When it comes to linking motors and loads, there's nothing simpler and more efficient than "direct drive." Properly applied, it's faster, more accurate, and sometimes, even less expensive than using conventional mechanical transmission components.
There are two types of direct-drive systems. One type, based on direct-drive linear (DDL) motors, moves loads along a straight line. Like rotary motors, DDL motors consist essentially of magnets and a coil assembly. The magnets lay sideby- side, forming a "magnet way" that spans the entire length of travel. The coil assembly – sometimes called a platen or table – slides along the "way" with a clearance of about 0.015 to .0030 in.
Though linear motors differ in shape from their rotary counterparts, they are similar in principle with respect to operation. In fact, their feedback devices and servo drives, as well as the electromagnetic interaction itself, derive largely from rotary motors.
Another type of direct-drive system, based on rotary motors, provides rotational motion as in a drive table. Compared to traditional servomotors, direct-drive rotary (DDR) motors have no additional transmission components to provide mechanical advantage. As a result, they have to provide more torque, albeit at lower speeds. This requires more magnetic structure with a higher number of poles and coils, which makes DDR motors larger in size (diameter) and mass.
DDR motors are sometimes sold as complete motors, but more frequently come as separated components ("kit" or "frameless" form) which are integrated into the machine. Most DDR motors have a hole in the rotor to accommodate plumbing and wiring.
Accuracy may be the number one reason for using direct drive. In both DDL and DDR motors, the workpiece is rigidly and directly coupled. Thus, there are no mechanical transmission errors to contend with such as backlash, lead screw error, belt stretch, and gear tooth error. Eliminating mechanical transmission components and the associated friction and compliance also reduces or eliminates stick-slip, a phenomenon that makes it difficult to accurately move small distances.
In a direct-drive system, the main limitation on accuracy is actually the feedback device. Incidentally, feedback devices for DDL and DDR motors are quite accurate. The multispeed resolvers used in DDR motors are 10 to 20 times more accurate than traditional resolvers, while the scales used with DDL motors are often resolved to a 100 nm or less.
Another compelling reason for direct drive, especially in linear applications, is speed – as in higher velocity and acceleration. The acceleration rate of most lead screws is about 1 G tops. But a linear motor can accelerate many times faster, limited only by their bearings. Ordinary bearings can tolerate anywhere from 2 to 5 G, while air bearings can take linear motors up to 10 G. As for velocity, linear motors regularly travel up to 5 m/sec, where a lead-screw would be limited to about 1.5 m/sec.
Servo performance is another factor that favors direct drive. In last month's (October) column, we discussed mechanical resonance associated with compliant transmission components. Direct drive avoids such problems because it ensures a stiff coupling between the motor and load. Thus, servo gains can be much higher on direct-drive systems than in most transmission-based systems. What's more, with direct drive, load inertia can be hundreds of times greater than motor inertia without degrading system performance.
With fewer moving parts, direct-drive systems also run quieter. And since the only wear is that of the rotary or linear bearings – which are typically permanently lubricated – direct drive often achieves "zero maintenance."
Less obvious is the fact the support structures in a direct-drive system are usually simpler and often smaller. In some cases, especially where high-accuracy transmission components and feedback devices are employed, direct drive can actually reduce system cost.
Direct drive isn't for every machine, however. The seemingly unmatched capabilities are offset by technological limitations. And there's the issue of cost.
In most cases, direct-drive motors are more expensive than traditional motors and transmissions. That's especially true when the transmission is used to gain large mechanical advantage, as in large gear-ratio gearboxes and fine-pitch lead screws. Feedback devices for direct-drive systems are usually more expensive as well.
Another disadvantage of sorts stems from the fact that direct-drive systems are low in friction. While that's usually an advantage, it may be a problem when power is lost because some machines rely on friction to bring them to rest. For this and other reasons, engineers familiar with traditional machines will find that it takes time to learn how to best apply direct-drive technology.
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Time for a test drive
Ready to take a look at one of the advantages of direct drive for yourself? Then log on to www.ptdesign.com and download the free control system simulator, ModelQ.
After installation select the October/November model from the combo-box at top center and click "Run." What you'll see is an example of a transmission-based system where the load inertia, although it's only nine times that of the motor, causes resonance.
To observe the effect of direct drive, you need to do the following: First, raise the load inertial JL to 0.02 kg m2, which puts it 100 times higher than the motor inertia JM. Next, to maintain servo performance in light of the increase in inertia, raise the gain KVP by a factor of ten to 7.2. Notice that the system goes completely unstable.
Now, make the "switch" to direct drive by raising Ks, the stiffness of the mechanical coupling, from 200 Nm/rad to 200,000 Nm/rad, which is typical of direct-drive system. See how the resonance problem disappears.
Benefits in packaging and assembly
Direct drive is particulary well suited for packaging and assembly machines as the following example illustrates. Designers working on a high-precision optical assembly machine were confronted with the challenge of finding a way to accurate place parts along a large diameter for an inserting process. The motion system was originally configured with a traditional motor and gearbox. At one point the designers considered switching to an antibacklash gearbox to get better accuracy, but the cost forced them to look at other solutions.
After some tests and calculations, the designers came to the conclusion that a direct-drive rotary motor would not only reduce the overall system cost, but provide better accuracy than transmission-based alternatives – as well as zero maintenance, quieter operation, fewer parts in the machine, and easier assembly.