Wireless Torque Sensors

Making a Difficult Task Easy

Wireless torque sensors are doing the seemingly impossible, forcing a complete turnaround in the way we can now get power to, and a torque signal from a sensor attached to a rapidly rotating shaft.

When measuring torque on rotating shafts, the old ways just aren’t up to it... wires are impossible and slip rings impractical.

Using wires is OK, but only if the shaft has to turn through one circle and then back again… not if it has to turn continuously... the wires would feel the strain after the first couple of rotations!

Now, wireless torque sensing is totally taking over from the older technology where slip rings, used to transfer power and torque signals, would often wear out or become unreliable if exposed to dirt and grease.

There are two main types of wireless torque sensors.

• In-line
• Clamp-on

Here’s a quick description on what torque is, but if you know already, just skip to the next heading.

Torque is a turning force

You probably already know that the longer the handle, the easier it is to turn

That’s why the lever was invented. The longer the lever, the more turning force (torque) you can apply. Torque is just force times the length of the lever. Double the length of the lever and you get twice the torque.

Torque is usually measured using strain gauges.

Find out about strain measurement

A shaft that’s used to transfer the torque is a convenient place to attach wireless torque sensors and carry out a torque measurement.

Car manufacturers specify the torque from their engines. This tells you how much turning force is available to turn the wheels.

Many vehicles now use torque sensors to sense the speed difference between the two rear wheels and adjust a set of helical gears to minimize the effect.

• Is it easy to fit?
  
  

  Simplicity means that less can go wrong and the measurement should be of better quality (other things being equal). And it doesn’t take long.





• Are there any limits?
  
  

  What’s the fastest the shaft will rotate? How hot (or cold) can the equipment get before it malfunctions. Check the operational temperature range in the manufacturer’s specification. If the spec says it won’t work below 0oC, and your sawmill needs to operate throughout the winter at temperatures below this, then look for another product with a wider temperature spec.





• Is it resistant to moisture and dirt?
  
  

  To find out, check the IP (Ingress Protection) rating. IP67 is good. In this case, the first number, 6, means it's dustproof. The 7 means you should be able to immerse it in shallow water without causing any problems. More on IP rating -link coming.





• Does it comply with a quality standard?
  
  

  If a product complies with an international standard such as ISO9001 for design and manufacture, that’s good. It doesn’t guarantee the product will be perfect, but does mean that the processes involved in its design and manufacture at least meet certain minimum standards…and that’s a great start…certainly significantly better than not having it. It also means that if you have any problems, you can tell the company’s Quality Assurance department and they will need to respond formally…and yes, these responses are audited by an independent authority.




Find out more about ISO9001



• Can you be sure the torque is really what it says it is?
  
  

  The way to check this is to find its calibration traceability, for example back to a NIST standard. Is it current?




Find out more about NIST



• Does the manufacturer recommend a calibration check period?
  
  

  Most sensors will drift over time and need to be checked regularly against an appropriate standard by an independent authority. They take the sensor and subject it to a known torque, then compare the numbers coming out of your sensor with what it should be. If they don’t agree, they’ll give you some corrections to put into your system. This is an extremely important check. You get a calibration certificate and this is the proof that your wireless torque measurement has been regularly verified.





• No moving parts that can wear out.
  
  

  No friction, no mechanical failure. A nice situation to be in, but of course we’d expect that in the 21st century!





• Will it be accurate enough?
  
  

  If you need your torque measurements to be accurate to say within 5% then you need to ensure that the accuracy, specified by the manufacturer, is better than this.





• Make sure that the torque measurement range is right.
  
  

  Find one that matches the equipment you want to measure. This may be in different units depending on the country of origin of the equipment. Often it’s specified in Newton metres (Nm) where the m is how long the lever is and the N is how hard you have to push to turn it.





• Is the accuracy affected by temperature?
  
  

  If it’s operating in a hot or cold environment, does the temperature extreme affect the measurement...making it look like an apparent increase or decrease in torque...when in fact it isn’t!

The clamp-on sensors are superior to the in-line sensors. The in-line sensors are not the best option.

Because the shaft needs to be cut and the sensor inserted, making the whole transmission chain longer and the installation rather permanent! And if the shaft ever needs to be replaced...!

We consider the clamp-on wireless torque sensors the best solution for the following reasons:

• Easy to fit.
  
  

  Just clamp them around the shaft and do up the fasteners. You may introduce some pre-load offset when you do up the fasteners, but the manufacturer should tell you how to null this out. It’s no big deal. If it’s reading zero when the shaft isn’t turning, it means it’s properly zeroed.

  
  

• Easy to relocate.
  
  

  If you want to measure the torque on several machines, simply remove the clamp-on sensor and relocate it to the next machine. It takes very little time. In this way, a single wireless torque sensor can be rotated around many different machines.

  
  

• Easy to calibrate.
  
  

  Simply remove the clamp-on wireless torque sensor components and sent them to someone who has the authority to carry out a calibration check.

  
  

• No moving parts.
  
  

  Well, OK the sensor will move, but this is OK...there’s no friction, so no wear and tear...unlike when you use slip rings.

  
  

• Resistant to dirt and oil.
  
  

  These are well sealed and can’t easily be affected by contaminants that might degrade the torque measurements.

  
  

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