How does the AMT line of capacitive encoders work compared to standard optical encoders?
In an optical encoder, very small LEDs are used to transmit light through a disk with notches at specified intervals. These intervals determine the resolution of the encoder. Once the light passes through the notches, light sensitive receivers receive the signal and send it out as position data.
AMT encoders can be described in a similar manner; but instead of transmitting light via LEDs, an electrical field is transmitted. In the place of an optical disk is a PCB rotor containing a metal trace sinusoidal pattern that modulates the electrical field. The receiving end of the modulated signal is then passed back to the transmitter where it is compared against the original via a proprietary ASIC. Performance of the ASIC is boosted further in many AMT products with the coupling of an onboard MCU. Because of this design, the AMT capacitive encoder allows for a much higher range of accuracy and flexibility compared to other encoders on the market. The rotor PCB is designed for a high resolution of position. This high base resolution allows the ASIC and the MCU to computationally derive a wide range of final resolutions that the customer may select for their solution. Because the resolution selection occurs in the ASIC and MCU, the customer needs only to purchase one part number, yet receives a product capable of many resolutions.
Will I need to make changes to my system in order to switch from an optical to a capacitive encoder?
The outputs waveforms of an AMT capacitive encoder have the same characteristics of any optical encoder so the transition should be seamless. Every AMT encoder has quadrature A/B signals as well as an index pulse. These are 5 volt logic waveforms that are 90 degrees out of phase to indicate direction of rotation.
And in fact, an AMT capacitive encoder may require even less external circuitry than an optical encoder. Many encoders use an open-collector output which leaves the output transistor’s collector floating (neither high nor low). In order to operate the encoder you need a pull-up resistor to pull the signal to the supply voltage, and then the active output will pull the signal down to switch the output pulses on and off to create the expected square waves. All AMT capacitive encoders use a CMOS push-pull drive that does not require external circuitry to create the expected square waves. And the added benefit of a CMOS driven output is its compatibility with any type of input.
What types of encoders do you offer in the AMT line?
We currently offer a standard incremental encoder, an absolute encoder, and a commutation encoder for brushless dc motors. All encoders include quadrature A/B signals with index pulses. Encoders are offered in radial and axial configurations and with various line driver options.
How does the AMT series hold up against dust, dirt, and harsh environments?
The AMT encoder series is quite rugged thanks to the underlying technology. Optical encoders have two main drawbacks; durability and life expectancy. An optical disk is typically made of glass which must be handled very carefully and doesn’t hold up well to any sort of shock or vibration. Furthermore, the signal will be compromised if dirt or particulates get on the disk and interfere with the light signal. Additionally, LEDs inside an optical encoder are susceptible to burn-out over time.
AMT encoders have neither of these problems. Because they use capacitive technology, they have no need for LEDs or line of sight. As long as the PCB rotor is allowed to rotate freely, the electrical field will pass from the transmitting side to the receiving side regardless of any non-conductive particulate matter in between. This means that dust, dirt, grime, or pretty much anything else you may find in an industrial environment will have no effect on an AMT encoder.
Without the need for a glass disk that an optical encoder would use, the AMT encoder is much more durable. A drop, vibration, or shock has a reduced effect. Further information regarding shock and vibration testing is available on the AMT datasheets.
Will the AMT operate in fluids such as gear oil?
You can submerge and AMT encoder in a nonconductive fluid such as gear oil and it will operate just the same. Optical encoders do not work in oils because the light is blocked or distorted from the transmitting side to the receiving side. This failure is due to an optical encoder’s requirement for line of sight. Since the AMT capacitive encoder doesn’t use line of sight optics and relies on an electromagnetic field, it makes it ideal for motor or gear assemblies that require the entire unit to be sealed in oil. Many optical encoder manufacturers will offer a seal that can be used on the shaft to block oil from entering the encoder, but this solution introduces another potential failure from wear over time.
I am having issues using an AMT10 Series encoder with a stepper motor, why is that?
The AMT10 series encoder uses the same proprietary ASIC available on all AMT products, but is unassisted by any sort of MCU for absolute position tracking. To compensate for the lack of absolute position tracking, a Hall Effect sensor is used to create the encoder’s index pulse. Without the Hall Effect sensor, the encoder would output 8 index pulses per revolution. This is due to the sinusoidal waveforms etched onto the encoder’s rotor board; there are 8 wavelengths, or lambdas (λ). The AMT10 series encoder interprets its rotational movement based on the capacitive sensors with 8 cycles, or lambdas, per turn. To generate the index pulse, the Hall Effect signal is gated together with its nearest wavelength pulse to generate an index pulse once instead of eight times.
When the encoder is mounted to a motor with a strong stray magnetic field flux, it is possible for the index pulse to become compromised. If the strong magnetic field is greater than 40 Gauss on the motor (usually stepper motors), it could influence the Hall Effect sensor. This will affect the gating of the eight wavelength pulses and could either create more or fewer index pulses per revolution than are expected.
In this case, an AMT10 series encoder can be purchased without a magnetic Hall Effect sensor. In this configuration the index output from the encoder will occur 8 times per revolution, since there is no Hall Effect sensor gating, as shown above. As an alternative to the AMT10 series, the next-generation AMT11 series incremental encoder utilizes an onboard MCU for generating the index pulse; this has the added bonus of allowing the user to zero-set index pulse at any angular position.
I am having some problems with noise affecting my encoder signals; what should I do?
In some applications noise may leak into and AMT encoder via its cabling. To counter this noise the user should take care to ensure proper grounding at all locations. If noise is leaking in, it is suggested that the user try a shielded cable. The cable shielding can be grounded at the driver end, and then encoder ground can be tapped into and grounded at the motor end. If the motor chassis needs to be isolated from the driver ground, the encoder may be capacitively grounded via a small electrolytic capacitor. If noise persists, CUI offers versions with a differential line driver, ideal for noisy environments and long cabling distances. Currently the AMT11 and AMT31 series encoders are both offered in various differential line driver output configurations for incremental and commutation outputs.
Can I use an adhesive to mount an AMT encoder instead of screws?
If you are mounting an AMT encoder in a location that cannot be drilled into an adhesive can be used. Any sort of glue or epoxy that is meant for use with plastic will be sufficient. We do suggest taking some extra time to ensure proper alignment during installation with this method however since it is not easily undone and adjusted.
You mentioned previously that the encoder is set at one base resolution, but how can I select my final resolution from the large range of options?
AMT10 Series Encoders:
Each AMT10 encoder has a 4 position DIP switch on the backside. This allows the user to select from 16 resolutions just by adjusting which switches are on. See the AMT10 series datasheet for the table of resolution options.
AMT20 Series Encoders:
This encoder’s resolution is programmable via SPI or more easily with the AMT20 Demo Board. Setting the encoder resolution via the demo board is a simple as a few button presses. For more information on using SPI with the AMT20 Series encoder, see the AMT20 Series datasheet.
AMT11 and AMT31 Series Encoders:
Programming the AMT11 and AMT31 series encoders is a very simple task using the AMT Insight™ PC software. With the AMT Insight™ application you can easily select your incremental resolution, as well as the motor pole pairs and rotation direction for AMT31 commutation encoders. Not sure what resolution you need? The AMT Insight™ application will help you decide with waveform plots and calculated pulse width values that automatically adjust when you select a resolution. An AMT programming cable is required.
All AMT encoders can be custom ordered preconfigured to any available resolution.
How is position accuracy determined?
When an encoder is rotating at near constant speed the output has no error. This is due to a design that incorporates what is best described as a moving average. This average helps stabilize by reducing the number of samples required to generate output. As the shaft rotates the current position is sampled at a designated rate and the speed is derived each time. This current velocity is stored into a register in the ASIC and is used to produce the output signals. As the encoder moves as constant speed, there will be no error…but if acceleration is introduced, this causes a slight latency in the time it takes the encoder to sample and recalculate velocity. The value caused by this latency is expressed as the position accuracy and is measured in degrees. As the ASIC measures changes in speed, it also measures the previous error value and uses that to compensate in the next correction that it makes. All of this happens very fast; the accuracy for AMT encoders is typically only around ±0.2 degrees; see encoder datasheets for exact numbers.