Understanding RC Servos and DC Motors

What You’ll Learn

• How an RC servo and DC motor operate• Understand the electrical and mechanical details

• How to interpret datasheet specifications and properly apply them in your application

• How your controller selection can impact performance

• Pros and Cons of selecting an RC Servo or DC Motor

•LifeApe Difference•Provide low cost controllers with easy to use software•Develop customized hardware and software products

starting with small volumes •Deliver free application assistance for component

selection, wiring, software development

LifeApe Brief Overview

What Does It Mean to “Servo”?

• Strictly speaking – there is no such thing as a servo motor• Servomechanism – is a system, not a “thing”. Consists of:• Feedback device (sensor) to measure external phenomena

• Position, speed, torque, current, voltage, pressure, altitude, etc…

• Compensator - Mathematical function that governs how the process is regulated• Plant – Physical thing you are trying to control

PLANT

-+COMPENSATOR

PositionCommand

MOTOR GEARHEAD

POTENTIOMETER

Error

ActualPosition

FEEDBACK

Quick RC Servo Introduction

• RC servo → DC motor plus additional components needed for a servomechanism

• RC Servos come in many different sizes• Standard, Pico, Micro, Giant, Tiny, Mini, Low Profile, Jumbo, …

• Rotate either 90° or 180° total• Specialty servos can rotate continuously

• Consist of 3 Wires• Power (4.5 VDC to 6 VDC), Ground, Signal (Control)

• Standard pinout but different wire colors

• Connector is not keyed so it can be plugged in backwards or misaligned

RC Servo Physical Construction

• RC Servo Components• Gears (plastic or metal)• Bearings or Bushings• DC motor• Potentiometer• Electronics

360⁰ Hack

RC Servo Electrical Interface

• Control is performed using Pulse Width Modulation (PWM)• Digital pulse train with constant

frequency and variable duty cycle• Duration of the duty cycle (on time)

controls the position of the RC servo

• RC Servo Typical Timing:• Frequency → 20ms (50 Hz)• Neutral Position → 1.5 ms (7.5%

Duty Cycle) → 0° position• Position Limit(max) → 1.0 ms (5%

Duty Cycle) → -45° to -90° position• Position Limit(min) → 2.0 ms (10%

Duty Cycle) → 45° to 90° position

Analog and Digital RC Servos

• No physical or component differences

• Difference is all electrical• Analog Servo Update Rate = 50 Hz (20 ms)

• Digital Servo Update Rate = 300 Hz (3.5 ms)

Characteristic Advantage Comment

Servo Response Digital Digital servos produce higher acceleration and better overall response

Deadband Digital Digital servos can provide programmable deadband down to zero

Disturbance Response Digital Generate “snappier” response to load change

Position Accuracy Digital Lower deadband results in more accurate positioning

Cost Analog Analog servos are substantially lower cost

Holding Torque Digital Provides stiffer feel when stationary

Power Consumption Analog Digital servos consumer 2x to 4x their analog equivalent

Analog Vs. Digital Servos Continued

Understanding the Specs

• Speed• Measurement of the time it takes the servo to rotate

a certain number of degrees.

• Has been standardized in most specifications to 60 degrees. In other words, the time it takes the servo wheel/arm to turn 60°.

• Torque • Determines the maximum amount of rotational force

the servo can apply at a right angle to a lever

Type Voltage Pulse Width Speed TorqueDeadbandWidth

CurrentGear Material

Weight Dimensions

AnalogDigital

4.8 to 6.0 500us to 2ms 0.12 sec/60° 48 oz-in 4 us No LoadStall

MetalPlastic

0.3 oz L x W x H

Application Considerations

• Controller• Software ease of use and flexibility

• Adjust the min and max pulse widths

• Set limits to guard against mechanical stops

• Controller Resolution (see table)• Servo Power Supply

• Require an additional dedicated servo supply?

• Does the supply provide adequate current?• Assume ~600 mA per analog servo and double for digital servo.

• Protection against cable miswiring

• Servo Motor Sizing• Torque -> Apply torque wrench to shaft. Adjust torque until wrench is able to rotate the motor shaft.• Speed -> Determine your travel distance and maximum allowable time. “Typical” servo speed is 60°

in 150 ms or approximately 180° in 500 ms at NO load.• Range of motion -> ±180°, ±90°, continuous rotation• Gear Material -> Cheap plastic, better plastic, metal

Controller Resolution

RC Servo Accuracy

Accuracy Difference

16-bit 0.054° NA

12-bit 0.88° 16x

8-bit 14° 256x

• Stator• Permanent magnets that surround the rotor which

generate a stationary magnetic field

• Rotor (Armature)• One or more windings that when energized (current

flowing) produce a magnetic field

• Brushes and Commutator• Work together to switch (commutate) the motor

windings. As the motor turns, carbon brushes slide over the commutator closing the circuit.

DC Motor (Brushed Motor) Construction

DC Motor Electrical Interface

• H-Bridge circuit used to allow bidirectional current flow enabling motor to move both forward and reverse

• Controller turns on appropriate switches with precise timing to ensure no cross conduction occurs

• Switches are typically controlled with PWM

DC Motor – Understanding the Specs

• Stall torque

• No load speed

• No load current

• Nominal voltage

• Torque constant

• Speed constant

• Electrical time constant

• Mechanical time constant

• Rotor inertia

• Ambient temperature

Why and Why Not Use DC Motors?

• Why?• Performance! Performance! Performance!

• Much higher torque, speed, and acceleration in similar dimensions

• Capable of replacing pneumatics in many applications

• Smoother movement than RC Servos, MUCH smoother than pneumatics

• Continuous rotation

• Why Not?• Few (if any) animatronics controllers are designed to control DC motors

• Feedback device needs to be supplied to provide positional control

• Requires more mechanical know-how to attach load to shaft

• More expensive IF an RC servo can meet your application requirements

Summary Analysis

Characteristic Advantage

Torque DC Motor

Speed DC Motor

Cost RC Servo Motor

Controller Options RC Servo Motor

Range of Motion DC Motor

Motion Smoothness DC Motor

Mechanical Knowledge RC Servo Motor

Ease of Use RC Servo Motor

support@lifeape.com 512.230.0468 www.lifeape.com

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