Use of electronics and capacitive-based position sensing have transformed the caliper, a fundamental and essential instrument of precision linear-dimension measurement.
Part 1 of this FAQ looked briefly at the basic mechanical, Vernier, and dial-readout calipers, as well as the micrometer. All these mechanical-only embodiments of the caliper were made obsolete within a few years with the development of the electronic caliper with a digital readout, Figure 1.
Q: How was the digital caliper developed?
A: Its invention is largely due to one man, Ingvar Andermo, an electrical engineer at the IM Research Institute in Stockholm, who was working on a bill-reading application using capacitive-sensing technology.
Q: What does that have to do with metrology and calipers?
A: The C.E. Johansson company asked Andermo to develop a digital caliper using sensing based on magnetorestrictive principles, but he thought that approach was too complicated and decided to use his experience with capacitive sensing instead. Although capacitive sensing is widely used for on/off touch switches in “open” or public applications such as elevators, he adapted and extended it to precise continuous-motion linear-position sensing.
Q: How does capacitive-touch sensing work?
A: In brief, in touch sensing, the user’s finger acts as the second plate of a capacitor which is connected to an interface circuit. The change in capacitance which occurs when the finger touches that second plate is sensed by the circuit which then generates a trigger signal, thus emulating the function of a traditional electromechanical pushbutton. Instead of a finger, another conductive surface can be used — and it is the basis for the caliper’s implementation.
Q: What are the virtues of capacitive touch sensing?
A: Touch switches are resistant to dirt, water, and overall abuse since they have no direct-contact parts and their only exposed portion is a small metal tab which is flush with the mounting surface. This technology is used in many appliances, often as adjuncts to touch screens. Many IC vendors offer easy-to-apply components specifically for this application.
Q: I understand capacitive sensing for touch switches, but how does that relate to digital calipers?
A: For the caliper, capacitive sensing goes beyond the basic on/off switch. There is an etched copper pattern under the protective plastic top layer, and the part that slides also has a circuit board with a similar pattern. The resultant “sandwich” two conductive plates and a dielectric separating layer (also plastic) looks electrically like a grid of capacitors. As the comb slides over the copper pattern, the capacitance between the plates changes in a highly predictable, known manner.
Q: I still don’t see the relationship here, can you explain it further?
A: There is analog interface circuitry containing a timer (oscillator) with its frequency set by a resistor/capacitor (RC) time constant, and changes in capacitance change that frequency. These minute changes in capacitance are measured by a frequency-to-voltage converter; the resultant voltage is directly related to the caliper’s position, Figure 2.
Q: When was this commercially applied?
A: The first Johansson Caliper (Jocal), was shown at a Chicago exhibition in 1980. Johansson then licensed the technology to Mitutoyo Corp. of Japan, who introduced a digital caliper based on this technology some years later. (In 1986 Andermo founded Micro Encoder Inc. to do work with Mitutoyo to further develop the caliper’s encoder technology for use in the field of dimensional metrology.)
Q: How does that translate the motion?
A: As the rotor turns, its pattern modulates the high-frequency signal in a predictable fashion. The receiver board reads these modulations and digital circuitry then translates them into readouts of rotary motion increments with resolution of up to 4,096 steps/turn, which is needed for high-performance motor-positioning and -speed control.
Q: Did the digital caliper “take off” in the market?
A: It did so absolutely and quickly, with millions of digital calipers sold worldwide, priced in the range of $25 to $100 depending on materials, body stiffness, and length. They are widely available to contractors and machine shops as well as ordinary hobbyists and homeowners through hardware, home improvement, and industrial/scientific/engineering suppliers.
Q: Is this implementation of capacitive-sensing technology limited to calipers?
A: No, it goes far beyond digital caliper. Andermo worked with CUI Inc., (Tualatin, Oregon) to develop capacitive rotary encoders for determining shaft rotary angle (position), using the same underlying technology as the caliper. These encoders have three parts: a high-frequency transmitter, a rotor etched with a sinusoidal-metal pattern, and a receiver board, Figure 3. The rotor sits between the transmitter and receiver boards.
It has also been adapted to digital-readout micrometers, Figure 4.
Q: What are some of the other advantages of the digital caliper?
A: In addition to accuracy, precision, and ease of reading, it can be switched between English and metric units via a simple pushbutton – very handy but not essential. More useful, it can be set to indicate a “zero point” at any position along its length, so it is easy to make relative-difference measurements in addition to absolute readings from the mechanical zero point.
Q: Can the digital caliper be “interfaced” with data-acquisition systems?
A: Yes, some models include an output-port connector so the readings can be automatically uploaded to a computer for data collection and analysis using one of several industry-defined data formats. Wireless units using Bluetooth and BLE (Bluetooth Low Energy) are now available as well.
Q: Are digital calipers power-hungry, given their physical principle and required circuitry?
A: Not at all. They typically use a single, small button cell which can last for a few years due to their very low-power design and ICs for both the analog sensor front-end and the digital processing/readout sections. Also, when not in use, the caliper goes into a deep-sleep, microamp range power-down mode yet wakes itself up within milliseconds when the slider is moved.
Q: Where does this leave the old-fashioned mechanical caliper, Vernier caliper, and dial-readout caliper?
A: Due to the digital caliper’s excellent performance, ease of use, and low price, the non-electronic calipers are largely obsolete (although they are still sold by metrology suppliers). Other than personal preference, there are few situations where the mechanical unit would be the preferred choice, except perhaps in unique situations where no electronics are allowed, or the battery has “died” and no replacement is available.
This FAQ has examined how low-cost, low-power, high-precision sensors and electronics, along with clever use of basic principles of physics, have resulted in a metrology device which has supplanted its mechanical predecessor, which was costlier and more difficult to use. This is a scenario we have seen many times, as electronics and innovation support each other to provide radically new approaches to solving long-standing application requirements.
- “A Brief History of the Micrometer,” (Mitutoyo)
- “Tech Essential: Outside Micrometers” (MSC Industrial Direct Co.)
- “How to read a vernier (caliper), ” (Autodesk, Inc.)
- “Small Tool Instruments,” (Mitutoyo)
- “Pierre Vernier,” MacTutor History of Mathematics (School of Mathematics and Statistics, University of St Andrews, Scotland)
- FDC1004: Basics of Capacitive Sensing and Applications
- The Advantages of Capacitive vs. Optical Encoders