Direct Drives for Inertia Matching

DN Staff

November 9, 2010

6 Min Read
Direct Drives for Inertia Matching

Imagineyou are designing a machine that needs to move a 200-pound load. You do quickmath to identify the properly sized motor with an inertia that preciselymatches the load. You specify the motor, but will the motor achieve the desiredmovement of the load in the required time? Will the machine provide optimumstability and efficiency? Maybe or maybe not, but often the key is the mysteryof inertia matching.

Most technical manuals recommend a1:1 match between the motor and load, and usually advise designers to never goabove 10:1 load-to-motor inertia ratio. The reason why this "rule" has workedso well in the past is relatively simple. Imagine a servo motor driving a loadthrough two meshed gears that aren't perfectly matched, allowing a certain amountof backlash between the motor and the load.

When the motor turns one direction, the gearteeth engage and the controller sees the full load inertia. When the motorstops and reverses direction, it is temporarily decoupled from the load due tothe backlash. Therefore, the controller only sees the motor inertia. Since thecontroller doesn't have the ability to store multiple gain settings fordifferent loading conditions, the system has to be either de-tunedsignificantly or may be simply un-tunable. Minimizing the difference betweenthe loaded and unloaded conditions, such as minimizing the inertia ratio, canhelp maximize the possibility of a successful outcome.

However, adhering to these "rules"may result in sub-optimal solutions and can greatly increase the cost of amotion solution. For example, the 1:1 ratio often causes engineers to specifylarger motors than necessary. While adding a gearbox or belt drive can reducethe reflected inertia and required torque, these mechanical components candemand frequent adjustment and can introduce backlash and compliance into theservo system.

Today we have a much better understanding ofthe variables that influence the level of acceptable inertia mismatch,primarily requirements for dynamic system performance and mechanical systemstiffness. At one extreme, if an extremely sloppy mechanical design is appliedin a highly dynamic application, such as a delta robot, even a 1:1 inertiamatch may not result in a solution that meets performance expectations. At the otherextreme, if there is an extremely stiff connection between the motor and theload in a low dynamic application, such as an indexing table bolted directly toa motor, inertia mismatch can effectively be ignored completely with a highlysuccessful outcome.

Inertia matching should really bethought of more as a spectrum, rather than a discrete numerical ratio. Withmodern, high performance servo controllers, high resolution feedback devices,and improved mechanical designs, inertia ratios of 20:1 or 30:1 are common forhighly dynamic applications. For less dynamic applications, 50:1 or 60:1 is notout of the question. When motors are rigidly mounted directly to the load inrelatively low dynamic applications, inertia ratios of 1000:1 and higher havebeen effective.

The Beauty of Simplicity
One approach many designers take to overcomethe uncertainties and inefficiencies of traditional machine design is tosimplify the mechanics through the use of direct-drive motor technology. In thepast, direct-drive solutions have been difficult to integrate andcost-prohibitive for many mainstream industrial applications. However, today'stechnology is available at a reasonable price from suppliers who understandboth the mechanical and electrical control environment and help machinebuilders and end users realize a much faster return on investment.

Mechanical transmission componentsused with traditional motion control systems can cause excessive heat, frictionand noise, all of which consume power. In fact, even a new, precisionservo-rated gearhead is at best 90 to 95 percent efficient on average.Direct-drive technology practically eliminates all three forms of lost power,which often allows machine builders to specify a smaller motor and drive. Thistranslates into savings in the initial costs of hardware as well as ongoingenergy savings from lower current draw.

By connecting directly to the load,direct-drive motors eliminate the need for gearboxes, timing belts, pulleys andother mechanical components. This helps eliminate backlash and compliance,improves overall control of the load and optimizes system efficiency andperformance. Because there are fewer parts that require ongoing maintenance,end users could also see increased productivity and can reduce the number andcost of spare parts.

Simplified direct-drive technologyalso provides a smaller machine footprint and can help cut assembly time. Anexample is that by installing direct-drive motors on a rotary knife system, oneconverting machine builder was able to reduce assembly time from 4.5 hours to30 minutes by cutting out mechanical transmission components. Eliminatingmechanical transmission inefficiencies also increased the percent of motortorque producing useful work (moving the load) by 11 percent. The improvedefficiency resulted in an annual energy savings of more than 50 percent in thissingle application.

A Perfect Match
With its reliable performance andsimplified mechanics, direct-drive solutions are becoming increasingly popularin mechatronic machine design. In traditional machine building, individualmechanical, control and electrical design teams often work independently toproduce separate pieces of the overall machine. Trying to size and specify theelectrical components before the mechanics are defined can lead to wasted timeand rework, since parameters such as inertia and torque are heavily influencedby the choice of mechanical components.

By optimizing the mechanical designfrom the start using direct-drive motors, designers can more easily addressconfiguration and integration issues up front and minimize the chance ofencountering power sizing problems in subsequent stages. A concurrentengineering approach can produce higher performance machines that run on lessenergy and are easier to maintain and update for product or design changes.

Software tools also help machinebuilders reap the full benefits of mechatronics by making it faster and easierto select, size and optimize motion control systems. With Motion Analyzersoftware from Rockwell Automation,for example, engineers simply enter information about the load and how it needsto be moved, and the software selects the most efficient motor-drivecombination. From a pull-down menu, designers can then select an actuator, forinstance, without having to figure out complex calculations or look upspecifications in the manufacturer's data sheets.

In addition to sizing and selection,the software provides performance and simulation analysis that helps engineersmore effectively investigate machine behavior and select a mechanical design -along with the optimum controls and software - that will maximize machineperformance. These simulation tools help reduce design time and help minimizeerrors that are more difficult and expensive to correct farther along in thedevelopment process.

Cost savings alone provide amplemotivation for mechatronic development. Forward-thinking machine builders arerelying on the latest tools and technologies, including new direct-drivemotors, to optimize their mechanical designs from the start. The result: higherperformance, faster time to market and reduced business risk. For end users,more accurate, reliable and optimized machines mean more uptime, increasedthroughput and better product quality, all of which translate into a morefavorable bottom line.

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