Are you choosing a position sensor for motor control that will operate in a dirty, high vibration or otherwise harsh environment?
In this second part of our series on position sensing in motor control, we’re looking at the impact of challenging conditions on the cost of your sensor system and the advantages of inductive position sensors in these situations.
Many environments create challenges for position or speed sensing:
An added challenge for engineers is that position sensors for motors in harsh environments are often performing important tasks; failures matter, commercially and technically. Although it’s tempting to specify the highest performance sensor available and move onto the next challenge, engineers need to be aware of extra costs associated with different types of sensor: sometimes high performance comes with high, and hidden, costs.
The table below sums up the situation for absolute linear position sensors (and the situation is similar for absolute rotary position sensors).
|Potentiometer||Magnetostrictive||Optical Encoder||LVDT||Resonant Inductive Position Sensor|
Works in dust or dirt without seals
|Tolerant to misalignment|
|Operates at big gaps|
|Short inactive zones|
Potentiometers are a common contact position sensing solution but the contacts tend to wear over time, so they are seldom chosen for harsh or critical environments.
This leaves engineers with options for non contact position sensors, such as optical encoders or magnetic effect sensors (Hall effect is typical for rotary position sensors, magnetostrictive or a Hall sensor array for linear position sensors).
Optical encoders, although they can offer high accuracy and resolution, suffer in any environment where dirt, dust, swarf or moisture can affect the optical path. This has a particularly high impact on higher resolution encoders because the feature sizes are similar to dust particles or water droplets. We’ll dig into the implications of this further down the page.
For situations that don’t call for high resolution position sensing, magnetic effect sensors are a popular solution. But nothing is perfect and there are some drawbacks that aren’t typically mentioned in data sheets. Over time the magnet can attract swarf (metallic particles and dust in engine oil) which can interfere with operation. These solutions are also sensitive to shock and vibration, both because magnets are brittle and because alignment of the sensing elements is particularly important.
In higher budget applications LVDTs and RVDTs offer high levels of accuracy and immunity to dust and dirt. Such accuracy does come at the price of expensive manufacturing (precision coil winding is never cheap) and heavy, bulky packaging which makes it these an unattractive solution for higher volume or smaller footprint designs.
Of course, many of these solutions can be made to work in extreme environments. Additional housing, seals and bearings can go a long way to improving sensor performance, although these elements come at a cost:
An optical encoder requires a clear optical path to work reliably, and optical emitter, receiver and code disc must remain free from dust, dirt and moisture to operate reliably. This means that an enclosure is needed when the product environment is anything except clean. Since optical encoders require precise alignment of the optical path, the sensing parts are usually housed together with bearings. Bearings do not seal out dirt and moisture, so seals are added for operation when these contaminants are present. Finally, a coupling is usually added between the product’s rotation axis (e.g. motor shaft) and the optical encoder to avoid “fighting” between them due to misalignments.
LVDTs are better suited to harsh environments, particularly in aerospace applications experiencing vibration and temperature extremes. Their operating principle is inductive rather than optical, meaning they are fundamentally more tolerant of dirt and moisture. However they need sliding bearings, which are themselves susceptible to dirt. It is also difficult to seal the sensing coils due to the small gap required between coils and moving element.
Hall Effect encoders are also fundamentally better suited to dirty environments than optical encoders due to their magnetic operating principle, which is not susceptible to moisture and dirt. However they require careful alignment between moving magnet and sensor chip to obtain specified performance, particularly in the presence of vibration, and this is usually achieved by adding bearings.
When a Hall Effect Encoder based sensor module includes bearings, seals and couplings are required when the product environment becomes harsh. They are particularly important for operation when metal swarf might be encountered, which might otherwise be attracted to the magnet and cause faulty operation and wear. Metal swarf can be produced from a variety of sources, not just machining, for example wear of moving parts inside machines.
CambridgeIC’s resonant inductive technology is unique among these position sensing technologies, because its components can be individually sealed and tolerate so much misalignment that bearings, seals and couplings are not required. Sensor elements are built from PCBs, which are robust and cost effective to manufacture. They usually do not require any protection when used inside products sharing the same space as other electronics. Condensing moisture requires only a conformal coating. Operation through metal swarf is normally achieved by mounting the sensor PCB behind plastic.
Since the technology can operate with big gaps between sensor and target, moulded plastic thicknesses of 1mm are practical. Sensors detect the position of a moving target which comprises an inductively coupled resonator. CambridgeIC’s Standard Target is a sealed part, and other available targets are easy to seal behind plastic as required.
At extremes of environmental harshness, CambridgeIC’s resonant inductive technology is used in underwater applications. CambridgeIC sensors, targets and processor chips are immersed in oil to exclude air pockets, and experience the full pressure of deep underwater operation. Tolerance of misalignment and big gaps means that mechanical complexity is minimized, so that costs are kept in control and reliability is assured.
This intrinsic robustness not only bears down on the costs of the whole solution, but also opens up a range of other opportunities:
We hope this post has shed some lights on the issues that drive position sensor choices. Remember, these are called challenging environments for a reason; perfect solutions don’t exist.
So if you’re still uncertain about the best choice for your project, please do get in touch with our engineers for a practical discussion of the best solutions available.
This is part 2 of a short series on contactless position sensors. In part 3 we’re going to be looking at some of the challenges raised where you need to measure position on multiple axes. If this is useful for you but you don’t want to keep checking back, sign up below for our regular round up. Sent out quarterly, it involves latest product news, along with hints and tips from engineers working with CambridgeIC products.