Feature: Industrial electronics
Potentiometer basics Feedback voltage from a potentiometer typically changes from 0.5 to 4.5Vdc as the actuator extends. Potentiometers are generally rated in ohms of resistance across their entire range and on the shaft’s number of turns (revolutions) from zero to full coverage. Common units include 10k (ohms) at ten turns. As the actuator screw extends or retracts, there is a discrete
step change in resistance, which can be converted into linear positions. The more turns the potentiometer takes before it reaches its full resistance range at the end of the stroke, the more precise is its position measurement capability. A contact-based potentiometer reads these voltage changes
by gliding its wiper across wire-round resistance coils, counting each move from one turn to the next. Non-contact potentiometers read voltage changes as well, but do so with electromagnetic sensors instead of contacting wipers. The designer’s degree of flexibility in implanting contact or non- contact-based potentiometer position sensing depends largely on user requirements. Customer requirements for resolution and durability will influence the designer’s decision of which position measurement to use; see Figure 1.
Designing for high resolution With contact potentiometers, the designer’s primary option to increase resolution is to modify the gearing to correlate potentiometer turns with motor or actuator screw turns. Designers who need position feedback across a full long- stroke application might gear the potentiometer to turn less as it moves. For example, in a short-stroke application, if the customer required position information for only 40% of travel, they would sacrifice resolution because there would be less resistance change over the shorter travel time. Because the number of turns in the coil of a contact-
based architecture is physically limited, there are only so many step changes to count, limiting the resolution of the position information it can deliver. And once a manufacturer has configured it for a specific number of turns needed to achieve the desired resolution, the design is set. On the other hand, non-contact potentiometers enable
higher resolution because they do not rely on physical wipers or gearing. Like contact-based potentiometers, non- contact sensors determine position by recording resistance changes across the stroke length, but instead of getting this information from a physical wiper touching a coil, they use an electromagnetic sensor that detects pulse changes in the surrounding magnetic field. The sensor outputs this digital signal to a non-contact
potentiometer’s integrated circuit, which converts it to analogue signal for reading by the actuator’s onboard electronics, representing it as position. Because these signals don’t need to correlate with physical constraints, the non- contact potentiometer gives virtually infinite granularity in measuring stroke position.
Figure 2: By eliminating gearing and wipers, non-contact potentiometers better absorb shock and vibration from heavy-duty applications. These potentiometers, along with many other control and performance features, can be found in select linear actuators already on the market
Programmability translates to design flexibility This ability to record infinitesimally-small voltage changes without physical contact and manage them through a software interface makes resolution programmable. If the end user needs different resolution, the designer can change it at the interface. For instance, if the application requires a high resolution between 0.5 and 4.5V, they can program the system to accomplish that. If they didn’t need as much resolution, they might program it to go to 2.5V at the end of the travel – all achievable through software rather than physical gear reductions. Such programmability can be valuable, certainly in
applications needing stroke changes to compensate for different geometries. Actuators with contact potentiometers, for example, might return different feedback from different strokes, which would require dedicated gearing for each stroke length. With a programmable non-contact device, the designer can maintain the same feedback over the stroke, regardless of changes in geometry. A user can keep that consistency without having to modify the physical architecture. Non-contact potentiometers also make it easier for the
designer to program more complex movement profiles, for example seeking forward a few millimetres or making a small set of movements back and forth to fine tune into the desired position. Non-contact potentiometers can be valuable in any application
requiring the actuator to move repeatedly and consistently to an exact location, such as managing the flow gate in a seeding
www.electronicsworld.co.uk December/January 2023 35
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52
