How to select a motor for robot joints | frameless motors

Modern robotic joints integrate various components, including motors, encoders, gearing systems, and occasionally brakes. A frequently used approach involves a direct drive frameless torque motor kit combined with a high-precision gear system. Although the terms "direct drive" and "precision gear" might seem contradictory, their synergy effectively maximizes both size and performance.

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The following five guidelines will assist you in selecting an appropriate direct drive frameless torque motor kit and outline essential engineering requirements for motor selection.

1. SYSTEM ARCHITECTURE

The motor selection process is primarily influenced by control types, feedback necessities, and mechanical characteristics.

a. The typical motorized joint consists of a brushless permanent magnet frameless torque motor kit, an absolute encoder kit, and a zero-backlash gear with a high ratio.

b. Often, a high-precision absolute encoder is necessary at the gear output to mitigate any motion loss and winding-up issues inherent in low-stiffness gear systems.

c. Most systems operate on low voltage levels (typically < 50 volts), which impacts motor impedance and determines input speed ranges.

d. Field-oriented sinusoidal servo drives with integrated safety functions and serial communication interfaces such as EtherCat are becoming common. These drives are also shrinking in size and are installed closer to the motor.

e. Force and torque sensors may be required, especially if the robot operates in collaborative scenarios.

f. If the robot must maintain a specific pose, fail-safe power-off brakes might be essential.

2. MOTOR SELECTION

Key attributes for a frameless motor kit include mechanical form factor, motor constant (Km), and the torque versus speed characteristics under power constraints. It’s crucial to understand the required motor constant, Km, before making a selection.

a. There are no industry standards for motor form factors, including diameter, length, and through-hole dimensions.

b. The Motor Constant, Km, is the most accurate measure of a motor's capacity to deliver torque while under thermal limits, calculated as Km = Kt/sqrt(R), where Kt represents the torque constant in NM/Amp, and R is the resistance in Ohms. Ensure that you maintain unit consistency throughout your calculations, as datasheets can often be unclear.

c. Cogging torque can significantly affect operational smoothness and is typically not included in motor datasheets. The absolute value and frequency of cogging torque are crucial for system dynamics, with lower values being preferable.

d. Sinusoidal torque versus angle curves and phase balance are essential for seamless motion. These metrics are rarely found in datasheets, so it’s beneficial to request this information from the manufacturer.

e. Understanding thermal resistance and thermal time constants is vital, yet often overlooked in datasheets that may rely on unrealistic testing conditions. Certain robots may have specific temperature conditions that differ from standard motor ratings.

f. The electrical time constant often goes unnoticed during selection, although it considerably affects servo system response and PWM frequency, which very much influences overall performance.

3. MOTOR INTEGRATION

Incorporating a frameless motor kit into a mechanical joint entails complex mechanical design that considers rotation mechanics, dynamics, and thermal aspects.

a. High mechanical integration often leads to reduced joint size, involving fewer bearings, eliminating couplings, and shared rotating shafts and housings, improving reliability while lowering component count.

b. Direct drive brushless frameless torque motors yield the highest Km; ensuring maximum torque output under the thermal budget. If torque requirements are unclear, it’s wise to opt for the highest Km motor possible that fits within your designated space. Continuous torque ratings from suppliers often serve as mere references, linked to thermal conditions.

c. Choose the largest diameter frameless motor kit with the shortest length to achieve the desired Km, as Km increases more rapidly with diameter than with axial length.

d. Larger diameter, high Km motors usually feature substantial through-holes, which may integrate bearings or house holding brakes co-axial to the motor, helping to reduce the joint's axial length.

e. Radial and axial magnetic forces affect all motors yet are seldom present in datasheets; remember to inquire about these from your supplier.

f. Opt for motors with minimal or zero cogging torque to reduce diagnostic and vibration issues historically tied to cogging torque.

4. TEMPERATURE AND THERMAL LIMITS

a. The thermal resistance associated with mounting a motor in a robot joint is usually unknown, making typical torque output figures in datasheets somewhat irrelevant. Thus, opting for the highest available Km in the given size allows for greater flexibility.

b. If integrating a power-off brake—often required for safety—the brake itself generates heat during motor operation, further constraining the thermal budget.

c. A complete thermal model should account for all heating sources and the available thermal budget for every joint, necessitating significant engineering for each joint.

d. Typically, the motors must be optimized to operate under less than ideal thermal conditions, which may result in derated performance.

5. OFF-THE-SHELF, MODIFIED STANDARD, OR CUSTOM MOTOR SOLUTION

a. While off-the-shelf products are appealing for their availability and cost-effectiveness, they may not align with the precision needs of robots manufactured under strict quality controls.

b. Tight control over design and revisions can contradict the off-the-shelf supplier approach, leading to unexpected changes in materials or designs, thus complicating matters.

c. Inexpensive hobby motors from global distributors may enter advanced systems without clear manufacturer identification, which places quality control at risk.

ARCSEC DRIVE prides itself on delivering excellent and reliable service.

d. It’s advisable to source from respected suppliers with transparency in operations, allowing factory visits and change controls. At a minimum, pursue modified standard products that include design revision controls.

e. If annual units approach a few hundred, opting for a custom-designed solution could be advantageous. This route ensures control over design, materials, and manufacturing processes, centralizing the requirements needed for your application.

A standard frameless motor exemplifies this with motors like ThinGap's slotless LS Series, known for zero cogging and substantial through-hole designs; their Km can rival traditional slotted motors in performance.

For compact integrated Robot Joints utilizing a frameless torque motor kit, one can expect high-resolution, accurate absolute encoders along with precision gears.

Comparatively, traditional servo motor setups are bulkier and less reliable due to increased components, highlighting the design benefits of modern frameless solutions.

How to Select the Best Motor for a Jointed Arm Robot