acs214 lecture 01

Discrete Systems

An Introduc2on to Mechatronics ACS214/271, Lecture 1 (R. Gross)

ACS214/271 Discrete Systems 2011/12

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Mechatronic engineering •  “Mecha-‐tronics” (Yaskawa Electric Company, 1969)

•  Mechatronics is an interdisciplinary area of engineering that combines mechanics, electronics, controls, and computer engineering.

•  Concerned with the development of ‘smart’ electromechanical products and systems through an integrated design approach

•  Mechatronic systems employ microprocessors and soUware as well as special-‐purpose electronics


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Mechatronics: Disciplines and Applica2ons

Application areas

•  Transportation

•  Manufacturing and production engineering

•  Medical and healthcare applications

•  Modern office environments

•  Household applications

•  Space applications


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Mars rover Spirit (2003 – 2010)


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Mechatronic systems •  Consist of mechanical elements, actuators, sensors, signal condi2oning elements, interface devices, digital hardware and soUware

•  Mechatronic systems are designed by a mul2disciplinary team of engineers

•  All components of a mechatronic system are designed and integrated ‘concurrently’ leading to a more efficient, cost effec2ve, flexible and reliable system


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Examples of mechatronic systems: Drive by wire •  Replaces mechanical

connec2ons (push rods, rack & pinion, steering columns etc.) by mechatronic connec2ons – sensors, actuators, embedded microprocessors, control soUware


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Examples of mechatronic systems: Smart skis •  Incorporate vibra2on control technology •  Employ piezoelectric materials which act both as sensors and

actuators

•  Embedded controller chip programmed to ac2vely dampen vibra2ons


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Examples of mechatronic systems: Humanoid robots

•  The ul2mate mechatronic system


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Examples of mechatronic systems: Swarm robots

A swarm rescuing a child


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12 Mechatronic system: Basic structure


13 Mechatronic system: Basic components

•  System: The system block represents everything performed by the process/equipment which is being controlled

•  Sensors: The sensors measure the value of the controlled variable and convert it into a usable signal. Generally it will consist of a primary sensing element and a signal transducer (or converter)

•  Controller: The controller computes the difference between the measured value and the desired value of the controlled variable and implements the control algorithm that converts error into a control ac2on

•  Actuators: The actuators translate the controller output into changes of the manipulated variable


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•  Signal Condi;oning Elements: prepare a signal to be used by another component. The input to a signal condi2oner is the output from a sensor or from the Analogue-‐to-‐Digital or Digital-‐to-‐Analogue Converter. The opera2ons performed by a signal condi2oner include: isola2on, impedance conversion, noise reduc2on, amplifica2on, lineariza2on and conversion.

•  Communica;on Interface: consists of a subsystem that sends or receives signals to or from other systems or subsystems and a set of standard func2onal characteris2cs, common physical interconnec2on characteris2cs and signal characteris2cs for the exchange of signals or data.


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•  Analog Signal: is any variable signal con2nuous in both 2me and amplitude. It differs from a digital signal in that small fluctua2ons in the signal are meaningful.

•  Digital Signal: A digital signal is a signal that is both discrete and quan2zed. Most modern control systems are digital.

•  Disturbance: The disturbance variables are process inputs that affect the controlled variable but cannot be manipulated. Disturbance variable are capable of changing the load on the process and are the main reason for using a closed loop control system.

•  Noise: accounts for the measurement inaccuracies of the sensor. Typically, measurement errors are randomly distributed.


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•  Analog-‐to-‐Digital Converter (A/D or ADC): a device that converts con2nuous signals to discrete digital numbers. Typically, an ADC is an electronic device that converts a voltage to a binary digital number. However, some non-‐electronic devices, such as shaU encoders, can be considered as ADCs.

•  Digital-‐to-‐Analog Converter (D/A or DAC): a device for conver2ng a digital (usually binary) code to an analog signal (current, voltage or charges). Digital-‐to-‐Analog Converters are the interface between the abstract digital world and the analog real life. Simple switches, a network of resistors, current sources or capacitors may implement this conversion.


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Mechatronic systems

•  Most mechatronic systems are in fact control systems

•  Control system: any group of components that maintains a desired result or value by manipula2ng the value of another variable in the system

•  The variable whose value is controlled is called the controlled variable

•  The variable that is adjusted in order to achieve the desired result is called the manipulated variable.


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Mechatronic systems classifica2on

Mechatronic control systems can be classified in a number of different ways according to:

Type of signal

Depending on the nature of the signal, control systems can be classified in analogue and digital control systems.

Feedback

Depending on whether feedback is used or not, control systems can be classified in open-‐loop and closed-‐loop control systems.


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A closed-‐loop control system measures the difference between the actual result and the desired result. The difference or error is used to determine the changes needed in the manipulated variable in order to drive the system toward the desired result.

A closed-‐loop control system performs the following basic opera2ons:

•  Measurement: measure the value of the controlled variable

•  Decision: derive a control ac2on to reduce the control error •  Manipula;on: Change the value of the manipulated variable according to the computed control ac2on.


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An open loop control system does not compare the actual result with the desired result to determine the control ac2on.

An open loop control system achieves the desired result based on a calibrated sedng.

Open loop control is easier and cheaper to implement but cannot correct unexpected disturbances or parameter varia2ons.


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Setpoint

Control systems are classified into regulator systems and follow up systems, depending on how they are used.

A regulator system: closed-‐loop control system in which the setpoint (desired value of the controlled variable) is seldomly changed. The prime func2on is to maintain the controlled variable constant, despite unwanted load changes. Examples include pressure regulators, cruise control systems, etc.

A follow-‐up system is a closed-‐loop control system in which the setpoint is frequently changing. Its prime func2on is to keep the controlled variable in close correspondence with a 2me-‐varying reference signal. Examples include radar tracking systems, antenna posi2on control systems, remotely operated robots.


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Applica;on area

Control systems can also be classified according to main applica2on areas as:

Process control: involves the regula2on of variables in a process defined as designed sequence of opera2ons or events, which through changes of physical or chemical proper2es of raw materials produces some desirable product. Examples include petroleum refinery, food processing plant, electric power plant etc.

The most commonly controlled variables in a process are temperature, pressure, flow rate and level. Others include density, viscosity, colour, conduc2vity, pH and hardness.

Process control involves rela2vely slow processes characteris2c 2me constants of the order of seconds, minutes or even hours.


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Servomechanisms: closed-‐loop control systems in which the controlled variable is physical posi2on or mo2on. Many servomechanisms are used to maintain an output posi2on in close correspondence with an input reference signal and hence are follow-‐up systems.

Servomechanisms usually involve rela2vely fast processes with 2me constants less than 1 second.

Cruise control


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Robo;cs: commonly involve programmable manipulators designed to move materials, parts, tools or other devices through a sequence of tasks to accomplish a specific task.

The closed-‐loop systems are usually follow-‐up posi2oning control systems which use posi2on and velocity feedback signals to control the movement of the manipulator. However, sight, tac2le sensing and voice recogni2on are also being used as inputs to the controller.

These sensory signals are used to detect the presence, dimension or even the iden2ty of an object.


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Controller loca;on

According to the loca2on of the controllers in a central control room or near the sensors and actuators, control systems can be classified into centralized and distributed.

Distributed control systems (DCS): are used in industrial and civil engineering applica2ons to monitor and control distributed equipment with remote human interven2on.

DCS use a network to interconnect sensors/field instruments, controllers, operator terminals and actuators.

DCS describe solu2ons across a variety of industries, including:

• Electrical power distribu2on grids and genera2on plants • Environmental control systems

• Water management systems

• Refining and chemical plants

• Pharmaceu2cal manufacturing


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Contents

•  Sensors •  Interconnec2on and signal condi2oning •  Digital logic and hardware • Microprocessors

•  Basic control implementa2on

•  Actuators

Stepper motors


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Stepper motors •  To control accurately position or speed of

a load or device •  Computer periphery, satellite dish positioning

systems, medical applications, robotics, etc.

•  Principle: •  Driven in fixed angular steps

•  One increment per electric pulse

•  Open loop control common •  Non-cumulative positioning error


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Different types

•  Variable reluctance

•  Permanent magnet •  low cost

•  for low power applications

•  Hybrid •  for industrial applications


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Stator/rotor system

•  Stator is the stationary outer or inner housing of the motor that supports the material that generates the appropriate stator magnetic field.

•  Rotor is the rotating part of the motor.


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Variable reluctance (VR) stepper motor •  Non-magnetized, soft-iron

rotor [6-pole]

•  Stator [8-pole], 4 phases

•  1 phase is energized at a time

•  Rotor motion is result of the minimization of the magnetic reluctance (resistance) between the rotor and stator poles.


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http://www.wisc-online.com/objects/ViewObject.aspx?ID=IAU14208

Stepping sequence

•  Step angle = 360/(a*b) •  a: number of rotor poles

•  b: number of phases ACS214/271 Discrete Systems 2011/12

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Stepping modes •  Full-stepping (previous slide)

•  Half-stepping

•  Microstepping (by changing the phase currents by small increments)


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Permanent magnet (PM) stepper motor •  Magnetized rotor

•  Electric circuit alternately switches polarity of stator poles (-1, 0, 1)

•  Rotor PM field will align to match induced stator field, reaching the stable equilibrium (detent position).

•  45° / 90° steps ACS214/271 Discrete Systems 2011/12

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Terminology •  Holding torque

•  The maximum steady torque that can be applied to the shaft of an energized motor without causing continuous rotation.

•  Detent torque (not for VR stepper motor) •  The maximum torque that can be applied to

the shaft of a non-energized motor without causing continuous rotation.

•  Maximum starting frequency ACS214/271 Discrete Systems 2011/12

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Drawbacks

•  If holding torque is exceeded, all knowledge of position is lost.

•  Produces much less torque, for a given size, than the equivalent DC/AC motor

•  Resonances can occur if not properly controlled.


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Example (from A2 assignment)

•  Control of stepper motor


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First assignment (A1)


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•  Programming of LED Display Units

•  Important: C programming skills (loops, array access, Boolean logic, hexadecimal)

References

•  Clarence W. de Silva, Mechatronics: A Founda2on Course, CRC Press, 2010 •  Today: Chapters 1 & 7.0 -‐ 7.2

•  T.G. Constandinou, “Stepper Motors Uncovered (1),” Elektor Electronics 11/2003, pp. 36-‐40


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acs214 lecture 01

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