CHAPTER 1FUNDAMENTAL OF MOTION CONTROLSYSTEMS AND THEIR APPLICATIONS
OUTLINE INTRODUCTION TO MOTION CONTROL COMPONENTS OF MOTION CONTROL SYSTEM MOTION CONTROL VARIABLES: POSITION, ANGLE, SPEED APPLICATION OF MOTION CONTROL
INTRODUCTION TO MOTIONCONTROL Subfield of automation in which the position and/or velocity of machines are controlled using some type of device such as hydraulic pump, linear actuator or servo. Widely used in the packaging, printing, textile, semiconductor production and assembly industries.
INTRODUCTION TO MOTIONCONTROL The interface between the motion controller and the drives it controls is very critical when coordinated motion is required as it must provide tight synchronization. Previously only used analog signal but later more interfaces were developed for coordinated motion control. ◦ SERCOS in 1991 ◦ Profinet ◦ EtherCAT
Common control functions: Velocity control. Position (point-to-point) control: Several methods for computing a motion trajectory. often based on the velocity profiles of a move such as a triangular profile, trapezoidal profile, or an S-curve profile. Pressure or Force control.
Common control functions:Electronic gearing (or cam profiling): ◦ The position of a slave axis is mathematically linked to the position of a master axis. A good example of this would be in a system where two rotating drums turn at a given ratio to each other. ◦ A more advanced case of electronic gearing is electronic camming. With electronic camming, a slave axis follows a profile that is a function of the master position.
INDUSTRIAL MOTIONCONTROL CATEGORIES: Sequencing Speed control Point-to-point control
SEQUENCING refers to the control of several operations so that they all occur in a particular order. simplest example: ◦ progression of events that take place through the mechanical linkages of a player piano. ◦ opening and closing valves can be sequenced mechanically with cam shafts.
SEQUENCING Sequencing generally becomes too complicated to be handled mechanically in industrial equipment such as conveyor lines. ◦ Option: using time delay relays ◦ Better alternative: using PLC
SPEED CONTROL Refers to applications involving machines run at varying speeds or torques. Source of power for such applications is generally either an internal combustion engine, or an electric, hydraulic, or pneumatic motor.
SPEED CONTROL Speed can be controlled either mechanically or, in the case of electric motors, electronically. Mechanical speed-control components include clutches and brakes, adjustable speed drives, traction drives, transmissions, and fluid coupled drives. Electronic speed control manipulates applied electrical power to control velocity and torque.
SPEED CONTROL Electronic speed control in ac motors employs special amplifiers or drives. These generally vary ac motor speed with adjustable-frequency inverters. More expensive than mechanical speed controls, they provide the advantage of reduced energy costs.
POINT TO POINT CONTROL Refers to applications where something must move from one point to another at a constant speed. There are two factors that must be controlled: ◦ Speed ◦ Distance.
POINT TO POINT CONTROL Examples: ◦ in x-y tables and in machining, where a tool moves in a straight line while it touches a work piece along one axis.
MOTION CONTROL SYSTEMCOMPONENTS
MOTION CONTROL SYSTEMCOMPONENTS Application software – You can use application software to command target positions and motion control profiles. Motion controller – The motion controller acts as brain of the of the system by taking the desired target positions and motion profiles and creating the trajectories for the motors to follow, but outputting a ±10 V signal for servo motors, or a step and direction pulses for stepper motors. Amplifier or drive – Amplifiers (also called drives) take the commands from the controller and generate the current required to drive or turn the motor.
MOTION CONTROL SYSTEMCOMPONENTS Motor – Motors turn electrical energy into mechanical energy and produce the torque required to move to the desired target position. Mechanical elements – Motors are designed to provide torque to some mechanics. These include linear slides, robotic arms, and special actuators.
MOTION CONTROL SYSTEMCOMPONENTS Feedback device or position sensor – A position feedback device is not required for some motion control applications (such as controlling stepper motors), but is vital for servo motors. The feedback device, usually a quadrature encoder, senses the motor position and reports the result to the controller, thereby closing the loop to the motion controller.
APPLICATION SOFTWARE Divided into three categories: ◦ Configuration ◦ Prototype ◦ Application development environment (ADE)
CONFIGURATION One of the first things to do is configure your system for all your motion control and other hardware.
PROTOTYPE Prototyping and developing your application. In this phase, you create your motion control profiles and test them on your system to make sure they are what you intended.
APPLICATION DEVELOPMENTENVIRONMENT For this, you use driver-level software in an ADE such as LabVIEW, C, or Visual Basic.
MOTION CONTROLLER A motion controller acts as the brain of the motion control system and calculates each commanded move trajectory. Because this task is vital, it often takes place on a digital signal processor (DSP) on the board itself to prevent
MOTION CONTROLLER The motion controller uses the trajectories it calculates to determine the proper torque command to send to the motor amplifier and actually cause motion. The motion controller must also close the PID control loop. Because this requires a high level of determinism and is vital to consistent operation, the control loop typically closes on the board itself.
MOTION CONTROLLER Along with closing the control loop, the motion controller also manages supervisory control by monitoring the limits and emergency stops to ensure safe operation.
CALCULATING TRAJECTORY The motion trajectory describes the motion controller board control or command signal output to the driver/amplifier, resulting in motor/motion action that follows the profile. The typical motion controller calculates the motion profile trajectory segments based on the parameter values you program.
CALCULATING TRAJECTORY The motion controller uses the desired target position, maximum target velocity, and acceleration values you give it to determine how much time it spends in the three primary move segments (which include acceleration, constant velocity, and deceleration).
Typical trapezoidal velocity profile
MOTOR AMPLIFIERS & DRIVES Part of the system that takes commands from the motion controller in the form of analogue voltage signals with low current Converts them into signals with high current to drive the motor.
MOTOR AMPLIFIERS & DRIVES Motor drives come in many different varieties and are matched to the specific type of motor they drive. ◦ For example, a stepper motor drive connects to stepper motors, and not servo motors. Along with matching the motor technology, the drive must also provide the correct peak current, continuous current, and voltage to drive the motor.
MOTORS & MECHANICAL ELEMENTS Motor selection and mechanical design is a critical part of designing your motion control system. Many motor companies offer assistance in choosing the right motor.
MOTORS & MECHANICAL ELEMENTS After determining which technology you want to use, you need to determine the torque and inertia at the motor shaft.
FEEDBACK DEVICES Help the motion controller know the motor location. The most common position feedback device is the quadrature encoder, which gives positions relative to the starting point.
FEEDBACK DEVICES Other feedback devices: ◦ potentiometers that give analogue position feedback ◦ tachometers that provide velocity feedback ◦ absolute encoders for absolute position measurements, and ◦ resolvers that also give absolute position measurements.
MOTION IO Protection from damaging the system. Includes limit switches, home switches, position triggers, and position capture inputs. Limit switches provide information about the end of travel to help you avoid damaging your system.
MOTION CONTROLLER VARIABLES:POSITION,ANGLE & SPEED A system with a feedback controller will attempt to drive the system to a state described by the desired input, such as a velocity. In practical applications this setpoint needs to be generated automatically. A simple motion control system is used to generate setpoints over time.
The motion profile is then used to generate a set of setpoints, and times they should be output. The setpoint scheduler will then use a real-time clock to output these setpoints to the motor drive.
MOTION PROFILES Consist of: ◦ Velocity profile ◦ Position profile
Trapezoidal velocity profile A trapezoidal velocity profile is shown. The area under the curve is the total distance moved. The slope of the initial and final ramp is the maximum acceleration and deceleration. The top level of the trapezoid is the maximum velocity.
Trapezoidal velocity profile Some controllers allow the user to use the acceleration and deceleration times instead of the maximum acceleration and deceleration. This profile gives a continuous acceleration, but there will be a jerk (third order derivative) at the four sharp corners.
Example 2 The motion in Example 2 is so short the axis never reaches the maximum velocity. This is made obvious by the negative time at maximum velocity.
Example 4 In some cases the jerk should be minimized. ◦ can be achieved by replacing the acceleration ramps with a smooth polynomial. Two quadratic polynomials will be used for the acceleration, and another two for the deceleration.
MULTI AXIS MOTION In a machine with multiple axes the motions of individual axes must often be coordinated. ◦ A simple example >robot that needs to move two joints to reach a new position. We could extend the motion of the slower joints so that the motion of each joint would begin and end together. When the individual axis of a machine is not coordinated this is known as slew motion
SLEW MOTION Each of the axes will start moving at the same time, but finish at separate times. Consider the example :A three axis motion is required from the starting angles of (40, 80, -40) deg, and must end at (120, 0, 0) deg. The maximum absolute accelerations and decelerations are (50, 100, 150)degrees/sec2, and the maximum velocities are (20, 40, 50) degrees/sec.
Example 5 These are done in vector format for simplicity. All of the joints reach the maximum acceleration. The fastest motion is complete in 1.13s, while the longest motion takes 4.4s.
Interpolated motion In interpolated motion the faster joints are slowed so that they finish in coordination with the slowest. Essential in devices such as CNC milling machines. If this did not occur a straight line cut in the x-y plane would actually be two straight lines.
Interpolated motion The slew motion example 5 can be extended where all joints finish their motion at 4.4s. This can be done by accelerating at the maximum acceleration, but setting a new maximum velocity. This is shown in the Example 6 using the results from the Example 5.