Servo Motor Control

Design Project You are to design a mechanical device that can tilt a given labyrinth board in two directions.

One Concept Could use two motors. Must be able to control the rotation angle of each.

Another Example Control the steering angle of an autonomous ground vehicle. Must be able to control the rotation angle.

Design Choices What power source will be used? How much power is needed? Is a speed reducer needed? Open-loop or closed-loop control?

Design Choices What power source will be used? electric servo motor electric stepper motor hydraulic actuator pneumatics

Design Choices How much power is needed? determine the maximum product of motor torque times angular velocity

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

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

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

How does this work? motor gear reducer desired steering angle how is the motor connected to the steering shaft? how does the computer cause the motor to turn? how does the computer know the current steering angle?

how is the motor connected to the steering shaft? Motor and Gear Box how is the motor connected to the steering shaft?

Motor and Gear Box

motor with gear reducer flex coupling

how does the computer know the current steering angle? Servo Motor Control select means of angle measurement optical encoder incremental absolute potentiometer resolver how does the computer know the current steering angle?

Incremental Optical Encoder this disk has 12 slots (30º resolution) typical incremental encoders have up to 2540 counts per revolution (0.142º resolution) ($488.41 each at McMaster-Carr)

Incremental Optical Encoder

Incremental Optical Encoder you can count pulses, but which way is it rotating?

Incremental Optical Encoder an encoder will typically also have an index that will ‘blink’ once each revolution

Incremental Optical Encoder 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 note: many encoders will have an index that will pulse once per revolution

Honeywell / Clarostat 600128CN1 Allied Electronics part # 753-0059 cost: $41.67

Incremental Optical Encoder problem: - you start up the device - where are you when you start?

Absolute Optical Encoder BEI brand encoder $455

How does this work? motor gear reducer desired steering angle how is the motor connected to the steering shaft? how does the computer cause the motor to turn? gear reducer and flex couplings (if needed) how does the computer know the current steering angle? computer controls the current going to the motor incremental optical encoder with homing switch to initialize position

How does the computer control the motor? control the current (via the voltage)

motor with gear reducer flex coupling limit switch homing switch optical encoder

Closed Loop Control

Homing and Limit Protection we need to add a switch to set the homing position we need to have overshoot switches

motor with gear reducer flex coupling limit switch homing switch optical encoder

Summary of Closed-Loop Control computer controls the current going to the motor motor gear reducer desired steering angle compared to current steering angle gear reducer and flex couplings (if needed) connect motor to steering shaft incremental optical encoder (with homing switch to initialize position) feeds back current motor position