Understanding RC Servos and DC Motors

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 Brief Overview 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

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 Position PLANT Command Error + - COMPENSATOR MOTOR FEEDBACK POTENTIOMETER GEARHEAD Actual Position

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 Type Voltage Pulse Width Speed Torque Deadband Width Current Gear Material Weight Dimensions Analog Digital 4.8 to 6.0 500us to 2ms 0.12 sec/60 48 oz-in 4 us No Load Stall Metal Plastic 0.3 oz L x W x H 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

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 Controller Resolution RC Servo Accuracy 16-bit 0.054 NA 12-bit 0.88 16x 8-bit 14 256x Accuracy Difference 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

DC Motor (Brushed Motor) Construction 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 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 Torque Speed Cost Controller Options Range of Motion Motion Smoothness Mechanical Knowledge Ease of Use Advantage DC Motor DC Motor RC Servo Motor RC Servo Motor DC Motor DC Motor RC Servo Motor RC Servo Motor

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