Direct drive motors with diameters of >1m are possible, able to produce torque of >10,000Nm. Many direct drive motors are ‘frameless’ which means that they are supplied without a housing, bearings or feedback sensor. This allows machine builders and system integrators to streamline their housing, shaft and bearing design to optimize overall size, shape, weight and dynamic performance.
The two main reasons for a design engineer to choose a direct drive are dynamic performance and shape factor. Rather than dealing with a coupling, gearbox, belts or chains, a direct drive motor attaches directly to the load so there is no hysteresis, backlash or ‘lost motion’ in any direction of movement. The design advantage that comes from motors which are fairly flat with a large hole in the middle – allowing slip-rings, pipes and cables to pass through – should not be underestimated.
The advantages of the direct drive approach include:
- Excellent dynamic performance and accurate control of position and/or speed
- No backlash or wear
- High reliability due to low part count & elimination of gears, pulleys, seals, bearings etc.
- Compact – with low axial height and large bore feasible
- Low torque ripple or ‘cogging’
- Energy efficiency from eradication of losses in intermediate mechanical elements
- Low acoustic noise or self-induced vibration
- No/low maintenance
- Low cooling requirements due to advantageous thermal geometry
- Relatively large airgaps – easy installation and resistance to shock.
The main disadvantage is often more perceived than actual – direct drive motors are often thought to be more expensive than traditional motors. While this may often be true in a simple 1:1 comparison, a more holistic view (taking in to account the eradication of intermediate gears, couplings, maintenance as well as reduction in overall mechanical simplification) shows that direct drive arrangements are, perhaps surprisingly, the optimal cost and performance solution in many applications.
Classic examples of direct drive applications are found in gimbals such as antenna systems (e.g. vehicle mounted satellite communications), surveillance & CCTV cameras, scanners, telescopes, electro-optics, rate tables, and radar systems. There are also applications in CNC machine tools, packaging equipment, robotics and even high end record turntables.
If the bore of the direct drive is fairly small (<2”) there is a wide choice of position feedback sensors based on optical, magnetic, capacitive and inductive technologies. For larger bores the primary options are frameless resolvers, ring encoders and inductive encoders.