How to Select a Robot Chassis Drive System ?

Currently, the drive systems for mobile robot platforms are primarily categorized into hub motor drive systems, servo motor drive systems, steering wheel drive systems, rear axle drive systems, and others. Many customers often ask: What are the differences between servo motor drive systems and hub motor drive systems? How should I choose and purchase? The key distinctions lie in their fundamental differences in application scenarios, control objectives, and system architecture.

The wheel hub motor with an integrated Hall sensor represents a highly integrated design that embeds high-precision position and speed sensors directly within the hub motor. Its core value lies in empowering control systems to achieve closed-loop precision drive (such as sensorless FOC speed regulation and electronic differential control), active safety protection (hill-hold and anti-lock braking), and functional optimization (regenerative braking energy recovery and high-precision mileage measurement) by providing real-time feedback on the wheel's precise rotational position, speed, and direction data. This enhances the equipment's dynamic responsiveness, operational reliability, and energy efficiency.

Purpose: As the power source driving the wheels, the primary objective is to provide propulsive/braking force and monitor wheel speed for vehicle control (e.g., anti-slip, anti-lock braking, traction control, energy recovery) and basic speed/mileage display.

Control Objectives: Direct micrometer/arc-second precision position control of the motor rotor is typically not required. The core requirement is a reliable wheel speed signal. Control loops (if present) are usually speed or torque loops, with significantly lower demands for dynamic response and absolute accuracy compared to industrial servo systems. Encoders (or equivalent sensors) primarily provide wheel speed and rotation direction information for vehicle-level control strategies.

Servo motors are intelligent actuators that integrate high-precision encoders with real-time feedback control, specifically engineered for closed-loop motion control, achieving micrometer-level positioning accuracy (±0.01mm), millisecond-level dynamic response, and 0.1% velocity stability. Their core value lies in empowering high-end manufacturing equipment to precisely execute complex trajectory planning, synchronized coordination, and active safety protection (such as safe torque off), thereby reducing critical component failure rates and optimizing energy efficiency.

Purpose: Achieve ultra-high precision control of position, velocity, and torque. The encoder serves as the core feedback component in closed-loop control systems.

Control Objective: The motor shaft (rotor) must precisely achieve the commanded position, velocity, or torque at any given moment, featuring rapid dynamic response (millisecond level) and high steady-state accuracy (micrometer/arcsecond level). The encoder provides real-time, high-resolution, low-latency rotor position/velocity information, forming the foundation for the drive's three-loop closed-loop control: current loop (torque), velocity loop, and position loop.

Encoder Function and Information Hierarchy:

Servo Motor:

Key Role: An indispensable component for achieving high-performance closed-loop control. Without a high-precision encoder, servo performance would severely degrade or become inoperable.

Information Level: Provides precise angular position and velocity information of the motor rotor body. This constitutes the most fundamental and core feedback signal within the control system.

Electric Wheel Hub Motor:

Key Function: Supplies wheel speed information to support vehicle-level safety and control functions (ABS, TCS, ESP, etc.). For motor control itself (e.g., FOC), it may provide rotor position data, but with significantly lower requirements than servo applications.

Information Hierarchy: Typically measures the actual rotational speed of the wheel (tire). In direct-drive hub motors, the motor rotor is rigidly connected to the hub/tire, making the encoder signal theoretically equivalent to wheel speed.

Installation Position:

Servo Motor: The encoder is directly and precisely mounted on the motor's rear shaft extension, coaxial with the motor rotor, to accurately measure rotor position. Installation requires extremely high precision.

Hub Motor:

(Direct-drive type) Motor rotor end: The encoder is mounted on the rotor shaft inside the motor. Its primary purpose is to provide position information required for commutation to the motor driver (e.g., FOC). Resolution/accuracy requirements are lower than for servo motors, but robustness demands are extremely high.

System Integration and Control Loops:

Servo Motor: Encoder signals are directly hardwired to the servo drive. The drive utilizes this signal to perform closed-loop calculations for current, speed, and position within microsecond time scales, directly controlling the motor's PWM output. The encoder serves as the core input for the drive's closed-loop system.

Wheel Hub Motors:

(For motor drive applications)If an encoder is mounted on the motor rotor for FOC control, its signal is connected to the motor controller (MCU). However, the control loop bandwidth and precision requirements for the MCU are typically lower than those for industrial servo drives.

Servo Motor + Encoder: The brain and eyes of precision instruments, pursuing micro-level control accuracy (measuring rotor rotation in micrometers).

Hub Motor + Wheel Speed Sensor (often broadly termed encoder): Primarily monitors wheel status (rotational speed, slip detection), with the primary requirement being reliable operation in harsh environments and relatively relaxed precision demands.

Understanding this fundamental difference in application scenarios is crucial for distinguishing their distinct requirements for encoders. The “encoder” used for vehicle control in hub motors essentially functions more like a highly robust wheel speed sensor rather than the precision position feedback component pursued for ultimate performance in servo systems.