A frameless torque motor is a direct drive motor built into a machine instead of placed inside a separate housing. In robotics, it lets a joint produce smooth rotational force without gears, belts, or bulky motor cases. This matters because robot arms, gimbals, and precision stages need compact joints that can move accurately and hold position under load.
The motor becomes part of the structure, with the stator fixed to the joint frame and the rotor attached to the moving link.
Key Facts
- Torque is rotational force: τ = rF sin θ.
- Mechanical power in rotation is P = τω, where τ is torque and ω is angular speed.
- A torque motor is designed for high torque at low speed, often with direct drive motion.
- In a frameless motor, the stator and rotor are supplied without a housing, bearings, or shaft.
- Direct drive joints reduce backlash because there are no gear teeth or belt stretch between motor and load.
- Motor torque is approximately proportional to current: τ = Kt I, where Kt is the torque constant.
Vocabulary
- Frameless motor
- A motor kit made of separate rotor and stator parts that are integrated directly into a machine structure.
- Torque
- Torque is the turning effect of a force applied at a distance from an axis of rotation.
- Rotor
- The rotor is the rotating part of a motor, often carrying permanent magnets in a brushless torque motor.
- Stator
- The stator is the fixed part of a motor that contains coils which create a controlled magnetic field.
- Direct drive
- Direct drive means the motor turns the load directly without gears, belts, or other transmission parts.
Common Mistakes to Avoid
- Treating a frameless motor as a complete motor is wrong because it does not include its own housing, bearings, or output shaft.
- Assuming high torque always means high speed is wrong because torque motors are often optimized for strong low speed motion and holding force.
- Ignoring heat removal is wrong because the stator coils can generate significant heat that must flow into the robot joint structure.
- Using gearmotor design rules for direct drive joints is wrong because frameless torque motors have little backlash but require careful alignment, sensing, and control.
Practice Questions
- 1 A robot joint uses a frameless torque motor with torque constant Kt = 0.85 N m/A. What current is needed to produce 17 N m of torque?
- 2 A direct drive joint produces 24 N m of torque while rotating at 3 rad/s. What mechanical power is delivered to the joint?
- 3 Explain why a frameless torque motor can improve precision in a robot arm joint compared with a motor connected through a gearbox.