Servos and Bus Servos

Introduction

A servo is a closed-loop actuator that integrates a motor, gear reducer, control circuitry, and position sensor into one unit. From simple PWM servos to high-performance bus servos, they cover a wide range of robotic applications from beginner to professional level.

PWM Servos

Operating Principle

PWM servos are controlled by pulse width modulation signals:

• Signal frequency: 50 Hz (20 ms period)
• Pulse width: 0.5 ms–2.5 ms corresponds to the angular range
• Typical mapping: 1 ms → 0°, 1.5 ms → 90°, 2 ms → 180°

     1ms       1.5ms      2ms
    ┌─┐       ┌──┐       ┌───┐
    │ │       │  │       │   │
────┘ └───────┘  └───────┘   └────
    0°        90°        180°

    ←──────── 20ms period ────────→

Internal Structure

PWM signal → Control IC → H-bridge driver → DC motor → Gear train → Output shaft
                        ↑                                ↓
                  Compare/PID ←── Potentiometer (angle feedback) ←─┘

Common Models

SG90 Micro Servo

Parameter	Value
Torque	1.8 kg·cm (4.8V)
Speed	0.1 s/60°
Weight	9 g
Angular range	180°
Gears	Plastic
Price	~5 RMB
Suitable for	Beginner experiments, light-load scenarios

MG996R Standard Servo

Parameter	Value
Torque	11 kg·cm (6V)
Speed	0.14 s/60°
Weight	55 g
Angular range	180°
Gears	Metal
Price	~20 RMB
Suitable for	Robotic arms, gimbals

MG90S Micro Metal Gear

Parameter	Value
Torque	2.2 kg·cm (4.8V)
Weight	13.4 g
Gears	Metal
Suitable for	Small-scale applications requiring durability

PWM Servo Control Code

// Arduino PWM servo control
#include <Servo.h>

Servo myServo;

void setup() {
    myServo.attach(9);  // PWM pin
}

void loop() {
    // Method 1: Set angle directly
    myServo.write(90);   // Move to 90°
    delay(1000);

    // Method 2: Set pulse width (microseconds)
    myServo.writeMicroseconds(1500);  // Center position
    delay(1000);

    // Smooth motion
    for (int angle = 0; angle <= 180; angle++) {
        myServo.write(angle);
        delay(15);
    }
}

Limitations of PWM Servos

• No feedback: Can only send target position; cannot confirm if reached
• Single-wire control: Each servo occupies one PWM pin
• Limited precision: Low potentiometer feedback accuracy
• No torque/speed control: Only position control
• Cannot be daisy-chained: Large numbers of servos require PCA9685 or similar PWM expansion boards

Serial Bus Servos

Bus servos use a serial communication protocol, addressing many limitations of PWM servos.

Core Advantages

Feature	PWM Servo	Bus Servo
Connection	Each occupies a PWM pin	Daisy-chained
Feedback	None	Position + speed + torque + temperature
Control modes	Position only	Position / speed / torque
ID addressing	None	Each servo has a unique ID
Parameter configuration	None	Configurable PID, limits, baud rate, etc.
Wiring	Complex (many wires)	Simple (one bus)

Waveshare ST3215 / ST3235

Feetech STS series, distributed by Waveshare, offering excellent cost-effectiveness:

Parameter	ST3215	ST3235
Torque	15 kg·cm	20 kg·cm
Speed	0.054 s/60°	0.068 s/60°
Resolution	4096 (0.088°)	4096
Communication	Half-duplex UART (TTL)	Half-duplex UART
Baud rate	1 Mbps (max)	1 Mbps
Feedback	Position + speed + load + temperature + voltage	Same
Control modes	Position / speed / PWM open-loop	Same
Price	~60 RMB	~80 RMB

Daisy-Chain Connection

MCU(TX/RX) → Servo1(ID=1) → Servo2(ID=2) → Servo3(ID=3) → ...
               │               │               │
              GND             GND             GND
              VCC             VCC             VCC

Half-Duplex Note

The STS series uses single-wire half-duplex communication where TX and RX share one data line. Direction control is needed (tri-state buffer or GPIO-based transmit/receive switching).

Control Example

# Waveshare ST3215 Python control example
# Using SCServo SDK

from scservo_sdk import *

# Initialize port
portHandler = PortHandler('/dev/ttyUSB0')
packetHandler = SMS_STS(portHandler)

portHandler.openPort()
portHandler.setBaudRate(1000000)  # 1Mbps

SERVO_ID = 1

# Write target position (0-4095 corresponds to 0-360°)
# Parameters: ID, target position, speed, acceleration
packetHandler.WritePosEx(SERVO_ID, 2048, 1000, 50)  # Center, speed 1000, accel 50

# Read current state
position = packetHandler.ReadPos(SERVO_ID)
speed = packetHandler.ReadSpeed(SERVO_ID)
load = packetHandler.ReadLoad(SERVO_ID)
temperature = packetHandler.ReadTemperature(SERVO_ID)

print(f"Position: {position}, Speed: {speed}, Load: {load}, Temperature: {temperature}°C")

# Sync write to multiple servos (simultaneous motion)
groupSyncWrite = GroupSyncWrite(portHandler, packetHandler, 
                                 SMS_STS_GOAL_POSITION, 4)
groupSyncWrite.addParam(1, [0x00, 0x08, 0xE8, 0x03])  # ID1: pos=2048, speed=1000
groupSyncWrite.addParam(2, [0x00, 0x04, 0xE8, 0x03])  # ID2: pos=1024, speed=1000
groupSyncWrite.txPacket()

Dynamixel Series

Robotis' Dynamixel is the most well-known bus servo brand, widely used in research and competitions.

Common Models

Model	Torque	Communication	Resolution	Features
AX-12A	15.3 kg·cm	TTL half-duplex	1024	Classic entry-level, discontinued
XL330-M288	0.43 N·m	TTL half-duplex	4096	Ultra-compact, suitable for dexterous hands
XL430-W250	1.4 N·m	TTL half-duplex	4096	Cost-effective entry-level
XM430-W350	4.1 N·m	RS-485	4096	Research workhorse
XM540-W270	10.6 N·m	RS-485	4096	High torque

Dynamixel Protocol 2.0

Communication protocol structure:

[Header] [Reserved] [ID] [Length] [Instruction] [Parameters...] [CRC]
 FF FF FD   00      0x01  0x07      0x03          ...            CRC

Main instructions:

Instruction	Value	Function
Ping	0x01	Check if servo is online
Read	0x02	Read control table data
Write	0x03	Write control table data
Sync Read	0x82	Synchronously read multiple servos
Sync Write	0x83	Synchronously write to multiple servos
Bulk Read	0x92	Bulk read (different addresses)

Dynamixel Python SDK

from dynamixel_sdk import *

ADDR_TORQUE_ENABLE = 64
ADDR_GOAL_POSITION = 116
ADDR_PRESENT_POSITION = 132
BAUDRATE = 57600
PROTOCOL_VERSION = 2.0

portHandler = PortHandler('/dev/ttyUSB0')
packetHandler = PacketHandler(PROTOCOL_VERSION)

portHandler.openPort()
portHandler.setBaudRate(BAUDRATE)

DXL_ID = 1

# Enable torque
packetHandler.write1ByteTxRx(portHandler, DXL_ID, ADDR_TORQUE_ENABLE, 1)

# Write goal position (0-4095)
packetHandler.write4ByteTxRx(portHandler, DXL_ID, ADDR_GOAL_POSITION, 2048)

# Read current position
position, result, error = packetHandler.read4ByteTxRx(
    portHandler, DXL_ID, ADDR_PRESENT_POSITION)
print(f"Current position: {position}")

# Disable torque
packetHandler.write1ByteTxRx(portHandler, DXL_ID, ADDR_TORQUE_ENABLE, 0)
portHandler.closePort()

RS-485 vs. TTL Comparison

Feature	TTL Half-Duplex	RS-485
Signal level	0/3.3V or 0/5V	Differential signal
Transmission distance	<1 m	<1200 m
Noise immunity	Weak	Strong
Number of nodes	Fewer	Up to 32/256
Typical application	Small robots	Industrial / large robots

See UART and I2C/SPI for details.

Bus Servo Comparison Summary

Feature	Feetech STS	Dynamixel X
Price	Low (60–100 RMB)	High (300–2000 RMB)
Ecosystem	Simpler SDK	Complete SDK, rich documentation
Community	Mainly domestic (China)	Global research community
Communication protocol	Proprietary	Protocol 2.0 (open)
Software tools	FD debug tool	Dynamixel Wizard 2.0
Suitable for	Cost-effective projects, education	Research, competitions, commercial

Servo Power Supply Considerations

Power Isolation

• Servos (especially multiple units) should use a separate power supply, not shared with the MCU
• Large transient currents may cause MCU resets
• Power wires must be thick enough to minimize voltage drop
• Bus servos can draw 3–5 times their rated current at startup

Typical Power Supply Configuration

Battery/PSU ─┬─→ Buck converter (5V/3.3V) ──→ MCU
             │
             └─→ Servo power (6-12V) ──→ Servo bus
                                          │
                                 Common GND ←─┘

Summary

• PWM servos are simple and inexpensive, suitable for beginner and light-load applications
• Bus servos support daisy-chaining, status feedback, and multi-mode control, suitable for complex robots
• The Feetech STS series offers excellent value; Dynamixel has a well-established ecosystem
• For projects with many servos, power supply design is critical
• Selection should consider torque, precision, communication method, and budget holistically

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