This article was the basis for the November, 1999 edition of20-minute Tune-Up printed in PT Design Magazine.

Direct drive motorsNovember 1999

The last few years have seen many developments in direct-drive technology, allowing a wide application ofmotors that, in the past, were reserved for the most expensive machines. Direct drive offers great promise.Properly applied, direct-drive motors are faster, more accurate, and, sometimes even less expensive thantheir transmission-based equivalents. However, direct drive is not right for every machine. To know if itsright in your application, you need to understand the capabilities and limitations of this technology.

There are two types of direct-drive motors: linear and rotary. Direct-drive linear (DDL) motors are usedwhen the work piece moves along a straight line. Magnets are laid side-by-side in a magnet way whichspans the entire length of travel. The coils are also laid side-by-side. The coil assembly, sometimes calledthe platen or table, slides along the magnet way with a clearance of .015 to .030 inches. While the shapesare different, the principles of linear motors are similar to those of rotary motors. The electro-magneticinteraction, feedback devices, and servo drives for linear motors are largely derived from rotary motors.

Direct-drive rotary (DDR) motors are used when the final motion is rotational, typically on drive tables.Compared to a traditional servo motor, DDR motors have no transmission components to providemechanical advantage, so they must provide more torque, albeit at lower speeds. This requires moremagnetic structure with a higher number of poles and coils, which makes the motor larger in diameter.DDR motors are sometimes sold as complete motors, but can be sold as separate components (sometimescalled kit or frameless form) which are integrated into the machine by the machine builder. In most cases,DDR motors have a hole in the rotor large enough to pass plumbing and wiring.

Figure 1. A typical linear motor

Why use direct drive?

Accuracy is a key reason direct-drive motors are chosen. In both DDL and DDR motors, the work piece isrigidly coupled to the motor. There are no transmission errors such as backlash, lead screw error, beltstretch, and gear tooth error. The main limitation on final accuracy is the feedback device. Feedbackdevices for DDL and DDR motors are very accurate. The multi-speed resolvers used in DDR motors are 10or 20 times more accurate than traditional resolvers. The scales used with DDL motors are often resolved to100 nanometers or less. The problem of stick-slip, a phenomenon where the load is hard to move smalldistances with accuracy, is reduced or eliminated with direct drive. This is because the relatively high-friction, high-compliance transmission components that commonly cause stick-slip in traditional mechanicaltransmissions are eliminated in direct-drive systems.

Another reason direct drive is chosen, especially in linear applications, is speedhigher velocity andacceleration. For example, the acceleration rate of most lead screws is limited to 1G, but a linear motor canaccelerate many times faster. Today, linear-motor acceleration rates are limited by the linear bearings, mostof which will tolerate 2 or 3 Gs, and those that will take 5Gs are now available. With air bearings,acceleration rates of 10Gs are attainable. As for velocity, linear motors regularly travel up to 5 meters/secwhere a lead-screw limits velocity to less than 1.5 m/sec.

A third advantage of direct-drive is its ability to provide superior servo performance. In last monthscolumn we discussed mechanical resonance, the problem associated with compliant transmissioncomponents. Direct drive has the ability to remove this problem because the coupling between motor andload is very stiff. This means the servo gains of the direct-drive systems can be much higher than they canbe in many transmission-based systems. Direct drive removes a related limitation of traditional systems:the load inertia need not be matched to the motor inertia. In direct drive systems, the load inertia can behundreds of times greater than the motor inertia without degrading system performance.

a) In a traditional system, mechanical resonance is a serious problem if the load is just 10x the motor inertia.b) In direct drive, the load can be 100x larger than the motor, and resonance is still not an issue.

Direct drive brings other advantages. Because there are fewer moving parts, direct-drive systems offerreduced audible noise. Zero maintenance is achieved since the only wearing component in the system is therotary or linear bearing. These are typically permanently lubricated and the assembly requires nomaintenance. The structures of direct-drive systems are usually simpler and, often times, are smaller. Insome cases, especially where highly accurate transmission components or feedback devices are alreadyemployed, direct drive can even reduce system cost.

Direct-drive motors do not fit every application. In most cases, they are more expensive than traditionalmotors and transmissions. Thats especially true when the transmission is used to gain large mechanicaladvantage such as with large gear-ratio gearboxes and fine-pitch lead screws. The feedback devices for

direct drive are usually more expensive as well. Also, direct drive systems do not have much friction.While this is usually an advantage, it can be a disadvantage when power is lost because some machines relyon friction to bring the machine to rest. Finally, for engineers who are used to the design of traditionalmachines, it takes time to learn how to best apply direct-drive technology.

How do you know if your application is right for direct-drive motors? Take the following survey:1. Is the accuracy of motor feedback and transmission limiting the accuracy of the machine?2. Does the machine have a feedback device, such as a glass scale, located directly on the load?3. Is the transmission limiting either acceleration or velocity below what the application could use?4. Does mechanical resonance limit the servo gains of the application?5. (linear only) Does the application move small masses quickly?6. Is the transmission limiting reliability below what the application needs?7. Is the transmission too noisy?8. Is maintenance of your system limiting the attractiveness of the machine or adding to the operating

cost?If you answered yes to two or more of these questions, you should take a look at direct-drive technology foryour next machine.

LaboratoryReady to take a look at one of the advantages of direct drive for yourself? Then log on towww.ptdesign.com and download the free control system simulator, ModelQ. After installation, click"Run." Select the October/November model from the combo-box at top center. Youll see a resonanceproblem. It comes about because the load inertia is high for a transmission-based system: its nine timesthe motor inertia. Now raise it even higher; set JL, the load inertia, 100 times higher than the motor (to 0.02kg-m2). Raise the gain, KVP, by 10 to 7.2. You must raise this gain to maintain servo performance when theinertia increases. Notice that the system goes completely unstable. Now raise KS (the stiffness of themechanical coupling) from 200 Nm/rad to a value more typical of direct-drive systems: 200,000 Nm/Radand watch the resonance problem disappear.

ApplicationAs an example, a recent application included the use of a direct drive rotary (DDR) motor in a high-precision assembly machine. The machine required very accurate placement of parts at a large diameter foran insertion process. The design was originally configured with a traditional motor/gearbox system.However, the accuracy required could not be solved without using a very expensive antibacklash gearbox.The DDR reduced the overall system cost and provided better accuracy than the transmission solution. TheDDR solution also added benefits of zero maintenance, quiet operation, fewer parts in the machine, andeasier assembly.