Direct Drive Systems: taking the direct route
New technologies are constantly being developed to improve quality and increase output rates for automated production and test equipment. Many manufacturers now prioritise higher throughput as a means to maintain their competitive edge and in order to optimise their operating costs they demand faster production rates in combination with higher quality output and a reduction in post processing operations. Other factors influencing manufacturing costs such as extending wear cycles, reducing machine downtime, eliminating maintenance, faster tool changing, quicker product loading and smaller machine footprints are also being targeted.
Just a few years ago many engineers and designers might have assumed that automated positioning system design had reached a plateau and a level of maturity that would limit increasing production rates or quality beyond a small percentage. Whilst important steps have been achieved in the fields of control software, process optimisation, ergonomics and tool design, arguably the most significant factor that has delivered results has been the advent of "direct drive" technology with both linear and rotary servo motors. At a conservative estimate, potential production rate increases have been raised by three to five times and direct drive technologies have essentially re-shaped positioning system engineering as we know it today. Thanks to its increased dynamic response, augmented precision and imperviousness to wear, direct drive technology is increasingly replacing conventional drive systems in motion control applications. This article explores some of its numerous advantages as well as some of its potential problems.
Unlike conventional drive technology, direct drive systems have no need for motion converters of any type (couplings, gearboxes, belts, friction wheels, toothed wheels, etc.). This brings real advantages, as all motion converters are sources of energy loss, loss of precision and suffer from wear. Furthermore, motion converters cause elasticity and hysteresis in terms of control technology, which reduces overall performance. The direct linking of drive, feedback system and load in motion increases rigidity in terms of mechanics and control, allowing loads to be positioned or moved more dynamically and therefore faster and more accurately. Both linear and rotary direct drive motors may be used. The direct drive linear motor obviously takes care of linear positioning applications and in the case of the direct drive rotary or "torque" motor, this produces sufficient torque to move the load at high speeds and position the load with enough accuracy, repeatability and angular resolution to negate the need for worm/wheel gearboxes or other transmission components. Aerotech also have some rotary positioning stages, designed with limited rotational travel, based upon "formed" linear stages. All of these direct drive motor types make possible the rigid and direct link to the payload in motion and also enable close coupling of the feedback system required for servo control.
A distinction is made between iron cored motors and ironless motors. Ironless motors have no magnetic attraction between the coil and the magnet track as the core contains no iron. In respect of linear motors, the longer the path to be traversed, the less expensive iron core motors become compared to ironless versions as the corresponding magnet tracks are normally flat rather than "U" shaped and can be produced more easily with a single row of magnets.
However, the iron core design conceals a number of disadvantages, the most obvious of which is the increased mass or weight of the coils, which has a negative effect for lightweight payloads in motion applications. Iron core rotors have a strong magnetic attraction with the stator, thereby subjecting the bearing to a sometimes excessive basic load. This not only makes assembly significantly more difficult when setting up and when replacing the motor during servicing, but also impairs synchronism due to the increased harmonics. Iron cored designs can produce the highest of continuous and peak forces but their lack of smoothness exemplified by their inherent magnetic "cogging" is a potential problem to high precision positioning. In comparison the "slotless" ironless motor is extremely smooth in performance and is ideally suited to fine and precise servo control. Furthermore, this motor type is considered better for applications where stray magnetic fields could cause problems as the U shaped channel provides better isolation.
Rotary Precision
The rotary "torque" motor offers direct drive technology crucial advantages for high precision rotary positioning applications. The generous inside diameter of these motors supports the design of rotary stages with large hollow shafts and therefore increased rigidity. In turn, this makes possible the close integration of a chuck or similar fixture for holding the payload and enables loading product from the rear, or the feeding through of cables, hoses and other objects. In the event of increased load moments of inertia, the relatively high motor moment of inertia is helpful, as the ratio between the load moment of inertia and the motor moment of inertia should not exceed 1:5 in order to optimise dynamic performance. The combination of this effect and the increased rigidity often improves dynamic performance to such an extent that even at low load moments of inertia, the disadvantage of the increased motor moment of inertia is cancelled out and dynamic performance is even better.
A distinct advantage over conventional geared positioning stages is the relatively lower height and often smaller footprint of the directly driven stage resulting in a reduced "stack" height for multi axis applications. Also in comparison to conventional rotary stages, the absence of a gear mechanism allows the full potential of the motors' dynamic acceleration and high speed performance to be realised, resulting in faster positioning. The increased diameter also enables more magnets to be fitted to the rotor, thereby increasing the number of pairs of poles and ultimately making the magnetic field more homogeneous, in turn increasing torque and reducing harmonics.
For linear direct drive systems the possibility of using multiple independent forcers with a single magnet track and guide rail, with a corresponding (multiple) number of encoder read heads and only a single feedback system scale, is another major advantage of this technology. It provides a cost-effective and very compact means of setting up a number of axes in parallel.
As both linear motors and rotary torque motors have been in mass production for many years, prices have also fallen accordingly, enabling the advantages of this technology to be used on an increasingly widespread basis.
For both linear and rotary direct drive systems, the direct connection between motor and load also enables a rigid connection for the linear or rotary encoder system used for positional feedback. This can significantly improve feedback analysis with the encoder being much closer to the load with the resulting increase in performance evident in improved path accuracy, more precise velocity control and the ability to traverse very small microdistances.
Feedback required
However, a word of caution - direct drive technology does not guarantee positioning accuracy alone, as abbe errors and compound mechanical errors away from the axis of measurement could still cause lack of positional performance. Feedback should not be taken for granted to improve accuracy performance, as this is only measured within the stage and not at the work piece.
In order to be able to use direct drive technology successfully, a number of issues, which are of minor importance in conventional drive technology, must be taken into account. The direct connection between drive and load in motion transfers power losses from the motor as thermal energy and can directly cause unwanted thermal expansion or distortion to the load or work-piece. This would obviously impair precision in an architecture of this type and it is critical that low power loss must be taken into account when selecting a motor and in some cases cooling systems may need to be used much more frequently than in conventional applications. Aerotech's linear and rotary motors are designed to offer the highest force or torque output based on size. This advantage can clearly be used to increase dynamics or to limit the thermal input into the mechanical system.
The high rigidity of the connection between feedback system and object means that external disturbances are injected directly into the controller with virtually no damping. In unfavourable conditions, this can cause the architecture to brace. Sufficient provision should be made to damp external vibrations of this type.
The same high rigidity of the feedback system / object connection means that external vibrations outside of the positioning system may be introduced into the support structure and travel through the mechanical system and into the controller with virtually no damping. In unfavourable conditions, this can cause the architecture to vibrate or ring. Sufficient provision should be made to damp and isolate external vibrations of this type.
Similarly external effects or effects generated within the system itself will cause problems if they are in the region of, or identical to the natural frequency of the architecture. In this context, it is important to understand that all objects connected to the motion axes are component parts of the architecture. For example, the resonance characteristics of a single axis may vary from one machine frame to another. The better the damping characteristics of the machine frame, the more scope there is for improved performance (dynamic response, speed, precision). This dependency on the mechanical environment is significantly higher with direct drive systems than on conventional drive systems.
Knowing your system
Due to the effects of external disturbances exciting mechanical resonances measured on the direct drive system, it is very important to be able to understand the response of the control system and which frequencies it can and cannot reject. By fully understanding the response of the complete system, decisions can be made on how to improve the mechanical system or enable software filters to be used in order to increase bandwidth, thereby enabling the use of the axis to be optimised.
It is therefore highly desirable that motion control systems used with direct drive technology are capable of both recording and making provision for the frequency response for the complete assembly. Aerotech's own A3200 Automation Platform features built-in analysis tools of this type to record the open-loop Bode diagram as well as provide appropriate tools for filter calculation.
The A3200 has four filters that can be used for each axis and the user can select notch filters for local resonance peaks or low-pass filters to isolate an entire frequency range.
All in all, for the reasons outlined above, direct drive technology is continuing to gain in popularity and supports numerous new possible solutions, not only in high throughput manufacture and test applications but wherever precision motion needs to be combined with high dynamic response.
If you are implementing a similar application, you should consider direct drive solutions.
