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Servo Motor Control
EML 2023Department of Mechanical and Aerospace Engineering
Design Project
• You are to design a mechanical device that can tilt a given labyrinth board in two directions.
EML 2023Department of Mechanical and Aerospace Engineering
One Concept
• Could use two motors.• Must be able to control
the rotation angle of each.
EML 2023Department of Mechanical and Aerospace Engineering
Another Example
• Control the steering angle of an autonomous ground vehicle.
• Must be able to control the rotation angle.
EML 2023Department of Mechanical and Aerospace Engineering
Design Choices
• What power source will be used?
• How much power is needed?
• Is a speed reducer needed?
• Open-loop or closed-loop control?
EML 2023Department of Mechanical and Aerospace Engineering
Design Choices
• What power source will be used?
• electric servo motor• electric stepper motor• hydraulic actuator• pneumatics
EML 2023Department of Mechanical and Aerospace Engineering
Design Choices
• How much power is needed?
• determine the maximum product of motor torque times angular velocity
EML 2023Department of Mechanical and Aerospace Engineering
Design Choices
• Is a speed reducer needed?
• a typical DC motor can rotate at a top speed of 2600 rpm (43⅓ rev/sec)
• gear box• epicyclic gear train• ball screw drive
gear box
worm gear drive
epicyclic gear train
ball screw drive
EML 2023Department of Mechanical and Aerospace Engineering
Design Choices
• Open-loop or closed-loop control?
• open-loop• well suited for remote
control using a joystick• computer control can be
implemented using a stepper motor
• closed-loop• requires a sensor to
measure the motor’s rotation angle
EML 2023Department of Mechanical and Aerospace Engineering
Design Choices
• What power source will be used?• electric servo motor
• How much power is needed?• experiment or perform analysis
(very little torque will be needed to rotate the labyrinth board ; further the rotational speed will be very low)
• Is a speed reducer needed?• yes
• Open-loop or closed-loop control?• closed-loop
EML 2023Department of Mechanical and Aerospace Engineering
how does the computer cause the motor to turn?
how does the computer cause the motor to turn?
How does this work?
desired steering angle
motorgear reducer
how is the motor connected to the steering shaft?
how is the motor connected to the steering shaft?
how does the computer know
the current steering angle?
how does the computer know
the current steering angle?
EML 2023Department of Mechanical and Aerospace Engineering
Motor and Gear Box
how is the motor connected to the steering shaft?
how is the motor connected to the steering shaft?
EML 2023Department of Mechanical and Aerospace Engineering
Motor and Gear Box
EML 2023Department of Mechanical and Aerospace Engineering
motor withgear reducer
flex coupling
EML 2023Department of Mechanical and Aerospace Engineering
Servo Motor Control
select means of angle measurement– optical encoder
• incremental• absolute
– potentiometer– resolver
how does the computer know
the current steering angle?
how does the computer know
the current steering angle?
EML 2023Department of Mechanical and Aerospace Engineering
Incremental Optical Encoder
• this disk has 12 slots (30º resolution)
• typical incremental encodershave up to 2540 counts perrevolution (0.142º resolution)($488.41 each at McMaster-Carr)
EML 2023Department of Mechanical and Aerospace Engineering
Incremental Optical Encoder
EML 2023Department of Mechanical and Aerospace Engineering
you can count pulses, but which way is it rotating?
Incremental Optical Encoder
EML 2023Department of Mechanical and Aerospace Engineering
an encoder will typically also have anindex that will ‘blink’ once each revolution
Incremental Optical Encoder
EML 2023Department of Mechanical and Aerospace Engineering
quadrature: - there are 4n ‘events’ per one revolution of the disk- the resolution can in effect be four times the number of slots
Incremental Optical Encoder
EML 2023Department of Mechanical and Aerospace Engineering
note: many encoders will have an index that will pulse once per revolution
Incremental Optical Encoder
EML 2023Department of Mechanical and Aerospace Engineering
cost: $41.67
Honeywell / Clarostat 600128CN1Allied Electronics part # 753-0059
EML 2023Department of Mechanical and Aerospace Engineering
problem: - you start up the device- where are you when you start?
Incremental Optical Encoder
EML 2023Department of Mechanical and Aerospace Engineering
Absolute Optical Encoder
BEI brand encoder $455
EML 2023Department of Mechanical and Aerospace Engineering
how does the computer cause the motor to turn?
how does the computer cause the motor to turn?
How does this work?
desired steering angle
motorgear reducer
how is the motor connected to the steering shaft?
how is the motor connected to the steering shaft?
how does the computer know
the current steering angle?
how does the computer know
the current steering angle?
incremental optical encoder with homing switch to initialize position
gear reducer and flex couplings (if needed)
computer controls the current going to the motor
EML 2023Department of Mechanical and Aerospace Engineering
How does the computer control the motor?
control the current (via the voltage)
EML 2023Department of Mechanical and Aerospace Engineering
optical encoder
motor withgear reducer
flex coupling
limit switchhoming switch
EML 2023Department of Mechanical and Aerospace Engineering
EML 2023Department of Mechanical and Aerospace Engineering
EML 2023Department of Mechanical and Aerospace Engineering
EML 2023Department of Mechanical and Aerospace Engineering
EML 2023Department of Mechanical and Aerospace Engineering
EML 2023Department of Mechanical and Aerospace Engineering
Servo Motor Control
• we will use a Galil motor control card to vary the current to the motor based on the error in the steering shaft angle
motorDesired angle Angle error controller ampDesired Current current signal Steering angleSteering angle
motorDesired angle Angle error controller ampDesired Current PWM signal Steering angleSteering angle
desired position
EML 2023Department of Mechanical and Aerospace Engineering
Homing and Limit Protection
• we need to add a switch to set the homing position• we need to have overshoot switches
desired angle
motorDesired angle Angle error controller ampDesired Current PWM signal Steering angleSteering angle
terminal program providesa user interface where we cansend commands to changethe steering angle
EML 2023Department of Mechanical and Aerospace Engineering
optical encoder
motor withgear reducer
flex coupling
limit switchhoming switch
EML 2023Department of Mechanical and Aerospace Engineering
