Limited Angle Servo Motors

Overview

Limited angle servo motors are brushless direct drive rotary motors designed for precision angular positioning applications. These frameless motors deliver smooth, cogging-free torque with high angular acceleration capabilities. The unhoused design allows direct mounting to customer bearing systems, making them ideal for space-constrained applications where conventional rotary motors won’t fit.

Available in two configurations: TMR Series (toroidally wound iron core with permanent magnet rotor) and TWR Series (arc segmented permanent magnet stator with wound wire rotor). Angular excursions range from less than 90° to full 360° rotation depending on configuration.

How Limited Angle Servo Motors Work

A current-carrying conductor placed in a magnetic field experiences a force proportional to the current magnitude and flux density. In limited angle servo motors, the permanent magnet creates a fixed flux density field. The direction of rotation depends on input current polarity, while torque output is directly proportional to current magnitude.

The system operates in closed-loop servo mode when coupled with a rotary encoder for position feedback. A DC linear servo amplifier provides power to the motor. The motion controller sends reference signals to the amplifier, which delivers the necessary current to generate required torque. Position feedback verifies the motor has reached and maintains the desired angular position.

Low rotor inertia enables high angular accelerations of the payload. The compact length-to-diameter ratio allows these motors to fit where conventional brush and brushless DC motors cannot.

Key Components

TMR Series (Toroidally Wound)

Rotor: Rare earth permanent magnet with 2, 4, or 6 poles. Maximum angular excursion is 180° (2-pole), 90° (4-pole), or 60° (6-pole). Torque drops to zero at travel extremes. Installed within the stator ID with magnetic attractive force between components that cancels when concentric.

Stator: Electrically insulated soft magnetic steel toroid with toroidally wound copper magnet wire sections. Two leads exit the assembly.

TWR Series (Arc Segmented)

Rotor: Single bondable copper magnet wire coil, preformed and held by bonding agents. Can be encapsulated with aluminum or plastic mounting brackets. Maximum angular excursion is less than 90°.

Stator: Multipole permanent magnets bonded to steel plates with end plate spacers creating a gap for rotor movement. No magnetic attractive force between stator and rotor.

TWR Configurations:

• Type L: Lower profile, fits compact spaces, less angular rotation per stator arc segment
• Type S: Larger overall height, approximately twice the angular rotation per stator arc segment

Performance Specifications

Torque Output:

• Continuous torque: Up to 1000 oz-in available
• Peak torque (10% duty): Up to 1440 in-lb [159 Nm]

Angular Range:

• TMR Series: Up to 180° (2-pole), 90° (4-pole), 60° (6-pole)
• TWR Series: Up to 90° per segment
• Full 360° achievable with modified stator winding (4-lead, commutated)

Example Specifications (TWR-070-410-1150-3PH):

• Continuous torque: 480 in-lb [53 Nm]
• Peak torque: 1440 in-lb [159 Nm]
• Angular range: 70° (expandable with additional 33° magnet track modules)
• Rotor inertia: 318.6 lb-in² [0.09 kg-m²]
• Moving mass: 2 lb [0.9 kg]

Advantages & Limitations

Control Options

Electronics Required:

• DC linear servo amplifier (due to low coil inductance)
• Programmable motion controller
• Rotary encoder or other feedback device

Control Loop Operation:

1. Motion profile downloaded to controller (speed, acceleration, position, PIDs)
2. Controller sends ±10V reference signal to servo amplifier
3. Amplifier provides current to generate required torque
4. Motor moves to desired position
5. Encoder feedback confirms position (typically 500 times/second)

Commutation Options (3-Phase TWR):

• Trapezoidal commutation using integral Hall Effects
• Sinusoidal commutation via software with appropriate amplifier/controller

Design Flexibility

• Frameless design mounts directly to customer bearing systems
• TMR or TWR series based on application requirements
• Angular excursions customizable up to 360°
• Multiple coils within single magnet assembly
• Modular magnet tracks expandable in 33° increments (TWR)
• Custom motors designed to specific application needs

Environmental & Mounting Considerations

Environment: Precision device requiring protection from wet or dirty conditions. Shield magnetic assembly from loose steel particles. Avoid ambient temperatures exceeding 50°C.

Mounting: Typically supplied without bearings, shaft, or housing for direct integration with customer rotary payload assembly. Customer must maintain proper clearances (0.025″ for TMR, 0.050″ for TWR). Motor wires must be strain relieved.

Applications

Limited angle servo motors excel in precision angular positioning where space is limited:

• Aerospace systems
• Semiconductor equipment
• Medical devices
• Military applications
• Coordinate measuring machines
• Pick and place machines
• Laser and water jet cutters
• Vision inspection equipment
• Warehouse sorting diverters

Why Choose Limited Angle Servo Motors

These motors are ideal when you need precise angular control in a compact package. The direct drive design eliminates backlash while the frameless construction integrates seamlessly with existing bearing systems. Zero cogging ensures smooth operation, and the brushless design requires no maintenance. Whether your application needs limited rotation or full 360° motion, these motors can be configured to meet your exact requirements.

Ordering Information

Torquers can be ordered for any angular excursion up to 360° and continuous torques up to 1000 oz-in. Motors are built to order based on customer specifications. Custom designs available—consult factory for details and drawings.