Servo Systems and servomotors

1. Jussi Suomela HUT/Automation 1
Servo SystemsServo Systems
Servo is mechanism based on
feedback control.
The controlled quantity is
mechanical.

2. Your Partner In Productivity
Closed Loop Control
• Controller is the “Brain”
• Drive is the “Muscle”
• Feedback Transducer is the “Eyes”
Motion
Command
Input
Controller
Drive or
Amplifier
Mechanical
Motion
Output
Feedback
Transducer

3. Your Partner In Productivity
• Uses feedback for automatic correction
• High performance
Servo systems are “Closed Loop” in design
feedback
control
control
Width of material
must be very
precisely controlled

4. Speed
Feedback
Load
Table
Movement
IMC S
Class
AC/DC
Servo
Amp
0 to ±10V
DC
Position
LoopVelocity
Loop
Encoder
Ballscrew
Reducer
Motor
Power
Example - Closed Loop Control System

5. Jussi Suomela HUT/Automation 5
Servo Control of an ElectricalServo Control of an Electrical
MotorMotor
servocontroller
Three feedback lops

6. Jussi Suomela HUT/Automation 6
Properties of Servo SystemsProperties of Servo Systems
high maximum torque/forcehigh maximum torque/force allows high (de)acceleration
high zero speed torque/force
high bandwidthhigh bandwidth provides accurate and fast control
robustness
Servo Systems areServo Systems are
not servosnot servos

7. How to design good Servo Systems?How to design good Servo Systems?
 Terminology:
 Do not confuse “servos” with “servo motors”
 DC motors (brushed or brushless) are also sometimes also referred to as
“servo motors”
 See: http://www.theproductfinder.com/motors/bruser.htm
 “So when does a motor become a servo motorSo when does a motor become a servo motor?
 There are certain design criteria that are desired when building a
servo motor, which enable the motor to more adequately handle
the demands placed on a closed loop system.
 First of all, servo systems need to rapidly respond to changes in
speed and position, which require high acceleration and
deceleration rates.
 This calls for extremely high intermittent torque.

8. Intermittent Torque in ServosIntermittent Torque in Servos
Torque is related to current in the brushed servo motor.
So the designers need to keep in mind the ability of the
motor to handle short bursts of very high current, which
can be many times greater than the continuous current
requirements.
Another key characteristic of the brushed servo motor is
a high ratio ofhigh ratio of torque to inertia.torque to inertia.
This ratio is an important factor in determining motor
responsiveness.
Further, servo motors need to respond to small changesrespond to small changes
in the control signal.
So the design requires reaction to small voltage variations.

9. ServosServos
Hobby radio-controlled servos

10. Small Servos (servo-motors) for smallSmall Servos (servo-motors) for small
robotsrobots
Positioning applications:

11. Three-wire interface of standardThree-wire interface of standard
servosservos

12. PWM control of ServosPWM control of Servos

13. Pulse between 0.6 ms and 2.0 ms
Pulse length controls the angle of the shaft rotationPulse length controls the angle of the shaft rotation

14. Speed Control PWMSpeed Control PWM is not theis not the
samesame as the Servo PWMas the Servo PWM
counterclockwise
center
clockwise
Single pulse width

15. What to remember when you write software to control?What to remember when you write software to control?
Writing software, take into account
mechanical constraints

16. ““Winch servo” andWinch servo” and how tohow to
convertconvert from standard servofrom standard servo

17. Pneumatic & Hydraulic ActuatorsPneumatic & Hydraulic Actuators
1.1. Large manipulators in industryLarge manipulators in industry frequently employ hydraulichydraulic drives,
• since such drives provide a higher torque-to-weight ratiohigher torque-to-weight ratio than electric motors
1. However, because of the maintenance problems associated with
pressurized oil (including leaks), hydraulic motors are not used in
smaller mobile robots
2. Pneumatic drives have been used as actuators in the past but are not
currently popular
3. Air is compressible, resulting in nonlinear behavior of the actuator