![Servo Section 5
27
3. Response Frequency of Position Controllers
One of the most important specifics required for position control is response frequency
characteristics.”
Now, higher speed and higher resolution are required for servo systems so that servo systems
meet response prior to constructing a servo system.
1) When the following specifications are required for positioning:
Positioning resolution: Ap [mm/p]……1 x 10ز [mm/p] → 10 [µm/p]
Max. positioning speed: Vmax [mm/sec]…… 500 [mm/sec]
Ball screw pitch: p[mm/r ……10 [mm/r]
2) First, get number of motor rotation.
3) Get resolution of the encoder.
4) Get oscillation frequency of the encoder and oscillation circuit.
Thus, the error counter and oscillation circuit require 50 Kpps of response frequency. In order
to get higher speed and higher resolution, higher response frequency circuit is required. As
shown above, 50 Kpps is an inevitable pre-condition to constructing the servo system in order
to meet the required specifications.
Note 1: When multiplication function of the encoder is used, number of encoder resolution is
enough with . In this case, response frequency of
command value oscillation circuit and the error counter should be calculated after
multiplication by this figure.
Oscillation Error Servo
driver
fa
+
-
fb
Number of rotation N [rpm]
Table of speed v [mm/sec]
Ball screw pitch: p [mm/r]
Electrical positioning precision: Ap (mm/P)
Encoder resolution: R [ppr]
N =
60
P
Vmax =
60 x 500
10
= 3,000 [rpm]
R =
P
Ap
=
10
1 x 10ز
= 1,000 [ppr] Note1
fa = fb = R x
N
60
= 1,000 x
3,000
60
= 50, 000 [pps] = 50 [Kpps]
1
(multiplication figure)
Command
E
M](https://image.slidesharecdn.com/1-150820050607-lva1-app6891/85/1-servo-basic-28-320.jpg)
1. servo basic
- 1. Introduction to Servo System CONTENTS 1 About Servo System...............................................................................................1 1. What is “Servo System”........................................................................................................2 2. Positioning Mechanisms .......................................................................................................3 3. Three Types of Control System ............................................................................................5 4. Configuration of the Servo System and Its Operation ..........................................................7 2. About Servo Motor ........................................................ .....................................10 1. Difference with Other Conventional Motor........................................... ........ .....................11 2. Types and Features of Servo Motors .................................................. .................................12 3. Construction of AC servo motor....................................................... ...................................13 3. About Detector.......................................................................................................18 1. Detector Classification According to Their Detection Method ...........................................19 2. Typical Detector ...................................................................................................................20 3. Features of Each Detector and Its Application. ....................................................................22 4. About Servo Driver...............................................................................................23 1. Typical Servo Driver....................................... ......................................................................23 5 About Position Control..........................................................................................24 1 Two types of Positioning Systems.........................................................................................25 2. Speed Control and Torque Control ......................................................................................26 3. Response Frequency of Position Controllers .......................................................................27
- 2. 1 Servo Section 1 1 About Servo System.....................................................................................................................2 1. What is “Servo System”................................................................................................................2 2. Positioning Mechanisms...............................................................................................................3 3. Three Types of Control System ....................................................................................................5 4. Configuration of the Servo System and Its Operation ..................................................................7
- 3. Servo Section 1 2 1 About Servo System 1. What is “Servo System” “Servo” derives from the Greek “Servus (servant).” The system is called “Servo System” as it responds faithfully to a command. It is a system to control mechanical instruments in compliance with variation of position or speed target value (designated value, command value).
- 4. Servo Section 1 3 2. Positioning Mechanisms The servo system is not the only alternative to control positioning and feed speed of mechanical facilities. Beside simple mechanical devices, however, the servo system is now the major control system to positioning and feed speed. Simple positioning Simple. Low cost. Available high speed operation. Flexible positioning by servo motor Precise. High speed. Easy to change target position and feed speed. No maintenance. High speed response Direct driving system Simple mechanism. No backlash. No trouble bout gear life. Stopper Cylinder Clutch & brake Motor Clutch & brake Cam system Position controllerM Stepping motor Position controllerM E Servo motor Position controllerM Servo motor Change to AC servo motor Open loop Semi-closed loop Full-closed loop Position controller Linear motor Direct drive Stator (coiling side) Rotor Position controller
- 5. Servo Section 1 4 Mechanical backlash and its correction Almost all mechanical devices have neutral zone between forward and reverse rotations. For example, when you change rotation direction of a mechanism from forward to reverse, an additional amount of rotation is required to cover a gap (free zone) between forward and reverse rotations. This gap or allowance is called “backlash”. The servo system has a function to compensate for this backlash. As shown above, the servo system sends command pulse adding correction amount of pulses. In this case, a current position value counter does not count this correction amount. Forward direction Forward /stop Start reverse rotation/ Send compensation pulses N amount of pulses Reverse (command pulse)/stop Start forward rotation Send compensation pulses N amount of pulses Forward command pulse/ stop (N-pulses) (N-pulses)
- 6. Servo Section 1 5 3. Three Types of Control System At present, there are three major control system: 1) open loop, 2) semi-closed loop, and 3) full- closed loop systems System Configuration Open loop Semi-closed loop Full-closed loop Features of each system Open loop Semi-closed loop Full-closed loop Control system Simple Little complicated Complicated Detection method None Not required as installed in motor Required Against load fluctuation Week Strong Strong Precision Mechanical difference Mechanical difference By precision of detector Difference (backlash pitch difference) Difficult to correct Correction available Correction not required Motor Stepping motor AC servo DC servo AC servo DC servo Feed rate Low High High Cost Cheap Little expensive Expensive Complicity of system configuration Simple Little complicated Complicated Position controller (NC controller) Stepping Table Ball screw (Reduction gear) Position controller (NC controller) Stepping Ball screw (Reduction gear) Linear scale Table Position controller (NC controller) Stepping Table Ball screw (Reduction gear) Rotary encoder
- 7. Servo Section 1 6 • Servo motor Major difference of the servo motor compared with general use induction motors is that it has a detector to detect rotation speed and position • Driver (analog input type) The driver controls servo motor rotation speed to rotate with a designated number of rotations in proportion to analog speed command voltage. Thus, it monitors motor rotation speed all the time • Positioner The positioner controls servo motor positioning. Thus, it monitors motor rotation position all the time Speed feedback signal Configuration example of servo system Output power shaft Motor section Detector (encoder etc.) Current Analog Command voltage (Ex.) Analog command Voltage +10V -5V Forward direction Reverse direction 3,000 rpm 1,500 rpm Number of rotation Position feedback signal Speed feedback signal Analog command voltage Position data setting Speed data setting Analog Command voltage Speed feedback signal Servo motor Position feedback signal Analog command voltage
- 8. Servo Section 1 7 4. Configuration of the Servo System and Its Operation These sections are same as the above configuration. Position controller OMNUC N515 ONNUC N116 TG Memory Controller Section Oscillation OscillationIndicator Direction judgement Error counter M D/A converter Pulse Multiplication circuit A B C D E E Encoder Motor Tacho-generator Position feedback Speed feedback Current feedback + _ + _ i Position command Speed command + _ MotorServo DriverPosition controller OMNUC N115 series M Motor + _ + _ i Re Speed feedbac k Resolver R/P MotorServo Driver Position feedback D E Position feedback M Motor + _ + _ i Speed feedback MotorServo Driver D E E Encoder F/V
- 9. Servo Section 1 8 Error counter operation and response characteristics 1) When an amount of command pulse is supplied with equivalent to command speed, the servo driver receives square shaped input. The motor, however, takes some interval to reach to the command speed as it has to overcome inertia. 2) When the motor reaches the command speed, command pulse frequency becomes return pulse frequency. The motor speed stays in the rated level. 3) When the driver stops supply of command pulse by ending number of pulse equivalent to the command position, the error counter and D/ A converter output power until stored pulses (an amount equivalent to delay pulse amount S1 at start) in the error counter are discharged, and the motor keeps rotation with this output. The motor stops rotation when number of position command pulse is equal to number of return pulses. Stored pulse amount S1 equals S2 in the figure above Note 1: Error counter (also called “deviation counter”) Command pulses are temporarily stored inside the servo driver until they turn to command voltage by the D/A converter. These pulses are called error pulses or deviation pulses. Stored section is called “error counter” or “deviation counter.” Difference after completion of positioning should theoretically become zero. However, this may vary by total amount of allowance of position detector (ex: one pulse of the encoder), allowance of the servo driver, and mechanical static friction torque. S Input command pulse Stop command pulse V (pps) y x This area measurement is not changed. (Stored pulse amount of the error counter) x S
- 10. Servo Section 1 9 In-position (nearly zero) When the servo system completes positioning, it goes to next step after receiving positioning completion signal. This positioning completion signal is called “in-position.” This in-position function enables the position control device (position controller or NC) to adjust its sensing width. By adjusting this in-position range and using this in-position signal, the system is able to go to next step in advance of completion of positioning or to shorten total cycle time The servo system, however, executes positioning regardless of in–position range and its output, and completes positioning until the error counter becomes zero. <Setting example of in-position range> This in-position is also called “nearly zero” as the error counter becomes almost zero. -4 -3 -2 -1 1 2 3 40-n …n Error counter stored pulse amount You can set any centering zero value. (Max. value is limited by the position controller) In-position signal output range
- 11. 10 Servo Section 2 2. About servo Motor.....................................................................................................................11 1. Difference with Other Conventional Motor................................................................................11 2. Types and Features of Servo Motors ..........................................................................................12 3. Construction of AC servo motor.................................................................................................13
- 12. Servo Section 2 11 2. About servo Motor 1. Difference with Other Conventional Motor Basic construction and operation principles of the servo motor are the same as general conventional induction motors. But they have been redesigned to meet high precision, high speed, high frequency positioning and speed control of mechanical facilities.
- 13. Servo Section 2 12 2. Types and Features of Servo Motors Servo motors are classified into DC servo motors, AC servo motors, and stepping motors. There are two varieties of AC servo motors; synchronous servo motor and induction type servo motor. Classification of servo motor Features of each servo motor Stepping motor DC servo motor Synchronous servo motor Induction type servo motor Capacity (watt) Less than 100 W Less than 500 W 100 to 2 kW 2 kW or up Advantages Compact and high output. Cheap. Smaller outside dimensions and large torque. Good operation efficiency. Good controllability. Cheap. High speed and high torque. Good operation efficiency. No maintenance required. High speed and high torque. No need maintenance Durable. Large peak torque. Disadvantages Out-of-step and magnet noise at low speed operation Limit at rectification. Low reliability. Requires maintenance. Expensive. Bad operation efficiency with medium capacity models. Complicated control circuit. Expensive Servo motor DC servo motor AC servo motor Stepping motor Synchronous servo (brushless DC servo) motor Induction type servo motor This is OMRON AC servo motor
- 14. Servo Section 2 13 3. Construction of AC servo motor • Features of AC servo motor compared with DC servo motor Permanent magnet is built-in the rotor….Rotating field type Coils are provided on the stator…………Static armature. In other word, electrical functions of rotor an stator are reversed. AC servo motor does not have the commutator and brushes which DC servo motor has.
- 15. Servo Section 2 14 Operation principle of AC servo motor Hall element Rotation angle (°)2400 120 360 480 H1 H2 H3 Rotation angle (°)2400 120 360 480 I1 I2 I3 Coiling magnetic current P1 P2 P1P3 Switching order and stator field rotation Stator field
- 16. Servo Section 2 15 (1)....................................................................................................................Features of Servo motor Characteristics of servo motor N ∝ V Motor speed: N varies in proportion to impressed voltage: V T ∝ I Motor torque: T varies in proportion to supplied current : I P ∝ N • T output ∝ V • I input Motor output power is nearly proportionate to product multiplied speed by torque, and product multiplied impressed voltage by current. Acceleration and deceleration time: t is in proportion to inertia moment: J of the whole mechanism and arriving speed: N, and in inverse proportion to torque: T. Th ∝ I² rms Operation limit of the motor is determined by temperature rise. Rising temperature is in proportion to square of effective value of current Motor rotation direction is determined by polarity of impressed voltage I N T T ∝N • J T deg Impressed voltage: V
- 17. Servo Section 2 16 (2) Comparison Between AC & DC Servo Ac servo DC servo Life <Bearing life> 20, 000 h or up. <Brush life> Normally, 3,000 to 5,000 h Varies considerably due to load and environmental conditions. Maintenance <Not required> No mechanical contact. (No brushes, commutators) <Required> Required periodical check and replacement of brushes. Sound noise <Quiet> <Noisy> Due to brush contacting noise. Electrical noise <None> No noise as no brushes. <Exist> Noise occurs due to actuation of brushes. Efficiency <Excellent? Good cooling efficiency as heat radiates from stator. <Good> Rectification loss occurs. Bad cooling efficiency due to rotor heat Against Overload <Good> Large thermal time constant. High speed and large torque. <Medium> Small thermal time constant. Limited current due to brush flashover Response Characteristics <Very quick> Large power rate. (Small rotor inertia and large torque until high speed range.) <Quick> Small power rate. (Large rotor inertia. Decrease torque at high speed range.) Cleanness <Good> Clean as no brush powder occurs <Bad> Brush powder occurs. AC servomotor is more suitable for high speed, high response, and high acceleration/ deceleration control than DC servomotor. It also does not require maintenance.
- 18. Servo Section 2 17 (3) Torque – number of rotation characteristics AC servo motor Number of rotation Rated number of rotation Instantaneous max. number of rotation Continuous operation zone Rated torque Torque Instantaneous max. torque Short time operation zone
- 19. 18 Servo Section 3 3. About detector............................................................................................................................19 1. Detector Classification According to Their Detection Method ..................................................19 2. Typical Detector .........................................................................................................................20 3. Features of Each Detector and Its Application. ..........................................................................22 4. About Servo Driver....................................................................................................................23 1. Typical Servo Driver...................................................................................................................23
- 20. Servo Section 3 19 3. About detector 1. Detector Classification According to Their Detection Method Detectors installed on servo motors are classified into three categories according to their detection method. Now, detectors are intended to carry out signal treatment of encoders’ and resolvers’ outputs, and function as both speed an position detectors. Classification of detectors Detection method Linear type Rotary type Detector Speed Position Use seed generated power Induction type speed detector. DC tacho generator. • Permanent magnet type • Other excited type AC tacho-generated Brushless DC tacho- generator • Permanent magnet type • Drag cup type Use pulse or phase equivalent to position and angle Photo type Magnetic type Electro-magnetic induction type Contact type Linear encoder Rotary encoder Magnet scale Rotary magnet scale Linear inductsyn Rotary inductsyn Sunchro Resolver • Brush type • Brushless type Contact type linear encoder Contact type rotary encoder
- 21. Servo Section 3 20 2. Typical Detector Rotary encoder, and resolver are examples of present day detectors for AC servo (1) Rotary Encoder Increment rotary encoder • Basic principle of rotary encoder Incremental rotary encoder The above figure shows detection system. A beam of light is directed at the rotation disc, which is installed on the fixed disc and the rotation disc installed on the input shaft. As the slits on both discs pass in rotation, this beam shuts off and on. This pulse is converted into electric signal by the photo diode. Both A and B slits on the fixed disc have phase difference of 90°. Thus, smoothed waveform output has two short waves having 90° phase difference. Counting this pulse output makes it possible to detect rotation angle. The outputs of both A and B enable detection of rotation direction when they pass through the direction judgement circuit, and are used to add or deduct measuring value. In addition, some rotary encoders have a slit per rotation to get zero position standard signal. Incremental type rotary encoder has simple construction and is cheap so that only a few output lines are required. On the contrary, it may accumulate differences generated by electrical noise at signal transportation. When power is OFF, the display disappears and does not store its data so that a separate counter is required.
- 22. Servo Section 3 21 (2) Resolver The resolver is a rotation angle detection sensor to detect mechanical rotation angle as rotor electro-magnetic induction voltage. Two stator coils are arranged to have a 90° angle to each other around the rotor coil. While AC voltage is charged with a rated frequency to the rotor coil, the rotor coil generates induction voltage waveform output relative to angle variation with the rotor. •Features of the resolver compared with the photo type rotary encoder are as follows: 1) Durable against environmental conditions like vibration, shock, and electrical noise as simple construction, no semiconductors are used. 2) Durable against power fluctuation and ambient temperature variation as no semiconductors are used. 3) Number of division per rotation can be selected by preparing an external circuit. 4) Complicated convert circuit from excitation voltage to digital, and increase peripheral circuit. KE sin(ωt + θ) θ E sin ωt K cos ωt E sin ωt θ KE sin(ωt + θ) Construction of synchronous resolver
- 23. Servo Section 3 22 3. Features of Each Detector and Its Application. Each detector has constructional, functional features and need required conditions to use it in compliance with those features. Item Photo type encoder Magnetic type encoder Resolver Features • Simple processing circuit. • Durable against electrical noise as it is digital signal. • Easy to get high resolution. • Weak against vibration and shock. • Not suitable for high temperature operation. • Simple processing circuit. • Durable against electrical noise as it is digital signal. • Relatively strong against vibration and shock. • Not suitable for high temperature operation. • Rugged • Strong against vibration and shock. • Available high temperature operation. • Adjustable resolution change by processing circuit. • Complex processing circuit. Speed Detection • By F/V converter • Usable as digital data • By F/V converter. • Usable as digital data • Uses phase variation by phase change at a rated interval as speed data. Position Detection • Use incremental pulse. • Easy to apply zero position pulse. • Use incremental pulse. • Easy to apply zero position pulse. • Two pole resolver has the same function as absolute encoder. • Incremental data is available by processing circuit Magnet pole sensor • Available by adding slit for magnetic pole sensor on rotation disc. • Available by adding track for magnetic pole sensor on magnetic drum. • Available by using same pole of motor or two pole resolver
- 24. Servo Section 4 23 4. About Servo Driver 1. Typical Servo Driver Let us become familiarized with the circuit and operation of PWM transistor driver which is one of major drivers for servo motors. Transistor PWM An example of main circuit Operation M Comparator M Command Standard triangle Tr3 Tr4 Transistor servo main circuit
- 25. 24 Servo Section 5 5 About position Control..............................................................................................................25 1 Two types of Positioning Systems..............................................................................................25 2. Speed Control and Torque Control.............................................................................................26 3. Response Frequency of Position Controllers..............................................................................27
- 26. Servo Section 5 25 5 About position Control 1 Two types of Positioning Systems There are two types of positioning systems: PTP (Point to Point) system and CP (Continuous Path) system. Each system is used in compliance with specific applications. • PTP (Point to Point) system • CP (Continuous Path) system PTP system and CP system Designate only target position. Do not designate route to arrive. Route may change in compliance with driving mechanism and speed of each axis. axis X axis Target position Start position In addition to designating start position and target position, also designate route between two points. X axis axis Linear interpolation Start position Arc interpolation Target position
- 27. Servo Section 5 26 2. Speed Control and Torque Control Beside position control, the system is also used to control motor speed by command voltage and torque by limiting current to the servo motor. Speed control (approx. 20%) (1) Rotate the motor with the rated speed (constant speed operation). (2) Match the motor speed with the standard speed of other feed mechanism (synchronous operation). Ex.) Torque control (approx. 5%) Motor torque changes in proportion to supply current. To limit motor torque, control current value to the motor M M V V Servo driver Servo motorGeneral induction motor M Contact Resistance with a volume Voltage input Current sensor Servo driver
- 28. Servo Section 5 27 3. Response Frequency of Position Controllers One of the most important specifics required for position control is response frequency characteristics.” Now, higher speed and higher resolution are required for servo systems so that servo systems meet response prior to constructing a servo system. 1) When the following specifications are required for positioning: Positioning resolution: Ap [mm/p]……1 x 10ز [mm/p] → 10 [µm/p] Max. positioning speed: Vmax [mm/sec]…… 500 [mm/sec] Ball screw pitch: p[mm/r ……10 [mm/r] 2) First, get number of motor rotation. 3) Get resolution of the encoder. 4) Get oscillation frequency of the encoder and oscillation circuit. Thus, the error counter and oscillation circuit require 50 Kpps of response frequency. In order to get higher speed and higher resolution, higher response frequency circuit is required. As shown above, 50 Kpps is an inevitable pre-condition to constructing the servo system in order to meet the required specifications. Note 1: When multiplication function of the encoder is used, number of encoder resolution is enough with . In this case, response frequency of command value oscillation circuit and the error counter should be calculated after multiplication by this figure. Oscillation Error Servo driver fa + - fb Number of rotation N [rpm] Table of speed v [mm/sec] Ball screw pitch: p [mm/r] Electrical positioning precision: Ap (mm/P) Encoder resolution: R [ppr] N = 60 P Vmax = 60 x 500 10 = 3,000 [rpm] R = P Ap = 10 1 x 10ز = 1,000 [ppr] Note1 fa = fb = R x N 60 = 1,000 x 3,000 60 = 50, 000 [pps] = 50 [Kpps] 1 (multiplication figure) Command E M