By Tom England
As Cartridge DDR direct drive rotary motors become more affordable, many OEM designers are turning to this technology for simpler, more reliable, and virtually maintenance-free machines.
This Cartridge DDR rotary servo motor
family represents a new approach to direct drives. They embrace the
advantages of frameless direct drive motors but are as easy to install
as full frame motors at less cost than conventional direct drive
Director of Product Management
Danaher Motion, Wood Dale, Ill.
Mechanical transmissions often limit the performance of conventional servo systems and increase the cost of operation. Belt or gear transmissions require periodic maintenance, cause backlash, increase compliance, and considerably reduce positioning accuracy. They make inertia matching more critical and are the primary source of audible noise. The Direct Drive Rotary or DDR motor technology eliminates the mechanical transmissions and many of the negative aspects associated with conventional servo systems.
Such components as gearboxes, belts, pulleys, couplings, and brackets become unnecessary, making system integration and maintenance considerably simpler. Belts and pulleys require tensioning and gearboxes require lubrication. All these parts require mounting and alignment during system integration, eventually break down, and need replacement. Machine down time is always expensive and sometimes results in more losses than just productivity. In today’s world when just-in-time delivery is a norm, missing a ship date can be extremely costly. The upfront cost of a DDR system had typically been higher than the cost of a comparable conventional motor and gearbox servo system, but today, the cost delta is becoming considerably smaller. Moreover, as the cost gap narrows, many OEM designers are turning to the DDR technology for simpler, more reliable, and virtually maintenance-free design solutions.
The DDR technology development was an evolutionary process where new motor configurations were developed to satisfy unique customer needs. The original Frameless DDR motors offered by the Danaher Motion Inland Motor Division are custom designed to become an integral part of the customer motion system. The initial development cost burden for the customer is relatively high, but the benefits of small space requirements, high reliability, and high-accuracy servo systems justify this technology for some specific applications.
The next generation, the Housed DDR motor, is fully enclosed and has all the components of a complete motor including the bearings and the feedback device and can operate on its own. The customer’s machine shaft slips through the bore in the motor and attaches to the rotor. Such an arrangement works well for some applications, but since the coupling between the motor and the load is stiff, applications where the machine currently had bearings in place presented a problem. The three or four bearings in the motor and the load in this configuration are directly coupled in a linear fashion making it nearly impossible to align the system components properly. So, even the slightest misalignment creates eccentric loading and leads to bearing failure.
This problem gave Danaher Motion the idea for the development of the Cartridge Direct Drive Rotary (Cartridge DDRTM) motor. This motor is fully housed and ready for application: it has no bearings of its own and uses the bearings of the host machine to support the motor’s rotor. The feedback device is pre-mounted and pre-aligned electrically during the motor production. Danaher holds a patent on the electromagnetic design of the cartridge motor, as well as the overall cartridge motor concept.
The cartridge DDR has all the necessary system components such as the feedback, rotor, housing, connectors, except for the motor bearings. The motor uses the bearings of the customer’s rotary mechanism, for example, rollers to support the rotor. The Cartridge DDRTM motor has a hole in the middle, which slips over the shaft of the customer’s machine. The motor housing bolts down to the machine frame and the rotor attaches to the machine shaft, so the motor drives the load directly.
The benefits of using the DDR
technology include an integrated, robust sine encoder and compression
coupling that fits easily over the customer-supplied shaft. This design
provides an easy to install, extremely compact unit that requires
virtually no maintenance.
The customer’s machine must comply with certain mechanical specifications, which are necessary for the Cartridge DDRTM motor to function properly. For example, the machine run-out for a smaller motor are specified at 0.005 in. with a ±0.010-in. axial movement, which are fairly flexible and can be achieved without much difficulty. As the motors become bigger, the tolerances become tighter.
The classic performance advantages are common across the DDR technology, including the cartridge motors. When the transmission components such as gearboxes, belts, rack and pinion, and pulleys are eliminated, the servo system becomes free of such negative factors as compliance, backlash, and component wear. The accuracy and torque increase, inertia-matching requirements relax, acceleration and deceleration improve, maintenance becomes unnecessary, and the product life increases by a significant degree. Cartridge motors also add the benefits of fast and easy installation. The elimination of the backlash and inaccuracies produced by mechanical transmissions considerably increase accuracy.
Even a relatively large CDDR unit
requires only a fraction of the space that is needed by a conventional
direct drive or servo motor assembly.
In general, the Cartridge DDRTM systems can achieve up to 50 times higher accuracy than the conventional geared servo systems. An absolute position sine encoder with a maximum resolution of 2,097,152 counts per revolution is a typical choice for the feedback device. Unlike the conventional servo system where a gearbox with backlash or a belt with compliance is installed between the load and the feedback device and thus adds inaccuracies, the feedback device of the cartridge motor is mounted directly at the load. So the position is measured with high resolution and directly at the load significantly improving the accuracy. For example, a ± 25 arc seconds system accuracy is fairly typical for Cartridge DDRTM motors. For comparison, a gearbox system will typically have between ± 1 arc minutes and ± 10 arc minutes accuracy.
The DDR advantage
In a conventional geared system the acceleration and deceleration are limited by the gear-train backlash. Increasing acceleration past the safe level will lead either to instability or to gear damage. However, when the load is directly coupled, the servo loop gain can be raised without the risk of loosing stability, thus achieving high bandwidth. The stiffness of the servo system increases and the settling time becomes very short, because, unlike the conventional system, the settling time is no longer limited by the transmission. These factors translate into a machine with a faster throughput and higher accuracy, which operates without maintenance and has a much longer life cycle.
The direct drive motors have very high magnetic pole count and lower speeds. The lowest number of poles is 10 and the number increases to more than 40 poles in the large motors. Since there is no gearbox, the direct-drive motor torque must match the required load torque. The higher pole count and the large rotor diameter increase the total flux and generate higher torque output. For example, the larger Danaher Motion Cartridge DDRTM motors have 25 to 30% higher torque per volume rating than any other product available today. Cartridge motors available now range from 5 Nm to 510 Nm of continuous torque.
However, as the number of poles increase so does the electrical losses in the steel at higher rpm. That is why most of the direct-drive motors are rated at 1000 rpm for larger motors and 2500 RPM for smaller motors. For a large percentage of the applications, this speed limit is not a problem. Loads are typically driven at 1000 rpm or less after the motor speed is reduced by a gearbox or a belt. An interesting aspect is that when considering the type of a servo system, it is common to think in terms of a 3000, 4000, or 5000 RPM servo motor, but since most motors are attached to a mechanical transmission the actual speed at which the work is done is often much lower. For example, a widely used combination of a 3000 rpm standard motor and a 5:1 gearbox results in the load speed of 600 rpm, which is well within the range of a DDR motor.
Typically, the inertia of the cartridge motor is considerably smaller than the inertia of the load. The motor size is based on the peak torque required for achieving the desired acceleration time specifications. When the motor and load are directly coupled, the traditional servo system limitations of 5:1 or even 10:1 inertia mismatch are no longer applicable. A servo system equipped with a DDR motor can typically function properly and control the load with the inertia mismatch of 50:1, 100:1, and even several hundred to one.
This freedom changes the fundamental design rules for the DDR servo systems. For example, in many printing presses or converting machines with rollers, the size of the motor is dictated by the requirements of the inertia matching. To inertia match the load to the motor designers routinely select large air-cooled motors to drive relatively low-inertia loads. When a directly coupled motor is used instead, as long as the motor torque equals to j8, and the motor can accelerate and decelerate the load as required by the application, the motor can typically be downsized. This is one of the major advantages when designing systems with the Cartridge DDRTM motors.
The ease of installation
makes the Cartridge DDRTM technology much simpler to use than
conventional geared systems and more compatible with many applications.
The customer can purchase the cartridge motor and be up and running
after a few simple steps:
• Slide the motor onto the machine
shaft • Bolt the motor housing to the machine frame • Torque the motor
compression coupling attaching the rotor to the shaft • Plug in the
cables from the drive amplifier • Test the system
The benefits gained from this procedure include:
Installation can be less than 30 minutes: mount motor and connect
cables • Increased machine up time by virtue of maintenance-free
operation • Quieter operation by eliminating the transmission (to 20dB
reduction) • Increased accuracy (to 50 times greater) • No life cycle
The Cartridge DDRTM motors are based on the high-energy
permanent magnet brushless DC configuration and consequently do not
require periodic replacement of brushes. In addition, since they have
no bearings, the motor itself has no wearing components, so only the
bearings and other mechanical components of the machine will require
maintenance. Thus, periodic maintenance, lubrication, adjustments, and
replacements are no longer required.
Direct drive motors share the cogging problem with other permanent magnet designs. But modern design techniques reduced cogging to less than 1% of the continuous force. In addition, since the servo loop gain of a directly coupled system can be increased, the low frequency cogging can be overridden. As the gain increases, the velocity profile starts to look like the error signal generated by the feedback device. So the servo loop overrides the low frequency cogging making the motion extremely smooth. That is why direct-drive motors are often used in coating and laminating applications: they can achieve exceptionally high gain and smooth machine performance.
The servo system package size is often a major issue for machines installed in tight spaces. Conventional rotary motors are often long and have a relatively small diameter in order to minimize the torque to inertia ratio. So, attaching a coupling, gearbox, and a motor to a roller creates a long assembly mounted on the side of the machine. When a cartridge motor is used instead, the conventional coupling and transmission are eliminated and the overall servo package is considerably shorter, which reduces the machine footprint. The trade-off, however, is that the cartridge motor diameter is larger and must not interfere with the adjacent parts.
Today, many machine specifications, even in the industrial environment, list the maximum allowable audible noise levels. The audible noise level of a direct drive system is typically 20 dB lower compared to a geared system since transmission components generate considerable noise levels. So, installing a direct-drive system can help achieve the required audible noise specifications.
A typical installation plainly shows the
neat arrangement (bottom) of the CDDR and obvious lack of wiring and
components that are needed in the more conventional arrangement.
A typical precision servo system equipped with gearboxes or belts requires tuning adjustments made annually or even more frequently for some demanding applications. In comparison, when the motor is directly coupled to the load and the coupling is mechanically sound, no periodic tuning is needed. The tuning settings, even after several years of service, are the same as on the day the servo system was originally installed. The elimination of the periodic tuning is a big relief for the user.
Consequently, cartridge motors extend the life cycle of the entire machine. A servo system equipped with a cartridge motor is expected to work for 10 years without any maintenance. Although the initial Cartridge DDRTM system cost might be higher compared to a conventional geared system, over a period of several years, eliminating the cost of repairs and periodic maintenance makes the overall cost of purchasing and operating a Cartridge DDRTM system lower. The immediate cost saving benefit might not be obvious at the time of the machine purchase, but ultimately the cost saving goes directly to the end user. Even with the slightly higher initial cost, over a five-year period, Cartridge DDR motors can reduce operationg costs to $10,000 per motion axis compared to conventional geared servo systems.
Indexing press feed mechanisms wear out mechanical transmissions of conventional motor/gear box systems due to constant acceleration and deceleration at high frequency. The cartridge motor has no wearing components and can be successfully used, either in a continuous run velocity profile or in a rapid acceleration/deceleration indexing application.
The printing and converting industries are viable candidates for cartridge motors. Applications that use rollers and require precision speed variation or precision coordination of axes such as flexo machines, corrugated processing machines, and coating and laminating machines can benefit the most from the Cartridge DDRTM technology. Other examples include robots, tables, and turrets.
For more information, visit www.danahermotion.com.
:: Design World ::
Filed Under: Appliance engineering + home automation, Factory automation, Mechanical, Motion control • motor controls, Motors • servo