Motion Control Overview
What is a Servo?
Servo control, which is also referred to as “motion control” or “robotics” is used in industrial processes to move a specific load in a controlled fashion. These systems can use either pneumatic, hydraulic, or electromechanical actuation technology. The choice of the actuator type (i.e. the device that provides the energy to move the load) is based on power, speed, precision, and cost requirements. Electromechanical systems are typically used in high precision, low to medium power, and high-speed applications. These systems are flexible, efficient, and cost-effective. Motors are the actuators used in electromechanical systems. Through the interaction of electromagnetic fields, they generate power. These motors provide either rotary or linear motion. Here is a graphical representation of a typical servo system:
This type of system is a feedback system, which is used to control position, velocity, and/or acceleration. The controller contains the algorithms to close the desired loop (typically position or velocity) and also handle machine interfacing with inputs/outputs, terminals, etc. The drive or amplifier closes the inner loop(s) (typically velocity or current) and represents the electrical power converter that drives the motor according to the controller reference signals. The motor can be of the brushed or brushless type, rotary or linear. The motor is the actual electromagnetic actuator, which generates the forces required to move the load. Feedback elements such as tachometers, lvdts, encoders and resolvers, are mounted on the motor and/or load in order to close the various servo loops.
ADVANCED Motion Controls designs and manufacturers servo drives and amplifiers for use in servo systems. Servo drives and amplifiers are used extensively in motion control systems where precise control of position and/or velocity is required. The drive/amplifier simply translates the low-energy reference signals from the controller into high-energy signals to provide motor voltage and current. In some cases the use of a digital drive replaces the controller/drive or controller/amplifier control system. The command signals represent either a motor torque, velocity or position and can be either analog or digital in nature. Analog +/-10 VDC command is still the most common reference signal but it is quickly giving way to digital network commands.
The controller is the “brains” of a servo system. It is responsible for generating the motion paths and for reacting to changes in the outside environment. Controllers can be something as simple as an ON/OFF switch or a dial controlled by an operator. They can also be as complex as a computer with the ability to actively control multiple servo axes as well as monitor I/O and maintain all of the programming for the machine.
Typically, the controller sends a signal to the drive; the drive provides power to the motor; and the feedback from the motor is sent back to the controller and drive. Feedback from the load is also routed to the controller. The controller analyzes the feedback and corrects for errors by updating the signal to the amplifier. The controller is considered to be the intelligent part of the servo, closing the velocity and/or position loops while the amplifier closes the current loop. However, many amplifiers will close the velocity and/or position loops reducing computational demands from the controller.
Physical Forms of Controllers
Controllers come in a variety of forms which people choose based on cost, performance, convenience, and ease of use. Most controllers fall into the category of Microcontrollers, PLCs, and Motion Controllers. Each is described below.
This is a small and low-cost type of computer that runs a program stored in non-volatile memory. Configuring a microcontroller for a system generally requires an experienced programmer, and closing loops such as position and velocity can be quite difficult. Often, when one designs a servo system using a microcontroller, one will have the amplifier/drive close the desired loops, while the microcontroller simply sends particular commands back to the amplifier. These commands may be dependent on inputs into the microcontroller (sensors, switches, etc).
In the late 1960’s, Programmable Logic Controllers (PLCs) were first used to eliminate the mess of wires and troubleshooting nightmares associated with sequential relay circuits. PLCs can take the place of mechanical relays, which have limited lifetimes. These controllers are more expensive than microcontrollers, but with good reason.
PLCs have a processor and memory to allow for commands to be programmed, saved and executed. It also has a rack and I/O slots so that I/O modules may be added to the PLC as needed. The modules may add such features as high-speed counters, real-time clocks, or servo control capabilities.
The benefits of PLCs include expandability and resistance to harsh environments. The price is generally lower than that of motion controllers.
Motion controllers are built specifically for the control of motion (hence the name). Therefore commands and I/O are specific to the needs of those in the motion industry. Unlike the others, motion controllers are often PC based, allowing for a graphical user interface. Usually, there are advanced features that allow ease of tuning, commutation sensing, and other functions. A motion controller, in general, will make your life easier than a PLC or microcontroller. Because of the added features, they are typically more expensive.
The command is the signal that is sent from the controller to the servo drive.
Digital servo drives can be controlled over various networks including CANopen, Ethernet, EtherCAT, Ethernet Powerlink, Synqnet, USB, RS232 and many more which allow you to control the motor by connecting the amplifier directly (or almost directly) to a computer. Network signals have the advantage of being able to communicate more than just the output command, including I/O status, drive status, position information and more.
Analog servo drives are controlled with +/-10V analog signals and PWM & Direction signals.
The servo drive is the link between the controller and motor. Also referred to as servo amplifiers, their job is to translate the low energy reference signals from the controller into high energy power signals to the motor. Originally, drives were simply the power stage that allowed a controller to drive a motor. They started out as single quadrant models that powered brushed motors. Later they incorporated four quadrant capabilities and the ability to power brushless motors. Four quadrant means the ability to both drive and regenerate a motor in both directions.
The current trend is to add more features and abilities to drives. Today drives can be expected to handle all of the system feedback including encoders, resolvers and tachometers, as well as limit switches and other sensors. Drives are also being asked to close the torque loop, velocity loop and position loop and being given the responsibility of path generation. As the line between controller and drive blurs, the drive will take on many of the more complex control functions that used to be the sole domain of the controller.
The future of drive technology will continue to build on the demands of the motion control industry. These demands include:
- Higher bandwidth to increase production throughput
- Increased velocity and position control to allow for more intricate and miniaturized manufacturing
- Increased network capability to closely coordinate axes within a machine and coordinate machines within a factory
- Simplified, user-friendly and universal operation
The motor converts the current and voltage that comes from the drive into mechanical motion. Most motors are rotary types but linear motors are also available. There are many types of motors that can be used in servo applications.
The following list of motors types are commonly found in servo applications.
Single phase motors have two power wires and are very easy to set up. Motors in this category can include brushed motors, inductive loads and voice coils. Amplifiers designed for brushed motors are typically used to drive single phase loads although, most three phase drives from AMC can also operate these motors.
The most common single phase motor. The brushes are a form of mechanical commutation that directs the current into the correct coils at the correct time.
Linear actuators use a rotary motor coupled to a gear box to move a linear shaft in and out. The motor in the actuator is often times a brushed motor.
A voice coil is conceptually similar to an audio speaker. Motion is linear and is usually limited to less than 0.5″ (13mm) of travel. Many voice coil applications require a high performance servo drive and ADVANCED Motion Controls is often the first choice.
Magnetic bearings float a rotating shaft on a magnetic cushion controlled with servos. They are used when low friction is required or when the shaft speeds are too high for conventional bearings. Magnetic bearings use electromagnets to levitate the rotating shaft so nothing is physically touching it. A typical magnetic bearing system will require 4 or 5 drives – an x and y on each side of the rotating shaft and an optional thrust bearing to keep the shaft from floating in and out. The performance requirements for the drives can be extremely high due to the dynamic nature of the system.
Inductive loads are often used by universities and scientists to create magnetic fields for their experiments. ADVANCED Motion Controls drives have successfully controlled inductive loads with less than 80uH of inductance to over 1H (1,000,000uH) of inductance. There are special considerations for the energy stored in a large inductor, and our technical support department would be happy to discuss these regarding your project.
Permanent magnet brushless servo motors have higher power density, better heat dissipation and require less maintenance than brushed motors. Brushless motors may be a little more difficult to set up due to the increased wiring so our digital line makes things easier by automating the commutation process.
The construction of a linear motor is the same as a rotary motor but opened up and flattened out. Configuring a drive for a linear motor is identical to configuring a drive for a rotary motor. Linear motors are used in direct drive applications where the speed and accuracy requirements are more than a rotary motor and ball screw can provide.
Load considerations should include the object that is being moved, the moving parts in the machine and anything that may cause unwanted instabilities such as couplings and backlash. The total mass of the moving parts in the machine all have inertias that will be reflected onto the motor. Friction points such as from linear stages and bearings will add to the motor load. Flexible couplings will add resonances that have to be considered.
In modern control systems, feedback devices are used to ensure that the motor or load reaches the commanded position or velocity. Servo amplifiers and controllers use this feedback to determine how much current to deliver to the motor at any time, based on its present position and velocity versus where it needs to be. There are two main types of feedback, absolute and relative (also known as ‘incremental’).
Absolute devices provide definitive position within a specified range upon power up (i.e. without a homing routine).
Relative Feedback (incremental)
These devices provide only incremental position updates. In order to know the motor or load’s position, incremental feedback needs to be used in conjunction with some type of absolute feedback (a limit switch, for example) to determine the initial position. Once the initial position is known, relative feedback can provide position information throughout the range of motion.
Within these two general types of feedback, there are many different feedback devices. Here are some of the devices most commonly used in motion control.
Encoders are the most prevalent position feedback device in motion control. Linear encoders can go to sub-micron resolutions and rotary encoders can have resolutions exceeding 100,000 counts per revolution. These are relative feedback devices.
Sinusoidal encoders use sine waves in place of the square waves seen on quadrature encoders. This allows intermediate encoder counts to be interpolated to over 1024 times. Resolutions of over 4 million counts per resolution are possible. These are relative feedback devices.
Absolute Sinusoidal Encoder
These use the same sinusoidal encoders as above in addition to a mechanical device or electrical circuit that can maintain absolute position information over many thousands of revolutions. These devices transfer the position information over a serial protocol such as: Hiperface®, EnDat® and BiSS.
This is a low resolution feedback that is often necessary for commutation control. This can also be used for velocity feedback at higher velocities. These provide 6 units of absolute feedback within each electrical cycle.
A resolver is essentially a rotary transformer. This feedback is capable of resolutions above 16bit. Resolvers are the feedback of choice for high temperature and high vibration environment. These provide absolute feedback within one revolution.
The purpose of all servo systems is to move some kind of load. The way in which the load is moved is known as the motion profile. A motion profile can be as simple as a movement from point A to point B on a single axis, or it may be a complex move in which multiple axes need to move precisely in coordination. An example profile is shown in Figure 1. The total distance traveled, D, is found by calculating the area under the curve. T is the total time required for the move. The slope of the velocity curve represents the acceleration or deceleration at that particular instant. There are several types of motion profiles used with servo control systems. The most often used are Constant Velocity, Trapezoidal, and S-Curve motion profiles.
Things to remember:
- Velocity proportional to 1/T
- Acceleration proportional to 1/T2
- Power (peak) proportional to 1/T3
The implications of the last bullet point is profound. For example if you have an existing system and you want the moves to complete twice as fast, the system will require 8x the power!
This motion profile maintains a constant velocity between points (see figure 2a). This is the most basic motion profile because only a velocity command is used.
Constant velocity would be used in something like a conveyor or a fan.
Precision positioning machines do not use the constant velocity profile because a real world machine cannot change velocity instantly. There will be a time delay that will fluctuate with changes in the load and system. In figure 2B, the dotted line represents the actual velocity path the load will take. Ta and Td represent the time required to accelerate and decelerate. These times may vary with fluctuations in the load.
The trapezoidal motion profile slopes the velocity curve to create predictable acceleration and deceleration rates. A trapezoidal motion profile is shown in figure 3. The time to accelerate and decelerate is precise and repeatable. Ta and Td still exist, but they are now specified values instead of random values.
- If ta = td = T/3 for a trapezoidal move profile, the overall power used is a minimum
- Overshoot error still exists for a trapezoidal move, but this error is negligible for many systems.
- Higher precision machines require a different motion profile.
The S-curve motion profile allows for a gradual change in acceleration. This helps to reduce or eliminate the problems caused from overshoot, and the result is a great deal less mechanical vibration seen by the system. The minimum acceleration points occur at the beginning and end of the acceleration period, while the maximum acceleration occurs between these two points. This gives a motion profile that is fast, accurate and smooth.
Torque and Power Calculations
Starting with the velocity profile, the torque profile can be derived by taking the derivative of velocity. A positive slope in the velocity profile will be positive torque and negative slope will be negative torque. The steepness of the slop corresponds to the magnitude of the torque.
Next the power curve can be derived by multiplying the Velocity curve with the Torque curve (torque x speed = power).
Designing a System
The information within these three profiles is the foundation of system design.
- From the velocity and torque profiles you can narrow your motor selection to models that are able to provide the torque and speed required
- Based on the motor data (Kt – torque constant, Kv – voltage constant, Rm motor resistance) you can then determine the system current and voltage requirements
- From the peak torque you can calculate the peak current (I). I = T / Kt
- From the peak velocity you can calculate the peak voltage (V). V = Speed * Kv + I * Rm
- Based on the motor data (Kt – torque constant, Kv – voltage constant, Rm motor resistance) you can then determine the system current and voltage requirements
- When the current and voltage requirements are known you can then select a servo drive
- Torque is proportional to current.
- Torque*Speed = Power
- KT =Torque constant (lb-in/A)
- RMS torque is important for supply and thermal considerations.