01 May 2001
by Bruce Smith
Motion control guys like me have heard for years about the advantages of direct-drive torque motors over drive systems that incorporate belts, pulleys, and other means of gear reduction. The mantra goes something like, "No more messy gearboxes, no worrying about belt particles in a Class-10,000 clean room, achieving both lower maintenance and higher bandwidths in our control loops ." The fact is, pretty much all that direct-drive rotary motor hype is true. However, the automation and motion control industries are only just beginning to see that as goes the rotary world, so too goes the linear.
The beauty of belts, pulleys, and gearboxes is that they allow you to use a low-cost motor that delivers low torque but runs at high speeds. Typically, we put a gearbox at the output of one of these motors to reduce its shaft speed while gaining greater torque at the gearbox's output. You aren't increasing your power output with gear reduction; the power going into the system (motor or motor + gear reduction) is equal to the power coming out. Because x speed = power, gear reduction allows you to decrease the output speed and increase the torque without paying for the additional mechanical or electrical power. You employ the same basic principle when you shift your car or bicycle into a lower gear to climb a hill. The engine (or pedals) revs faster, but you get more torque delivered to the drive wheels.
However, you don't truly get something for nothing. Adding gear reduction increases the number of system components, requires more maintenance, and takes up more space. Fortunately, belts and pulleys can be replaced with direct-drive torque motors, yielding more torque-per-unit volume. In the linear-motor world, there's an analogous situation with direct-drive linear motors. Furthermore, the options for providing direct force are rapidly increasing. Companies such as Linear Drives Ltd. (www.lineardrives.com), LinMot (www.linmot.ch), and California Linear Devices (www.calinear.com) are at the forefront in providing novel means of direct-drive force.
LinMot offers a range of tubular linear motors, delivering peak forces up to about 50 pounds (lb) in a compact package that integrates the electronics, a feedback device, and an amplifier. LinMot's linear motors are waterproof and, given the appropriate connectors, can thus be used in a wide range of aggressive environments (including underwater), as well as in Class-10 clean rooms. They're offered in configurations that deliver nearly 60 inches of stroke in a light, efficient package.
Linear Drives' ThrustTube linear motor offers many of the same advantages. Its integrated package eliminates a good number of moving parts but requires external bearings. The ThrustTube also delivers low force-ripple and micron-positioning capability. Available in eight motor sizes, its 100+-inch stroke delivers intermittent peak forces up to about 200 lb (Figure 1).
California Linear Devices (CLD) offers tubular linear motors in six sizes, targeting those applications requiring up to 1,125-lb peak force. Forces at this level make CLD's products candidates for replacing the fluid-based actuators (e.g., hydraulics and pneumatics) that have reigned, as a rule, over the high-force domain. These motors are making fully programmable, high-force servo control available like never before.
The Playing Field
Conventional approaches have many things going for them, not the least of which is cost. Applying new technology is often seen as a risky enterprise. This risk includes the time investment required to come up to speed, as well as the possibility of exotic modes of failure. Accordingly, it seems easier to go with what you know, as even a good idea can be subject to a type of inertia. Yet the old way meant we had to drive cams and levers to ultimately get the forces we required.
Now, however, both options and incentives are growing for replacing not only hydraulics and pneumatics but also ball screws and other electromechanical motion solutions on the modern production floor (Table 1). You as the designer no longer have to first develop a means of converting rotary motion into linear motion via cranks, levers, or ball screws. You can more quickly prototype your design by incorporating fully programmable linear positioning. Users will be surprised to learn that a competitive advantage exists today for implementing linear power transmission.
Designers can choose among several linear motor manufacturers from which to make a purchase. Today's linear motor can follow arbitrary velocity and position commands (thus providing a programmable cam) or dynamically vary force control in pumps and compressors, vibratory applications, or a myriad of other applications, all done on the fly. (And you thought the wheel was a great invention!)
What we're seeing is that direct-drive linear motors, once thought to have serious limitations (not the least of which was high cost), are now gaining increased acceptance. Indeed, they're a desirable alternative to hydraulic fluids, pneumatics' lubricated exhaust, and the wear and subsequent backlash associated with traditional electromechanical devices such as ball screws, cams, belts, and pulleys.
Two recent factors have caused many motion system engineers to take notice of direct-drive linear motors. The first is that the magnets, which are key to the motors' operation, are now readily available, and at a considerably lower cost than in the past. Secondly, improvements made in digital control technology mean that these high-performance devices can be brought under a level of control that enables them to take full advantage of their speed, accuracy, and sensitivity. Consequently, systems integration has become much easier.
Magnetism, whether it's used for rotary or linear motion, is the key to direct-drive technology. In a direct-drive motor, a magnetic field provides the interaction between the stationary and moving parts. This is a very reliable, accurate method for achieving force and motion control.
There are several different types of direct-drive motor methodologies used throughout industry. Usually, some type of position feedback is required, and itas well as the method of controlwill depend on the type of motor technology being used. Because direct-drive motors are usually offered as part of a packaged system, the bundled feedback solution has considerable influence on the type of drive or programmable motion controller you'll be able to use.
Why choose a direct-drive motor? Primarily, it's to increase a production machine's accuracy and throughput. Because we've rigidly coupled the load to the motor, we've eliminated any errors caused by transmission components, so there's no backlash, belt stretch, or gear-tooth error. The limiting factor in achieving positioning accuracy thus typically becomes the feedback device. Fortunately, modern feedback devices for direct-drive motors have become extremely accurate and, when coupled with the right controller, can achieve accuracies on the order of microns.
The high stiffness between the motor and the load effectively absolves the motor as a source of mechanical resonancethe phenomenon in which a compliant load generates instability under high servo gains. Hence, the servo gains of direct-drive systems can be set very high, allowing faster servo response and greater resistance to load disturbances. System instability is therefore more likely to be found in a resonance mode of the load rather than in the linear power transmission. As an added benefit, having fewer moving parts also reduces noise.
Maintenance is likewise reduced because the only wearing component in a direct-drive system is the bearing, and if that bearing is permanently lubricated, the assembly can achieve zero maintenance. As a consequence, machines using direct-drive motors are often simpler and smaller because the transmission is eliminated. In addition, direct-drive motors actually reduce costs in cases where highly accurate transmission components or feedback devices would otherwise be needed.
Direct-drive Linear Motors
The North American market for direct-drive linear motors and drives is experiencing tremendous growth. The most significant driver behind this is the push toward greater efficiencies, often realized through increased machine throughput and improved manufacturing effectiveness.
Industry's need to increase both throughput and efficiency is resulting in an escalation of servo technology use. As OEMs make this shift, they're discovering that direct-drive linear technology meets their needs better than nondirect methods.
Direct-drive linear motors represent a departure from traditional electromechanical devices. Assemblies such as ball screws, gear trains, belts, and pulleys are all eliminated. As the name implies, the motor and load are directly and rigidly connected, improving simplicity, efficiency, and positioning accuracy. The acceleration available from direct-drive systems is remarkable compared with traditional motor drives that convert rotary motion to linear motion.
Direct-drive motors' inherent simplicity leads to a number of significant advantages over traditional approaches to linear motion and motion control. As I've stated, there are no gearboxes, ball screws, pulleys, or mechanical transmissions involved, so both failure potential and maintenance requirements are dramatically reduced. Decreasing the number of moving, wearing parts results in a longer cycle life.
The performance benefits are also substantial. There's no backlash, and because feedback resolution is high, direct-drive systems can be counted on to deliver superior repeatability and stiff, true positioning. By eliminating backlash and mechanical transmission considerations, we also realize much better overall servo performance.
All these factors add up to lower life-cycle costs, higher throughput, and greater productivity for a wide range of production applications.
Direct-drive Linear Motor Control
Drive electronics, including the motion controller, amplifier, feedback method, and control algorithm, play a major role in the overall performance of linear motion systems. Advances in digital technology have meant rapid improvements in controllability and control, allowing the accuracy of direct-drive linear motors to be fully utilized.
Recent advances have increased our options in direct-drive linear motion alternatives. A recently introduced device that's increasing in popularity is the permanent-magnet, tubular linear motor. Possessed of integrated bearings and high force capabilities, tubular linear motors are fast becoming a bigger part of the automation landscape.
CLD has developed a tubular, brushless, direct-drive-based linear motor technology that incorporates permanent magnets, making it an excellent fit for a broad range of industrial applications. LinMot has its own offering for lower-force applications. Each tubular motor incorporates the important design features of a single moving part; integral, built-in bearings; a mounted encoder; and exceptionally high force-per-unit volume. These features deliver significant advantages, including space and cost savings, over not only traditional methods but other linear motors as well.
The Tubular Concept
With one moving part and an integral bearing system, tubular linear motors can accelerate quickly to high velocities, even when handling heavy loads. Their delightfully simple design provides robust, reliable operation and longer cycle life. More importantly, the design generates force in a highly efficient manner.
The permanent-magnet tubular motor's shaft slides into a stator assembly containing electromechanical coils. The stator's length and diameter set the force level, while the shaft length determines the stroke (Figure 3). Linear motion is controlled directly through a precision feedback device, which relays detailed position information to a motion controller. There's neither backlash nor compressibility to compromise position accuracy.
Current designs incorporate three-phase, brushless DC technology and rare earth, neodymium-iron-boron permanent magnets wrapped around the shaft's periphery. Not unlike a conventional linear actuator, the rotor lies flat and is then formed into a tube (Figure 4). The distinct advantage of this approach is that the electromagnetic interaction takes place over the shaft's full 360° surface area, resulting in a high force-to-volume linear motion solution.
Simplicity's the Key
The simplicity of brushless, tubular, linear motors carries benefits for just about any factory motion application:
- One moving part
- Two wearing parts
- Integral bearing system
- Encoder mounted directly to the motor
- Compact size
- No backlash or compressibility to affect position accuracy
- No supporting mechanical systems, pumps, or tanks necessary
- Environmentally friendly: no oil or hydraulic fluids, quiet operation
In addition to the above benefits, tubular linear motors have numerous inherent characteristics that result in a lower life-cycle cost:
- Their simple design costs less to integrate and install.
- Reduced downtime is due to simplicity and robust construction.
- They have fewer maintenance requirements.
- No environmental control activities are necessary.
Tubular vs. Competing Technologies
Other direct-drive linear technologies, including flat linear motors, offer excellent accuracy, resolution, and speed. The downside is that they offer these benefits at a premium price. Additionally, many competitive designs require an external bearing system to support and position the moving member in relation to the magnets. Such a bearing system can cost as much as the motor itself.
In contrast, a tubular linear motor's bearing system is an integral part of the motor. By packing the motor with its own bearing structure, we can realize both easier installation and substantial cost savings. MC
Figures and Graphics
- Figure 1: This tubular linear motor's 100-in stroke can deliver peak force approaching 200 lb.
- Figure 2: Tubular motors like this one can provide significant space and cost savings.
- Figure 3: Stator length and diameter set the force level, while shaft length determines the stroke in this tubular motor.
- Figure 4: A rotary motor's magnets (a) laid flat, make a linear motor (b) rolled up, they're a tubular motor's shaft (c).
- Table 1: Comparative characteristics of motion control systems.