Important considerations when sizing linear and direct-drive technologies

Linear and direct-drive insights

• Direct-drive motors are more suitable for rotary applications, which require more torque at lower speeds than linear motors.
• While sizing an application, using the latest sizing software can reduce the chances of oversizing or undersizing a motor for an application. It also allows people to compare and contrast between motor capabilities.
• When considering both linear and direct-drives, it is important to factor in immediate costs compared to long-term costs.

Whether users are cross-referencing, integrating or retrofitting a linear or direct-drive application, feeling comfortable with sizing will not only speed up the automation process, but also greatly impact performance.

Oftentimes, both linear motors and direct-drives are overlooked by their just as capable counterpart, the rotary servo motor. In doing so, manufacturers could be missing out on the benefits a linear or direct-drive could provide.

Understanding mechanical data for a linear and direct-drive

When sizing for any application, it is best practice to start with the basics: involving the critical moving payload, external payloads, orientation, length and the motion profile. Further analyzing these topics provides solutions for determining importance in position settling time, desired accuracy and repeatability, rigidity and identifying space and environmental constraints.

By considering these performance metrics and gathering the mechanical data, users can compare linear and direct-drives with that of using a rotary servo motor.

Linear motor technologies are separated into two categories. One is a linear motor stage that creates direct-to-linear motion; its electrical components produce the motion and the other linear technologies involve rotary motors, which convert the rotary torque into linear motion by using a ballscrew, belt and/or rack and pinion transmission.

In comparing the two categories by some performance metrics, machine builders would consider that the linear motor stage overall outperformed the rotary-to-linear motion technologies, even after applying a fully closed loop encoder to enhance settling time, accuracy and repeatability.

For a further study in linear technologies and their application uses readers should read the Control Engineering article “Choosing linear servo motors for the right application” by Matt Pelletier from 2021.

While linear motors and direct-drives share many advanced precision similarities, the use of a direct-drive is more suitable for rotary applications, which require more torque at low speeds. In a similar comparison between a direct-drive and rotary motors that have a gearbox and or belt and pulley system, the direct-drive outperformed each in several performance metrics.

When applying a rotary servo motor gearbox and/or a belt and pulley mechanism, applications experience long-term effects like increased backlash, loss in rigidity and wear on precision capabilities. A direct-drive system simply bypasses all these long-term effects associated to transmission components, as a direct-drive can support the weight of the entire load.

For a further study on direct-drive technologies and their application, read the article “Direct-drive servo tutorial, application update” by Matt Pelletier from 2022.

When sizing any application, having the mechanical data at hand alongside the performance metrics that are considered important will help users feel more comfortable when considering a different approach to what they would have normally. For even further assistance in sizing, running the mechanical data in a sizing software will not only help produce the motors best for the application, but also provide application results for when considering a linear motor and/or direct-drive alongside the rotary counterpart.

Validate sizing with the support of software

In the course of sizing an application, there are many advantages of using the latest sizing software. Not only can machine builders reduce the chances of oversizing or under-sizing an application, they can also compare and contrast between motor capabilities to garner bill of materials, or BOM, information for next step decision matrices.

Sizing software offers a step-by-step platform that guides users through a wide range of application possibilities, complete with an intuitive editor to enter load data, mechanical transmissions and move profiles. Lastly, the software generates a finalized report that contains servo system capabilities versus application requirements with a complete BOM for processing.

In the initial “User Info” tab in the Yaskawa SigmaSelect software, users should enter application information and or sizing analyst information for future cross-referencing, machine design changes, etc.

The next tab, “Load Editor,” involves entering the application mechanical data. The following example consists of a “Pick and Place” — ballscrew application with the following mechanical data:

• Slide mass: 20 kg.

• Payload: 60 kg.

• Screw length: 1,200 mm.

• Screw lead: 20 mm.

• Screw shaft diameter: 20 mm.

• Stainless steel.

• Linear guides with nylon bushing.

• Coupler aluminum height: 60 mm.

• Coupler inner diameter: 20 mm.

• Coupler outer diameter: 38 mm.

The motion profile involves (see Figure 1):

• Intermediate dwells of 0.5 seconds.

• Placing speed: 5 inches/second with a slot of 3 inches.

• Transfer speed: 20 inches/second.

• Ending dwell for part remove/reload.

Figure 1: Motion profile milling application. Courtesy: Yaskawa

In the “Load Editor” tab, data was entered for: critical load mass (kg), the selected ballscrew lead (mm) and the slide’s mass (kg) (see Figure 2). The mechanical data is then used to determine important characteristics involving friction coefficient (N), ballscrew inertia (and coupling transmission inertia) (see Figure 3).

Figure 2: “Load Editor” tab: mechanical data implementation. Courtesy: Yaskawa

Figure 3: Coupler inertia, length, inner and external diameter and material. Courtesy: Yaskawa

In the preceding tab, “Profile Editor,” the expected motion path was entered. The motion profile generates graph options for position, velocity, acceleration, jerk and estimated torque. Performance metrics for accuracy, repeatability and time cycle are crucial assets in this stage. It is recommended to review not only mechanical data, but the expected motion profile for optimization possibilities.

Once the motion profile is optimized, the SigmaSelect software generates a list of potential motors and amplifiers capable of meeting the motion demand. Considering the four key sizing notes alongside categorizing for “Cost Factor” and/or filtering for speed, inertia, torque, etc. will help select and or compare motors for:

• Inertia ratio.

• Speed.

• Max torque at speed.

• Rms torque at speed.

From the list of sizing factors, it is generally recommended to keep the application inertia ratio mismatch value lower than the allowable ratio value, as this will assure the motor can control the load at the desired speeds. Keeping the application required torque within a general 80% of the rated or peak torque values allows for buffering for the application.

For this example, the motors were categorized by their cost factor starting from least to most. For this application the SGM7J-08A*A, was the suitable candidate, as it met all the requirements with the application gaining an additional 80% factor of safety, a wider scope creep for future machine changes. On the other hand, the SGM7G-03A*A is 11% more in initial costs and only provides a 60% factor of safety, lower values in rated/peak torques and peak speed of 3,000 revolutions per minute.

When a motor is selected for final revision, the “Motor Details” tab provides users an in-depth graphical and motor analysis, while displaying the root mean square (shown as RMS) and peak values in intermitted or continuous periods.

In this example, the motor values fall within the continuous cycle, while it satisfies both inertia mismatch and speed demands (see Figure 4). The next tab, “Regeneration,” would detail whether the application needs an external resistor for negative torque produced. It did not. Finalizing the assessment will generate a PDF with all of the application specifications.

Figure 4: Motor details are shown in the program here. Courtesy: Yaskawa

To compare the ballscrew application to a linear motor, SigmaSelect allows for intuitive edits to adjust applications without having to start over. Keeping previous information stored, allows for on-the-fly changes. The only edit other than changing mechanism type was in the application’s friction coefficient for a linear motor’s ball bearings.

The motor results were filtered specifically for the Sigma Trac II linear stage to demonstrate a complete solution comparable to the ballscrew approach. A linear stage system is specifically built to order, fully tested and ready to use out of the box.

Filtering for cost and the list of key sizing factors, the ST2F-A1A* meets the motion speed demands, while keeping RMS values continuous, with peak values allowing room for higher force and potentially increased application speed. The motor results also specify for an additional 5% more in initial costs, but the motor ST2F-A2A* has double the factor of safety and force values than the ST2F-A1A*. The ST2F-A1A* updated application did not require an external resistor.

SigmaSelect software has progressively developed to allow more possibilities that involve transmission components like belt actuator, belt conveyors, ball/lead screw and rack and pinion. It caters to a variety of other applications that can be optimized, while keeping machine builders as much a part of the process and detailing motor recommendations. Having the possibilities to asses and compare without extra work supports end users to consider sizing alongside software.

Consider design long-term versus initial costs

Using the ballscrew versus linear motor stage application, the design costs between the two systems are vastly different for the same application and will remain different for the entirety of the system’s life span. The SigmaSelect software solely offered a cost factor for the ballscrew’s motor and its associated servo amplifier. Machine builders would have to source for their own milled to length ballscrew, coupler, linear guide with nylon bearings and if performance metric accuracy is desired, a fully enclosed encoder. The system would also then require either outsourcing or in-house engineering installation support to initially mount, align, wire and tune the system.

As the system is in operation, machine builders will have to continue to supplement component replacements. For example, the ballscrew will incur increased backlash and decreased repeatability, rigidity, etc. over time. Machine builders could find themselves sacrificing speed and repeatability, as they allocate replacements.

In comparison, the SigmaSelect software provides the cost factor for the Sigma Trac II as a fully complete solution that is built to order and tested. Machine builders simply provide a flat mounting surface and bolt for their payload. The solution results in a repeatability of a 10 million double-stroke design life before the need for inspection. In comparison to the ballscrew, the Sigma Trac II is the more expensive of the two, but in the long run, it out lasts the ballscrew application, maintaining its speed, repeatability and rigidity longer.

In a further analysis when considering design costs, keep in mind the following: When the forces are high and the speeds are lower, ballscrews become a good option. Linear motors offer moderate force at high speed, while a ballscrew is capable of higher forces at lower speeds. The load-side feedback of the linear motor offers better accuracy and repeatability with fewer moving parts. Ballscrews are also limited in their length, as deflection occurs at high speeds. Increasing the diameter helps, but increases the amount of load inertia. Sigma Trac II stages can be up to 1920 mm long.

Overall upfront cost of linear motors is higher than actuators, but production factors, such as settling time and repeatability create payback scenarios that can be very short.

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