Frameless direct-drive servo motors are often the best choice in applications that require absolute precision and design flexibility.
Frameless motors: less is more
Compact, high-end actuators are found in many applications where precision is essential, such as robotic workstations and applications where 18-bit or higher resolution may be required for highly repeatable positioning.
For example, consider a gimbal that stabilizes and positions an electro-optical or infrared imaging system, used for example for fine targeting in aerospace and defense applications; its motors must react immediately and precisely to maintain a stable image despite the severe vibrations that occur during high-speed flight.
Many other applications depend on the ability to provide high torque, responsive acceleration/deceleration, and absolute precision, all in a compact footprint; these are where frameless direct-drive motors are the ideal choice.
Even for projects requiring an ultra-compact design with extremely high precision and responsiveness, or with the need to protect the motor from harsh environmental conditions, a frameless motor is often the best solution.
Precision, efficiency, compactness and resilience
Even when using transmission elements, frameless motors allow to minimize elasticity and backlash: a typical case is robotics applications, where frameless motors are combined with harmonic reducers.
With a high torque density, these motors are the most energy-efficient solution; they are able to meet the torque and speed requirements of the application because they consist exclusively of a stator and rotor. All the components associated with a traditional servomotor are integrated into the application mechanism; this allows to obtain a particularly small footprint, without compromising its performance.
By integrating frameless motors directly into the application, it is possible to protect them from environmental factors; in washdown applications, for example, a frameless motor can be recessed so that high-pressure fluids do not even touch it.
Furthermore, it is easy to optimize its size in relation to the characteristics required by the performance. Kollmorgen’s multiple frameless product lines offer shallow depths with standard diameters ranging from a few centimeters to nearly a meter. Bus voltages range from ≤ 48 VDC to 680 VDC (480 VAC rectified). Continuous torque capabilities range from a fraction of a Nm to several thousand.
Depending on the options, design engineering can determine the moveable mass, timing, application and spatial restrictions, including environmental challenges.
A feedback device provides rotor position to control timing and power. In its simplest form, this commutation signal can be provided by Hall-effect sensors. Another option is to provide closed-loop control feedback using an incremental encoder, which also incorporates Hall-effect output tracks. The best solution with frameless motors, however, is an absolute encoder, which provides 18-bit or better resolution.
Frameless motor integration and temperature control
Unlike a motor with a mounting flange that attaches to the machine, a frameless stator typically has a hollow cylindrical component that acts as the motor housing. This is mounted inside the machine close to the shaft to be driven, allowing for a compact design.
To ensure structural integrity and provide adequate heat dissipation, the housing is typically made of aluminum, with a minimum thickness of 4-6 mm.
Kollmorgen provides a rich set of design tools such as the Frameless Motor Performance Curve Generator, which provides detailed information about the motor’s speed and torque, available over a wide range of specific thermal conditions.
This allows you to properly size motors for each application, including design requirements for stator housing dimensions and thermal considerations for components such as bearings, gears, and feedback devices.
Some frameless motors are designed to operate well at a temperature significantly lower than their rated maximum temperature. For example, the Kollmorgen TBM2G series delivers exceptional performance at no more than 85°C, but can also maintain full performance continuously up to 155°C.
A linear thermal sensor is often useful during the design and prototyping phase to ensure that the motor can deliver the required continuous torque without exceeding a safe winding temperature. A motor operating at its maximum winding temperature, such as 155°C, can cause thermal damage to nearby sensitive components such as bearings and gears, as well as thermally sensitive feedback electronics.
A linear thermal sensor, such as a PT1000, can provide the information needed to prevent failures, and the data could be used to safely push the machine to high performance levels.
Many applications do not need the level of detail provided by a linear thermal sensor, in which case a simple PTC device, connected to the drive, can support several corrective actions in the event of overheating. For example, if a motor begins to overheat the control system can be programmed to provide an alarm, reduce current until the motor cools, or go through a slowdown/stop sequence.
Mechanical features for frameless motors
A traditional motor has internal bearings that allow the rotor to spin freely. These bearings are not intended to support the load, so an additional external set is required. Frameless motors do not require additional bearings, the bearings on the shaft support both the rotor and the load.
There is no need to change the overall design of the machine to accommodate a frameless motor. The designer simply needs to find a location on the shaft to mount the rotor; based on the position of the rotor, the stator housing is placed in the machine.
The movement of the rotor does not introduce any significant axial or radial load forces on the bearings.
Frameless motors are ideal for direct drive applications, but when it is desirable to increase torque by reducing speed, these motors can also be used with harmonic drive gears, cycloidal, spur and planetary gears. These gearboxes maintain high precision and high torque multiplication in a compact footprint.
For example, using a harmonic drive gear with a typical reduction ratio of 100:1 reduces the inertia of the load reflected on the motor shaft by the square of the ratio, or a factor of 10,000, without significantly affecting the overall size of the application.
Some applications require electromagnetic or mechanical brakes. In vertical applications, for example, gravity is a factor that could dislodge the load from its intended position if power to the motor were to be unexpectedly interrupted.
Another use for brakes is to maintain the integrity of the load position when the motor is intentionally turned off. Brakes can be supplied as an integral component of many motors; in a frameless motor, they are added to the primary shaft.
When designing, consider the manufacturing process, assembly order, and total costs. For example, if the application is expected to have extreme radial loads that could shorten the typical life of the shaft bearings, it may be appropriate to incorporate a disassembly system for easy replacement of the bearings.
