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Welcome to the CUI Devices product spotlight on capacitive commutation encoders. This presentation will provide an overview of encoder technology and their function, including an introduction to the features and benefits of CUI Devices' AMT capacitive commutation encoder series.
An encoder is a device that senses mechanical motion. It translates motion such as speed, direction, and shaft angle into electrical signals. There are many different types of encoders. Most encoders generate square waves, making them ideal for use in digital circuits. In this presentation we will consider only rotary encoders although encoders are also available in linear configurations.
Inside a rotary encoder there is a disc fixed to a shaft that is free to rotate. On one side of the disc is a signal source, on the other side a receiver. As the disc turns, the signal source is alternately allowed to pass and be blocked. When the signal is passed through the disc, an output pulse is generated.
Detection of shaft direction is very useful and even critical to some applications. In a radio, the rotational direction of the volume knob tells the receiving circuit whether to increase or decrease the volume with each square wave. In automation equipment, the rotational direction is detected and other operations are initiated when a pre-set number of pulses for that direction has been achieved. An elaborate and sophisticated set of movements can be executed automatically to perform tasks like placing components on a pc board, welding seams in an automobile body, moving the flaps of a jumbo jet or just about anything that involves a set of precise motions.
Each pulse from Channel A or B increases the counter in a users system by one when the encoder is turning counter-clockwise and reduces by one for each pulse from the encoder when it is turning clockwise. The pulse count can be converted into distance based on the relationship between the shaft the encoder is coupled to and the mechanics that convert rotary encoder motion to linear travel.
Encoders can detect speed when output pulses are counted in a specified time span. The number of pulses in one revolution must also be known. In the equation below, S represents speed in revolutions per minute (RPM), C represents the number of pulses counted, PPR represents the encoder's number of pulses per revolution, and T represents the time interval in seconds during which the pulses were counted. The second equation shows that if 60 pulses are counted in a time interval of 10 seconds using a 250 PPR encoder, the shaft speed is 1.44 RPM.
The equation for calculating speed is:
S = (C/PPR) / (T/60)Therefore if 60 pulses were counted in 10 seconds from a 250 PPR encoder, the speed can be calculated as:
S = (60/250) / (10/60) = (0.24) / (0.1667) = 1.44 RPMAll of the counting, timing and calculations can be done electronically in real time and used to monitor or control speed.
Encoders can detect distance traveled based on the number of pulses counted. In most applications, rotary motion is converted to linear travel by mechanical components like pulleys, drive gears and friction wheels. In this illustration of a cutting table, if the diameter of the friction wheel and the PPR of the encoder are known, linear travel can be calculated. Pulse count to achieve desired linear travel can be calculated in a similar fashion to the diagram above for devices that use ball screws, gears or pulleys to convert rotary motion to linear travel.
In the equation, C is the number of encoder pulses counted, L is the desired cut length in inches, D is the friction wheel diameter in inches, and PPR is the total pulses per revolution of the encoder. The second equation is based on a desired cut length of 12". Assuming the friction wheel diameter is 8" and encoder PPR is 2000 we can calculate that 955 pulses must be counted to achieve a cut length of 12".
Quadrature decoding is a means of increasing the accuracy of the encoder by counting every state change from both channels in one cycle. Both channel A and channel B produce two state changes (switching between high and low) per square wave cycle. The quadrature decoder circuit (shown on the right) uses logic gates to detect a state change on both the A and B channels and it compares the current state to the previous one. If it senses that the encoder is moving forward, it will send out a pulse on either the Count Up output; and conversely if it senses backwards movement it will send a pulse on the Count Down output. All the user needs to do is keep track of how many pulses occur on which line to know how many increments the encoder has moved.
Commutation is the process of applying current to a motor so that a rotating magnetic field may be established. In the illustration, current is commutated to windings in the stator. The stator is on the perimeter of the motor and does not move. The permanent magnets in the rotor are attracted and/or repelled by the magnetic fields in the stator created as a result of precisely timed application of current through the commutation process.
The illustration above is very simple. Notice that poles in the stator are alternately energized with a polarity that magnetically attracts the rotor to move in a clockwise direction. In the real world, both attraction and repulsion are used to increase speed, power and efficiency.
Motors with brushes and commutator are inexpensive but require a lot more maintenance than brushless motors. The brushes erode and fail, create friction, impede current and decrease efficiency. The advent of motor controllers has driven a strong and steady market shift toward brushless motors. Brushless motors require another method of current commutation. Hall devices are cheap but not very accurate, and are usually used with a position encoder increasing cost and BOM count. Resolvers are very accurate and durable but very expensive. Encoders with commutation output have become dominant because they are very accurate, already required in many cases for position feedback, and the cost is competitive.
 Permanent Magnet |
 Brushless DC |
|
| Voltage | DC | AC, DC (Control) |
| Speed | 1,000-5,000 | 1,000-5,000 |
| Horsepower | Medium | High |
| Efficiency | 60%-70% | 65%-80% |
| Life | Medium | Very High |
| Maintenance | Medium | Very High |
| Noise | Medium | Very Quiet |
| Speed Regulation | Fair | Excellent |
| Starting Torque | Very High | Very High |
Different from BLDC (brushless dc motors), brush motors use only two connections instead of the three phases that BLDC motors use. In the case of a brush motor, only positive or negative dc voltage is applied. The voltage level, and polarity (positive or negative) dictate speed and direction.
In Fig. 1 the ends of the armature are mainly repelled by both permanent magnet poles and attracted to the opposite magnet poles they are moving toward.
In Fig. 2 the ends of the armature are equally repelled and attracted by both magnets. If the rotor does not have enough inertia to rotate beyond this point the motor is in danger of locking. For this reason 2 pole motors are not very popular and not often used.
In Fig. 3 the ends of the armature are mainly attracted by both magnets and barely repelled by either magnet. The polarity of the magnets in the rotor will change when the split in the commutator ring passes beyond the brushes. At that point the current will be reversed changing the south pole of the rotor to north, and the north pole of the rotor to south.
Brushless servomotors, including many stepper motors, have become dominant in the automation market over the last twenty years. Their high efficiency, low maintenance, declining acquisition cost and high reliability have resulted in broad market acceptance and utilization. The AMT31 and AMT33 series, with their commutation outputs, have been designed to mate with brushless motors.
The UVW channels generate square waves that are configured based on the number of poles in the brushless motor. A motor controller reads the UVW pulses and vectors current to the stator windings accordingly. In order to properly time the incidence of current to the stator windings correctly, the encoder and motor must both begin at the zero point.
Most brushless motors have a feature that locks the rotor in the zero position. Usually the commutation encoder must be aligned manually by sight, which can be a tedious and time-consuming process. The AMT31 and AMT33 series eliminate this procedure. When the motor rotor is in the zero position, the AMT31 and AMT33 series are mounted and zero position is instantly established via serial communication using CUI Devices' AMT Viewpoint™ graphical user interface or the One Touch Zero™ module.
Just as there are many methods of commutating current to a motor, there are many types of encoders that can perform that task.
Optical encoders with commutation output currently dominate the market, most often used in precision applications and built in to electronic devices to control motion.
Magnetic encoders with commutation output are often used in applications where there are extreme temperatures, high humidity or exposure to particulates or liquids.
Fiber optic commutation encoders are sometimes called 'explosion proof' and are used in applications where methane, propane, or other highly combustible gases are present.
The AMT31 and AMT33 are not recommended for explosion-proof applications but can withstand similar environmental factors as magnetic encoders and generally outperform optical encoders thanks to their proprietary capacitive technology.
The AMT series consists of three basic parts as shown in the image. The ac field transmitter emits a signal that is modulated by the metal pattern on the rotor as it turns. The sinusoidal metal pattern on the rotor creates a signal modulation that is repetitive and predictable. This occurs as a result of varying capacitive reactance between the signal generated by the transmitter and the metal on the rotor. The field receiver uses a proprietary ASIC to convert the modulated signal into output pulses that can be read by the same circuits used to receive optical encoder output.
If you have ever used digital calipers, you are already familiar with capacitive encoding. The code generation used in digital calipers for decades is the same technology built into the AMT. This capacitive code-generation technology has been shown to be reliable, accurate, economical and rugged enough to outlast other types of optical encoders.
Encoders are used is a range of industries and applications where motion feedback is required. Industrial, medical devices, security, robotics, automation, and renewable energy are just a few of the market sectors that use brushless dc motors mated with commutation encoders.
The AMT encoder platform is unique among modular, commutation encoders because it is able to combine durability, accuracy and efficiency in one solution. Thanks to its capacitive design, the AMT is not susceptible to environmental contaminants such as dirt, dust and oil that would disable a typical optical encoder. Additional advantages include the lack of an LED which can eventually fail, a wider temperature range, higher vibration tolerances, and very low current consumption. The digital nature of the design also allows for increased flexibility through programmability of various features, ultimately reducing assembly time and cost compared to other encoders. And, compared to magnetic encoders typically valued for their rugged performance, the AMT Series offers higher accuracy and stable performance under various temperature conditions.
With 9 shaft diameter options per encoder, the AMT31 and AMT33 series can easily mount to almost any brushless motor. Their low mass disc means virtually no additional backlash or increased moment of inertia, making them a more reliable component for measuring and controlling the motor. Their compact packages also allow for mounting in space-constrained applications.
Zero position is easily set by CUI Devices' AMT Viewpoint GUI or by the AMT One Touch Zero Module accessory. Installing a commutation optical encoder onto a brushless dc motor can be an iterative and time consuming process. The optical disk must be physically and precisely rotated to align with the correct motor windings. Once aligned, the assembly must then be checked via back EMF to ensure mounting accuracy. This process can take upwards of 15 minutes per motor. The AMT, being ASIC and MCU based, reduces this time consuming process to a few seconds via the One Touch Zero feature, saving time and cost during the manufacturing process.
With the encoder's disk built-in to the assembly, mounting an AMT encoder is very quick and easy. Take a moment to watch the videos below for detailed instructions on mounting an AMT encoder. With just a few durable pieces, the encoder mounts on a motor in seconds without the risk of damaging components like with optical encoders.
AMT31 Mounting Instruction Video
AMT33 Mounting Instruction Video
The AMT31 and AMT33 series kits each support 9 different motor shaft diameters for added design flexibility. Typical commutation encoders on the market today fit only one motor size per SKU. For example, if a manufacturer is utilizing motors with 2 mm, 5 mm and 8 mm shafts in their system, they must purchase three separate encoders. Offering several popular mounting patterns and nine shaft size options, the AMT31 and AMT33 series kits can fit all three applications under one SKU. With the ability to adapt to almost any application, the AMT31 and AMT33 are the most flexible commutation encoders on the market today.
|
Hole Pattern mm/in |
Number of Holes | Recommended Screw | AMT Series |
| Ø16/0.63 | 2 | M1.6 | AMT31 |
| Ø19.05/0.75 | 2 | #4 | AMT31 |
| Ø21.55/0.848 | 3 | M1.6 or M2 | AMT31 |
| Ø25.4/1.0 | 4 | M1.6 or M2 | AMT31 |
| Ø32.44/1.277 | 2 | #4 or M2.5 | AMT31 |
| Ø43.84/1.726 | 2 | #4 or M2.5 | AMT33 |
| Ø46.02/1.812 | 2 | #4 or M2.5 | AMT31 |
Thanks to the AMT's innovative design, CUI Devices is able to deliver an unprecedented level of visibility and control through the AMT Viewpoint™ Graphical User Interface. Via the simple to use software, users are able to set and control a range of parameters, including resolution, zero position, and pole count, reducing development time and virtually eliminating tedious steps in the assembly process. Additionally, the software allows engineers access to a range of diagnostic data for quick analysis during design or in the field.
Supporting 9 different shaft sizes from 2 mm to 8 mm, the AMT31 series generates standard U/V/W commutation signals for vectoring current to brushless motors. The encoder offers 22 programmable resolutions from 48 to 4096 PPR and can commutate 2, 4, 6, 8, 10, 12, or 20 pole motors. For environments with significant electrical noise or long cabling distances, line driver versions are available for quadrature outputs, commutation outputs, or both. With radial and axial orientations, the AMT31 further features an operating temperature range from -40 to 125°C, high accuracy, low current draw, and added resilience to environmental contaminants.
Supporting 9 different shaft sizes from 9 mm to 15.875 mm (5/8 in), the AMT33 series generates standard U/V/W commutation signals for vectoring current to brushless motors. The encoder offers 22 programmable resolutions from 48 to 4096 PPR and can commutate 2, 4, 6, 8, 10, 12, or 20 pole motors. For environments with significant electrical noise or long cabling distances, line driver versions are available for quadrature outputs, commutation outputs, or both. With radial and axial orientations, the AMT33 further features an operating temperature range from -40 to 125°C, high accuracy, low current draw, and added resilience to environmental contaminants.
The AMT-OTZ-1 One Touch Zero module is a simple and intuitive alignment tool for the AMT31 and AMT33 series, allowing for unprecedented time savings during the encoder alignment process. With the simple press of a button, the AMT31 or AMT33 encoder can be instantly aligned to a brushless dc (BLDC) motor, eliminating the traditionally time-consuming alignment process and removing the need for a motor back-driving fixture and oscilloscope. The module also incorporates test points for quadrature and commutation signals, which can be used to verify encoder alignment or provide quick access to debugging the encoder.
In summary, the AMT31 and AMT33 series deliver the best of both worlds, combining levels of accuracy and durability unrivaled in other encoder technologies. The AMT encoders unique platform also delivers an unparalleled level of flexibility and intelligence thanks to the digital nature of the design. And, the encoders are easy to install, greatly reducing assembly time and cost.
