, and 20 mA being 3,000 psi, everywhere in between is just a factor of both. Each milliamp of energy translates to 187.5 psi of observed pressure, and even each fractional milliamp is further broken down. If your transducer has an accuracy of 0.5% (which high quality ones do), this means each 0.1 mA will accurate represent a change in 18.75 psi of pressure change.

Measuring more than just pressure
I’m picking on pressure in my examples, but it should be known a transducer can observe many physical properties and then provide an analogue output to suit. Temperature, flow, fluid level, position, water saturation and particle contamination are all common, as was mentioned earlier. Although most are still available with the same 0-5 V, 0-10 V or 4-20 mA output signals (or others such as CAN), how they arrive at those ranges can vary vastly.

A temperature transducer can read the changes in resistance as caused by a change in temperature, and it uses on-board electronics to improve accuracy, resolution and response time. However, a flow sensor uses a completely different principle altogether. The Hall effect uses a solid state pickup to measure the intensity of a magnetic field. An inline impeller can be made from magnetic material—which, as it spins, produces ever increasing voltage in the sensor, called the Hall Voltage. The downside of this sensor is the response time for the impeller to speed up and slow down, making it accurate for only steady-state flow. A flow sensor using a set of positive displacement gears (like a gear pump), is much more accurate, but is also much more costly.
One of the more interesting transducers is the particle contamination measuring transducer. Rather than use some sort of change in resistance, inductance or capacitance, it uses a laser to measure fluid passing across its orifice. As particles break the laser beam, the time and frequency of the broken beam tells the transducer the size and quantity of the particles passing by. As you can imagine, these units aren’t cheap, but they are a great technology that allows technicians the opportunity to measure fluid cleanliness on site.

As electronics advance, they actually continue to ensure that fluid power stays relevant. Productivity is rising more rapidly than what can be achieved with logsplitter-era technology, and we in the industry owe that to the sophisticated electronics permeating fluid power. Transducers are a piece of the electrification puzzle, and are key to the growth of fluid power as a whole.

 

Mobile Hydraulic Tips

Share