Check out a motion-control application and you’ll likely find a motor and position encoder. These ubiquitous transducers have long been the standard way of providing position feedback for servoloops.
It used to be the case that cost, resolution, and accuracy were the key yardsticks for assessing position encoders. That is beginning to change, however, as power drain has become an increasingly pressing concern. More and more positioning systems are in service within systems either powered by batteries, or where there is a spec dictating an overall energy efficiency goal. That puts the 50 to 100-mA current drain of most optical encoders under close scrutiny.
No surprise, then, that alternatives to traditional optical encoders are getting attention. One device in this category is the capacitive position encoder. Capacitive encoders now on the market operate down to 3.6 V — an advantage when operating from batteries — and draw only about 7 mA. In addition, they can be advantageous where the presence of dirt, mist, or other contamination render optical encoders unreliable.
One capacitive encoder now finding use in such areas comes from CUI Inc., Tualatin, Oreg. The technology was originally used in digital calipers and adapted to position encoding a few years ago. CUI says one point in its favor is that capacitive technology can be inherently less expensive than optical or magnetic encoders. The CUI encoder has also been incorporated as an option for position encoding in the product lineup of motor supplier MicroMo Electronics Inc., Clearwater, Fla. CUI encoders tend to get spec’d in where there is a need for ruggedness that might eliminate optical encoders with their glass disks, says MicroMo.
The capacitive encoder also incorporates features that simplify its assembly onto a motor shaft. A simple jumper change lets these encoders handle different resolutions, thanks to the use of the ASIC chip for most computation chores. MicroMo also makes use of the polycarbonate flange adapters that let the encoders fit 22 to 32-mm motor shafts.
It is interesting to examine the technology behind how the CUI capacitive transducer reads the position. It uses an encoder disk that modulates the capacitive coupling between electrodes on a transmitter disk and a receiver disk.
Thus depending on the rotary position of the disk, different parts of its conductive segments fully or partially couple signals between different parts of the transmitter and receiver boards. In this way, the relative amplitude of signals coupled from different parts of the transmitter board depend on the position of the disk and so serve as a means of measuring the shaft position. An ASIC uses this amplitude information to compute the sensor position for each measurement cycle. Additional computations on the ASIC create a continuous model of the rotational motion and derive quadrature signals similar to those from optical encoders.
To generate the transmitter signal, the encoder starts with a 10-MHz digital signal. It goes to a counter circuit that, together with other logic, produces a set of four digital patterns each displaced in phase by 90°. The digital signals each go to respective conductive areas on the transmitter. These areas are oriented such that they can capacitively couple to a conductive area on the receiver board as the disk rotates.
As the encoder disk moves, it effectively modulates the amplitude of the signal that the receiver picks up. This amplitude-modulated signal goes through a synchronous detector on the receiver. The detector strips away the 10-MHz carrier and leaves the amplitude modulation, resulting in a signal that is effectively sinusoidal at about 10 kHz. Its phase is a measure of the angular position of the encoder disk.
Other circuitry on the receiver detects the zero crossings of this signal. The resulting information is used to note the count at the time of the zero crossing. The count becomes a digital measure of the angular position of the encoder disk, and thus generates the digital representation of the shaft position.
The specially designed ASIC chip also adjusts the quadrature resolution among 16 different values from 2,048 to 48 ppr.
In depth: Capacitive-encoder internals
The transmitter signal is divided into four phase-shifted versions which each feed to a different electrode group. A receiver electrode picks up all four phases of the transmitter and adds them together after their relative strength has been modulated by the rotor disk which causes each phase to couple into the receiver and serves as a variable capacitor.
For example, when the rotor is in a position that gives phase one the strongest signal to the receiver, the receiver signal will have a phase position equal to transmitter phase one. When the rotor has moved so phase two is dominating, the receiver will be in phase with transmitter phase two. And so it goes with a linear transformation in space between phases.
The receiver decodes the signal beamed over by the transmitter through use of a synchronous demodulator followed by a low-pass filter. The demodulated receiver signal is a lowfrequency sinusoidal signal. The phase position of the resulting sinusoidal signal relative to the transmitter modulation follows the movement of the rotor.
The phase position of the demodulated receiver signal relative to the transmitter modulation is measured by letting the zero crossing of the signal trigger the reading of a counter that controls the transmitter modulation. This value represents the position of the rotor at that instant. A position-follower counter is used to get a continuous representation of the rotor position.
Its value is compared to the transmitter-control counter at the same instant. The difference (positive or negative) controls the frequency of a pulse generator that feeds the position-follower counter. By suitable filtering of this control loop, the position-follower counter follows the actual movement of the rotor with good accuracy.
The circuit creates the quadrature signals normally associated with position encoders by combining the two least-significant bits of that counter in a simple logic circuit. The position-follower counter is set up so its full count equals the number of counts for a full 360° turn of the encoder.
The position-follower counter is an up/down counter. When the encoder movement is forward, this counter counts up and its contents increase in value. When the encoder moves backwards, the counter reduces its count. This action lets the encoder keep track of whether the rotor is moving clockwise or counterclockwise. A quadrature circuit uses this information to generate A and B quadrature pulses, with the relative phase between them indicating the direction of shaft rotation.