Technical
Article
How to build a potentiometer with familiar outputs – and
unfamiliar qualities
Norwood Brown
The classic potentiometer (pot) has been used successfully in electronics system designs for decades: its strengths and weaknesses are very well understood. But what if the pot were able to be reinvented? Which characteristics would the design engineer choose to change?
Imagine, for instance, a pot that can be built into a system without concern for the rotational position
of the shaft during assembly, because the zero and maximum positions can be programmed after
assembly.
Imagine a pot that can produce a full dynamic-range output, no matter the zero-point and maximum
rotational angle required.
What about a pot which offers ±0.28% accuracy over a full 360° rotation, at 12-bit resolution?
Or a pot that can automatically go into a low-power mode when not in use, and automatically wake
upon detecting movement?
The wish-list might also include reliability: this would mean a pot in which the only drag and wear
mechanism is the bushing or bearing supporting the shaft. A pot which does not become oxidized,
does not vary its output over temperature, is undisturbed by liquid, dirt, dust and other environmental contaminants. A pot that does not degrade over time or in harsh operating conditions.
Imagine, as well, a pot which will allow continuous rotation through, and beyond, 360°, and maintain
contiguous output.
A pot which will work with a microcontroller that has no A/D input. And which has integrated diagnostic capabilities to support users’ maintenance processes and thus help to prevent unpredictable,
catastrophic failures.
Of course, this fictional potentiometer should share the familiar physical geometries and electrical
interfaces of a normal pot, so that it can be a direct drop-in replacement.
Pure imagination? Not exactly. As this article will show, a new sensor IC from ams makes it simple
to design and build a pot replacement with exactly the properties of the designer’s ideal pot, and
with an existing pot’s familiar ratiometric output so that the host application needs no modification at
all.
The potentiometer: new thinking about an old technology?
The imaginary potentiometer described above has very different characteristics from those of existing potentiometers. So why does the traditional pot, which boasts none of these characteristics, still
find its way into so many designs?
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A long history in the market is the traditional pot’s main advantage: design engineers are used to
handling the potentiometer’s ratiometric outputs, and can often re-use existing application code for
processing a pot’s outputs in the implementation of new designs.
This traditional potentiometer is a complex assembly which includes a resistive element, a wiper
and multiple mechanical components (see Figure 1). Its output can be a resistive divider, or a variable single-ended resistance.
Most commonly, it is used as a resistive divider to generate a voltage which is ratiometric to the rotational position of the pot shaft.
Fig. 1: [left] the traditional potentiometer contains many mechanical and electrical components
Fig. 2: [centre] Operational diagram of the traditional pot vs. the AS5600 solution
Fig. 3: [right] an AS5600 magnetic position sensor senses the rotation of a simple two-pole magnet
A ‘solid-state potentiometer’
As is so often the case in the field of electronics, the means to improve aging electro-mechanical
device designs are found in solid-state technology.
In many categories of electronics devices, a solid-state part, when used to replace an electro-mechanical equivalent, is:
•
more robust and reliable
•
more tolerant of shock and vibration
•
highly dynamically configurable
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Technical
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•
able to provide better management of the risk of failure
By thinking anew about the traditional pot, ams has provided a way for design engineers to implement a solid-state alternative with the introduction of a contactless, magnetic position sensor IC
(MPS), the AS5600.
This IC allows designers to build a contactless (magnetically coupled) potentiometer solution with
an IC and a magnet (see Figures 2 and 3). It provides the pot’s familiar ratiometric output, so that it
may operate with existing application software, while offering far greater longevity, reliability and
ease of implementation. This new ‘potentiometer IC’ also adds useful diagnostic capabilities.
Easy to mount at the exposed end of any rotating shaft or element, a solution based on the AS5600
MPS can be made to mimic the physical and electrical operation of a traditional potentiometer, as
well as providing the means to create new kinds of rotary sensors in non-traditional form factors
which better fit the designer’s mechanical solution.
A simple, non-contacting assembly
The AS5600 measures the rotational displacement of a magnet which is suspended over the top (or
bottom) surface of the IC. The magnet is affixed to the end of the rotating element (shaft, hingepoint, etc.), and is centered over the IC, which produces an output that is proportional to the rotational position of the magnet relative to the device, ratiometric to the IC’s supply voltage
The analogue output is electrically equivalent to that of a classic voltage-divider potentiometer. Its
resolution is 12 bits over the selected zero-point-to-maximum-point range. The point of maximum
rotation can be as small as 18°.
The device’s accuracy is rated at ±0.28% over a full 360° turn.
The AS5600 is also capable of producing a PWM-encoded output, or can provide a direct 12-bit position measurement via I2C.
Since it is a contactless, solid-state system, it suffers from none of the drawbacks that bedevil the
conventional potentiometer:
•
The only wear mechanism for the AS5600 pot replacement is the shaft bushing or bearing.
And this is a wear mechanism for the host device itself – it is not an additional mechanical
component introduced only to support the pot replacement.
•
The AS5600-based solution is unaffected by liquids, dirt, dust or other non-ferrous contaminants. This means that a fully sealed system may be assembled with the shaft and magnet
positioned outside the sealed envelope, eliminating the shaft/body seals required to protect
the electronic elements in an equivalent system based on a standard potentiometer.
•
The device may be in-line washed during the assembly process.
•
The AS5600 offers four power modes with different sampling frequencies, enabling the designer to reduce the average power requirement when application conditions allow. The device also offers an automatic low-power mode.
•
The AS5600 provides integrated fault indicators.
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•
The AS5600 readily allows unlimited rotation through angles greater than 360°, with no risk
of losing positional data, and no danger of causing mechanical damage to the sensor.
•
The AS5600 suffers no mechanical hysteresis when the direction of rotation changes, and
causes no drag or torque inconsistency in the movement of the shaft.
A system based on the AS5600 can provide advanced safety and operational information, enabling
the designer to intelligently manage system function in the event of variations in the expected operation of the (magnetically coupled) moving-element engagement.
Implementing a new-generation potentiometer
A design replacing a conventional potentiometer with an AS5600-based solution can be accomplished in a straightforward mechanical arrangement with a minimum of assembly components. The
solution can be accomplished in the same or even a smaller footprint than that of a traditional pot.
The AS5600-based potentiometer design calls for a diametrically magnetized magnet which produces a field strength as specified in the device’s datasheet. Such magnets are readily available at
low cost for solutions of this type.
The magnet will be mounted on the end of the rotating shaft. If this element is ferrous or magnetic,
the magnet may be mounted in a non-magnetic holder to isolate it. Then the rotating element is
placed into position over the proper location on the AS5600, and is secured so that x, y positioning
is stable. End-play is controlled to maintain the proper magnetic field strength at the AS5600.
Differential field measurement technology, developed and patented by ams, makes the AS5600
(and other MPS devices from ams) immune to stray magnetic fields, and allows designs to be implemented without any external magnetic shielding.
From an electrical point of view, implementing an AS5600-based potentiometer is extremely simple.
Figure 4 shows a complete – not a simplified – circuit example.
Fig. 4: [left] schematic of a typical application circuit using an AS5600 to implement a contactless
potentiometer
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Fig. 4: [right] an evaluation board from ams for the AS5600 to be mounted parallel to the face of a
rotating circular diametrically magnetized magnet (AS5600-SO_EK_AB)
The device can be mounted on a small PCB, together with a single bypass capacitor, and it can operate from either a 3.3V or a 5V power supply. The mechanical assembly (see example in Figure 5)
should be designed to ensure that the shaft remains centered over the AS5600, that the face of the
magnet remains parallel to the surface of the IC, and that the magnetic field at the IC remains within
the specified range (the device can provide diagnostic data to confirm the field strength at any time).
A common air gap between the magnet and the surface of the IC is in the range 0.5-1.5mm, based
on a standard, recommended magnet.
It is notable that this air gap can vary during operation with no effect on the accuracy of the measurements, provided the magnetic field strength remains within the specified limits. The required air
gap is dependent on the diameter and strength of the shaft-mounted magnet.
Fig. 5: a traditional style assembly for a rotating knob using the AS5600 is far simpler than the
equivalent device based on a conventional potentiometer (AS5600-SO_RD_ST)
The AS5600 can be permanently programmed for start and end (zero and maximum) positions and
other operating characteristics after mounting, without a dedicated programming unit. This greatly
simplifies the mechanical assembly and configuration of the complete unit.
And remember: Figure 5 shows the AS5600 and the rotating magnet configured exactly like a traditional potentiometer. But the implementation may be mechanically structured to specifically match
the target system, rather than having to force the system to match the conventional physical pot design.
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Conclusion
The AS5600 provides an easy means to implement a magnetic position sensing potentiometer
which offers the familiar ratiometric output of a classic potentiometer complemented by selectable
digital interfaces, while providing much greater robustness, reliability, configurability and manufacturability along with diagnostic capabilities. For the first time, design engineers now have a way to
build a potentiometer that they can appreciate because of its quality, cost-to-quality ratio, reliability,
and performance alongside its long-appreciated functional convenience.
For more information on the AS5600, please visit:
https://www.youtube.com/watch?v=Dwc4hFgNP5I
Summary
The classic potentiometer (pot) has been used successfully in electronics system designs for decades: its strengths and weaknesses are very well understood. But what if the pot were able to be reinvented? Which characteristics would the design engineer choose to change?
This article describes how a new sensor IC from ams makes it simple to design and build a pot replacement with exactly the properties of the designer’s ideal pot, and with an existing pot’s familiar
ratiometric output so that the host application needs no modification at all.
This reinvented potentiometer offers great advantages over the traditional device:
•
it offers high-resolution, accurate position measurement over a full 360° rotation
•
it is easy to assemble in the end equipment, since its start and stop positions can be programmed after assembly
•
it is highly reliable, since – unlike a traditional pot – it does not become oxidized, does not
vary its output over temperature, and is undisturbed by liquid, dirt, dust and other environmental contaminants.
•
it will work with a microcontroller that has no A/D input
•
it has has integrated diagnostic capabilities to support users’ maintenance processes and
thus help to prevent unpredictable, catastrophic failures.
This article describes the main operating features of the rotary magnetic position sensor at the heart
of a digital potentiometer, and outlines the approach to the implementation of such a device based
on the AS5600 sensor from ams.
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Biography
Norwood Brown is a Staff Field Application Engineer at ams in the US. He joined ams in 2012 as a
Regional Field Application Engineer. Between 2000 and 2012 he held application engineering roles
at electronics component distributors in the US. Before this, Norwood held various design engineering and engineering management roles at a medical equipment OEM.
Norwood holds a Bachelor of Science degree in electrical engineering from the University of Alabama at Birmingham.
For further information
ams AG
Norwood Brown
Staff Field Application Engineer
Tel: +41 469 29 84 221
norwood.brown@ams.com
www.ams.com
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