Servo Drives and Motors
Feedback terminology
Accuracy
Accuracy is the measure of the difference between the expected position and actual measured value. Rotary feedback accuracy is usually given as an angle representing the maximum deviation from the expected position. Linear feedback accuracy is usually given as a distance representing the maximum deviation from the expected. Generally, as accuracy increases the cost of the feedback device increases.
Absolute encoder
Absolute encoders output unique information for each mechanical measured position. With the motor shaft or plate in any position when the drive is turned on the feedback device will always be able to sense a unique position and transmit this value to the drive. For an absolute single turn rotary encoder these unique positions will be over one revolution.When power is removed from the encoder and the shaft or plate moves the device will know its current position when the power is restored.A non-absolute feedback mechanism must start from a known position, such as the index or marker pulse.
Bit
A bit is short for Binary Digit. It is the smallest unit of information in a machine/drive. A single bit has a binary value of either 0 or 1. These bits do not normally exist on their own, but usually in groups. The larger the number of bits in a group the larger the amount of information that is available and thus the higher the resolution. This group can be converted to decimal using binary arithmetic. The group of bits can be converted to decimal by starting at the right most bit and multiplying each successive bit to the left by two. So for example a 12 bit number would give a decimal equivalent of 4,096 and a 19 bit number would give a decimal equivalent of 524,288.
Commutation
All brushless AC permanent magnet motors require commutation information to enable the drive to synchronize the stator flux field with the rotor of the motor.To ensure optimum torque at all rotor positions both when stationary and at speed the drive is required to maintain motor current in phase with the peak of the motor’s sinusoidal waveform. The drive must therefore know the position of the rotor with respect to the stator at all times.
Commutation phase offset
Most drives, including the Unidrive M and Digitax ST, provide a “Phase Offset” adjustment as a means of correctly setting the commutation position.For feedback devices that are not aligned, the Unidrive M has an Encoder Phasing Test (Autotune) (Pr 5.012) that automatically creates a Phase Offset value (Encoder phase angle) (Pr 3.025).All fm motor feedback devices are set to match the Unidrive M definition of zero phase offset, so that the drive may operate with zero phase offset adjustment, thus allowing interchange of motors between drives without further adjustment.Note that not all drives have the same zero offset definition.
Commutation outputs
Commutation outputs are used on devices that are non-absolute. For AC Synchronous 3 phase motors there are 3 commutation output signal channels from the feedback device, for example S1, S2 and S3. The diagram below shows commutation outputs for 6 pole commutation (3 pole pairs). The 3 phase motor sinusoidal power from the drive runs synchronously with motor speed at N/2 cycles per revolution;
Where N = number of poles. For example a 6 pole motor the encoder commutation tracks will output 3 pulses per channel per revolution and for an 8 pole motor the encoder commutation tracks will give 4 pulses per channel per revolution. The commutation signals allow the drive to operate the motor at ‘switch on’ with only a small possible reduction in efficiency and torque in the motor. The best way to explain this is to use an example where an encoder is connected to a motor with 6 poles.On power up the drive would look at the S1, S2 and S3 signals to determine where the stator is relative to the rotor or magnetic plate. This would give a known position that is within 60 ° electrical of an electrical cycle (20 ° mechanical). During this initial period, the drive assumes that it is in the middle of this 60 ° unknown region. So the worse case error of this is 30 ° electrical (10 ° mechanical), which equates to a drop of 13.4 % in the rated torque when 100 % current is delivered into the motor winding. When the drive is commanded to move the motor position, the stator is energized causing the plate or rotor to move. While the rotor or plate is moving, the drive detects that a signal switch (edge detection) has occurred on one of the commutation channels (S1, S2 or S3). At this point the drive knows exactly where it is in the electrical cycle and adjusts the field orientation to compensate for the error. At this point the drive switches over to using only the incremental signals for commutation and the commutation channels are no longer used.
Electronic nameplate
Available on some feedback devices the electronic nameplate provides the facility to electronically store information about the motor and feedback device. This information can then automatically be used to configure the drive for operation.
Environment
The environment is the external conditions that physically surround the Feedback device. The main factors that affect the feedback device are temperature and mechanical shock and vibration.Motors are designed to allow the feedback devices to be within their operational temperature limits. Generally it is assumed that there is free air movement around the motor. If the motor is positioned where there is little or no airflow or it is connected to a heat source such as a gearbox, it can cause the air temperature around the feedback device to be operating outside its recommended operating temperature and can lead to problems.Mechanical shock and vibration tends to be transmitted from the load, through the motor shaft and into the feedback device. This should be considered when the motor and feedback device are being specified for the application.
Position
The defined position is the location in a coordinate system which is usually in two or more dimensions. For a rotary feedback device this is defined as the location within one revolution. If it is a multi-turn device it is the location within one revolution plus the location within a number of rotations.For a linear feedback device this is defined as the distance from a known point.
Resolution
The resolution of a feedback device is the smallest change in position or angle that it can detect in the quantity that it is measuring.Feedback resolution of the system is a function of the type of feedback device used and drive receiving the information.Generally, as the resolution of the feedback device increases the level of control that can be used in the servo system increases. As with accuracy, as the resolution of the device increases the cost increases.
Resolver
A passive wound device consisting of a stator and rotor elements excited from an external source, such as an SM-Resolver, the resolver produces two output signals that correspond to the sine and cosine angle of the motor shaft. This is a robust absolute device of low accuracy, capable of withstanding high temperature and high levels of vibration. Positional information is absolute within one turn - i.e. position is not lost when the drive is powered down.
Incremental encoder
An electronic device using an optical disc. The position is determined by counting steps or pulses. Two sequences of pulses in quadrature are used so the direction sensing may be determined and 4 x (pulses per rev) may be used for resolution in the drive. A marker pulse occurs once per revolution and is used to zero the position count. The encoder also provides commutation signals, which are required to determine the absolute position during the motor phasing test. This device is available in 4,096, 2,048 and 1024 ppr versions. Positional information is non absolute - i.e. position is lost when the drive is powered down.
SinCos/AbsoluteEncoders
Types available are: Optical or Inductive - which can be single or multi-turn.
1) Optical
An electronic device using an optical disc. An absolute encoder with high resolution that employs a combination of absolute information, transmitted via a serial link, and sine/cosine signals with incremental techniques.
2) Inductive/Capacitive:
An electronic device using inductively coupled PCBs. An absolute encoder with medium resolution that employs a combination of absolute information, transmitted via a serial link, and sine/cosine signals with incremental techniques. This encoder can be operated with the drive using either sine/consine or absolute (serial) values only. Positional information is absolute within 4,096 turns - i.e. position is not lost when the drive is powered down.
Multi-turn
As previous but with extra gear wheels included so that the output is unique for each shaft position and the encoder has the additional ability to count complete turns of the motor shaft up to 4,096 revolutions.
Serial Interface
Serial communication is available on some feedback devices. It is the process of sending data one bit at one time, sequentially, over a communication channel. The specification normally used to define this method of communication is the EIA485 specification. These can be synchronous, which means that they operate with additional clock channels. The main advantage of synchronous data transmission is that it can operate at high speed. A disadvantage is that if the receiver goes out of synchronization it can take time for it to resyncronize and data may be lost. Note that not all serial interfaces use the clock channels.Serial interface communication allows data to be sent and received from the feedback device. In addition to the position and speed data other information can be sent such as multi-turn count, absolute position and diagnostic information.
Synchronous
If something is synchronous it means that events are coordinated in time. For serial interfaces this means that clock channels are used.
Asynchronous
If something is asynchronous it means that events are not coordinated in time. For serial interfaces this means that clock channels are not used.
Speed
Speed is the rate of change in position which can be either angular or linear traveled per unit of time. For rotational motors this is usually defined as revolutions per minute (rpm).
Volatile
Stored information will be lost when power is removed.
Non volatile
Stored information will not be lost when power is removed.
The original servo drives are analog types that operate on ±10-Volt inputs. In contrast, digital servo drives operate over fieldbus networks that now dominate the market.
The primary difference in construction between digital and analog drives is that a digital drive includes a microprocessor to carry out computations — in turn to determine the output control signal based on a mathematical model of the system’s behavior.
Most digital drives can accept feedback from tachometers, resolvers, encoders, and various types of switches or sensors. In addition to managing the torque, velocity, and position control loops, digital servo drives often include higher-level functionality … including operations such as path generation that were traditionally handled by the machine controller.
While analog servo drives are relatively inexpensive and simple to set up, there are benefits of using digital servo drives. First, a digital drive is tuned via software, rather than being tuned manually with potentiometers.
Most digital drives can also be auto-tuned or self-tuned, which is especially helpful when the load or inertia parameters are difficult to model or predict. This also simplifies the tuning process and provides a system that is much more responsive. Because all of the configuration and tuning setting are stored in the drive, it’s also easier to replicate a specific setup across multiple drives.
Note that auto-tuning (self-tuning) is a process in which servo control-loop gains are set automatically. The drive excites an attached motor at various frequencies to sense the system’s inertia and response, and then determines and sets the appropriate gains to ensure stability at all of the various frequencies.
digital servo drives
Feature image courtesy Parker Hannifin Corp.
With a digital servo drive, voltage pulses are sent to the motor at a much higher frequency — often 5 times or more — than with an analog drive. This allows the motor to respond to commands faster and provides smoother acceleration and deceleration. It also gives the servo system much higher holding torque.
Digital servo drives are versatile in their function. Most digital drives are capable of operating with an analog voltage signal, like an analog servo drive, and some can even accept step and direction signals to operate as a stepper drive. They can also be used when master and slave axes are required, with electronic gearing or an electronic cam between the axes.
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