An often asked question from industrial machine builders or integrators is how they can effectively design or implement the conversion of a machine with servo technology to meet performance expectations. This is a specialized task filled with layers of complexity that can prove difficult to execute, even when the scope of work is fully understood.
Available and different technologies present various possible engineering variations and unique operating processes. If there is a misunderstanding or knowledge gap for any given process in the work to be performed, the possibilities expand exponentially and create further complexity with added risk. This article presents a simplified design approach for servomotor utilization to overcome many of the initial challenges. The approach is based on several different but typical mechanical axis configurations and requirements that highlight risk management, optimal performance, and reduced development time.
When considering a machine design, there are clearly many factors to address in the planning phase. Reviewing all possible situations to reduce risk of failure as well as working through the different combination scenarios — all the ifs, ands, and buts — is a demanding set of tasks. For this reason, it’s essential to build baseline knowledge of machine functions and each of its axes, relative to the overall chosen operating process and work to be performed. Start by developing a thorough understanding of the chosen process to complete the machine’s function, the full picture encompassing the ins and outs, identify any variables and tradeoffs, and recognize there likely will be some unknowns. This extends to the advantages and disadvantages of available motion technologies considered and applied to each axis of the machine.
Acquiring as much in-depth comprehension up front will undoubtedly alleviate potential issues downstream and greatly enhance the opportunity for successful execution. Also, at the center of the design is risk management of specific technologies available and their interface with each other related to the tradeoffs and decision priorities to be given to the machine’s function for the desired process.
Technologies and Degrees of Performance
What is considered high-technology performance for one manufacturing process is not necessarily high-performance for another. It is natural for the machine builder to deploy technologies they have experience with. However, new challenges often entail the utilization of newer technologies. When a retrofit or a new machine design requires the utilization of closed-loop (servo) motion control technologies, there are often misconceptions involved. For example, misconceptions often occur between what was required for a machine’s optimization utilizing previous technologies and what is now required for a machine’s optimized performance.
Proper deployment of closed-loop motion control technologies requires balancing its capabilities, tradeoffs, and other factors that will enhance the new machine’s performance. Previous technologies may include, but are not limited to, hydraulic actuators, variable-speed motors, pneumatics, or any number of the typical open-loop, ON-OFF control, and in some cases, semi- or pseudo-closed-loop technologies. Even newer closed-loop control concepts must be considered or balanced with older concepts to reduce risk. For example, it may be a great enhancement to run a machine and control all its axes by a virtual master axis. However, if one axis is essentially driven by two or more motors (hard-coupled or pseudo-coupled mechanically by the mechanism/load), the additional latency of one motor’s drive talking to another through the virtual master’s control, rather than directly to each other, will increase risk as a function of speed at which the machine is to operate.
In general, any process that is to run at a faster rate requires a machine with the capability of faster response times than its previous design to maintain quality. In other words, the machine must have the capability to move and act on the product at a faster rate, and to respond to all commands and disturbances within the limit of the product and process itself.
Often an actual process time is fixed and cannot be increased under an existing technology, leaving only product transfer times as the available time to be sped-up. In turn, this increases specific axis’ peak horsepower (hp) requirements during acceleration/deceleration times from its baseline by the product of the increased ratio: speed and torque (a 15% increase of each, speed and torque, during peak requirements is a 32.25% hp increase). Many of the issues involved, when converting a process machine from some form of open-loop, ON-OFF (bang-bang) control or pseudo-closed-loop control method to a closed-loop servomotor controlled machine, may not be particularly intuitive to a first-time servo machine designer.
Inertia. Inertia was not a concern or even a consideration in the past for some specific axes of a machine design. For some other axes, an optimal machine required a high system inertia (load and actuator) to dampen any disturbance from being seen by the product. We want to utilize a high-performance servo to increase speed and thus productivity, with the same or improved quality. This requires axes with higher-bandwidth (BW) capability than most previous designs in order to sense commands, product changes, and disturbances, such that we can respond to errors (delta (Δ) between command and actual) and make the appropriate corrections both quickly and easily. In order to accomplish these tasks, a lower system inertia is generally desirable and most frequently required. This is especially true of processes requiring point-to-point moves or on-the-fly corrections for continuous or pseudo-continuous processes. Production energy costs are often reduced by the higher levels of production efficiency.
Mechanical Advantage by Gears. Another issue that occurs, especially with previously designed machines, is backlash within an axis’ mechanism. Often this type of axis movement was only mildly considered a potential process issue. The reason is because the unidirectional driven advantaged mechanism driving against the load usually stays on one side of the mechanism’s backlash. However, with the constant velocity correction of a servomotor, the full ± displacement is repeatedly seen.
Mechanical Advantage by Timing Belt. For many previously designed machines (especially uni-directionally driven), the amount of compliance produced by a belt is typically not a major concern in regards to the process, as long as it is sized large enough so that it does not break. However, with the constant velocity correction of a servomotor, the full ± displacement of the belt’s compliance can be repeatedly seen. The typical doubling of the belt’s width (as calculated for a unidirectional mechanism) to reduce compliance may make the belt too wide. In this case, the designer may need to utilize as much width as the available space will allow and if possible, further reduce belt compliance (increase rigidity) by selecting a stronger or thicker belt. [Note: Be careful. A thicker belt reduces compliance (desirable), but powers natural resonance frequency (undesirable), depending on where the frequency is within the control system’s spectrum. Then there is another issue: a larger belt will have a greater side load that must be considered in the design (bearings, tensioners, pulleys, and/or motors could be affected).]
For many designers, these new issues can present hard concepts to get through at first. What worked for a host of different open-loop, ON-OFF control and pseudo-closed-loop control technologies is now in part or as a whole a potential hindrance against the new machine design, impacting the desired goal of increased production and quality. Thus this new design may need additional effort from the mechatronic designers with typical disciplines in mechanical, electrical, electronic, control, process, and programming fields in order to simplify and achieve the goals of risk management, optimal performance, and reduced development time.
Minimizing Potential Process Issues with the New Design
Typically, when utilizing a servo system technology to meet this overall goal, the designer will need to enhance the BW response capability for each axis of the new machine. To accomplish the task, we must consider a number of variables. They include frictional loads and any external loading (gravity or otherwise), the inertia between the load reflected back to the motor for a practical controllable solution within the process required tolerances, and also the backlash and compliance of each axis. For a typical servo mechanism, it is desirable to have a rigid style (compression, etc.) coupling to minimize compliance.
For many direct-drive axes, the steel’s compliance between the motor and load can be a limiting factor. The steel’s compliance can affect the ultimate BW of the servo control loops. Even a machine’s frame compliance can become a major player against axis BW capability, motion stability, and controllability, where with previous technologies it may not have been of any concern. For example, to achieve the best possible axis BW capability, controllability, and minimal risk of any issues for direct-drive cartridge motors, it is very important to design the driven shaft (if applicable) with an outside dimension (OD) as large as possible for as long as possible, with an overall shaft length as short as possible. (Use as large an ID bearing here as possible to help system BW.)
Direct-drive cartridge motor technology utilizes a machine’s bearings to support the rotor of a full-frame motor for the ease of installation, and can often eliminate the need for a mechanical advantaged mechanism (gearheads, pulleys, and belts, etc.) like other direct-drive motor designs.
Prioritizing for Risk Management and Tradeoffs
It cannot be stressed enough that controlling factors for risk management are the machine’s functions with the chosen process to accomplish the work of each axis, as they apply to the new product production requirements and not the new or original machine’s design. Remember, for all new designs and especially for proof-of-concept designs, cost reductions cannot be reasonably applied to a machine whose manufacturing process doesn’t yet work. Changing the machine’s motion technology and control by specifically keeping the machine functions and chosen process in the forefront for making decisions and tradeoffs for each axis, with available servo system technologies, will greatly reduce risk and enhance the success of any machine design.
After chosen process and machine functions are understood (ins and outs, and basic safety requirements), we can now begin asking questions to determine direction and possible solutions for the work to be performed by each axis. The following set of questions is not meant to be all-inclusive, but rather a strong start to simplifying the design approach of each axis for servomotor system utilization.
Does the axis in question require point-to-point moves (typical Position Mode operation)?
Reduce load inertia and mechanism inertia as much as possible. For example, utilize aluminum over steel if possible, and/or remove unnecessary metal from components, especially at the larger diameters where not otherwise needed. Remember, the moment of inertia of a rotating component about its center axis goes up by its diameter to the 4th power.
Reduce friction as much as possible: bearings versus bushings, ball screw versus acme style screw, etc.
Reduce mechanism compliance as much as possible. Use the knee of the cost curve versus capability, when applicable.
Reduce, minimize, or eliminate mechanism backlash as much as possible: belt versus gearhead, versus direct-drive, etc.
Minimize the number of moving bodies between the load and motor, and make the mechanism’s drive train as rigid as possible. For example, a rack-and-pinion mechanism must be locked together such that the rack/pinion does not rise up on its teeth during a high-speed acceleration or deceleration.
Use a rigid (compression style, etc.) or equivalent coupling when applicable for the mechanism, reducing potential for mechanical wind-up and otherwise relatively large coupling inertia.
For indexing applications (especially high-speed), increase feedback resolution to maximum (knee of the cost versus capability curve), if one has not done so already.
Ensure proposed control method(s) can achieve safety protocols and any other specific requirements.
Consider basic maintenance procedure requirements in harmony with the process and safety protocols up front.
Does the axis in question require a continuous operating velocity (typical Velocity Mode operation)?
One must consider velocity tolerances long term versus short term, if applicable. If very short-term tolerance is more critical/dominant (smaller short-term Δ tolerance required per some time unit), then a higher than normally desirable load inertia may still be more suitable. Process needs to be understood and for a specific process, it could go either way: minimized load inertia (with maximum feedback resolution) versus a purposely designed larger load inertia (to dampen short term response) — it is very hard to make a judgment call without specific process information.
If long-term tolerance is dominant (tight long-term Δ tolerance required per some time unit), then typically it is best to maximize feedback resolution, and reduce load and mechanism inertia, allowing the servo to maintain the best control with the highest BW.
If the process requires the best of both worlds, reduce load inertia and mechanism inertia, and increase feedback resolution to the maximum available (utilize knee of the cost curve vs. capability).
When applicable, reduce load inertia and mechanism inertia as much as possible to increase BW capability. Reduce friction as much as possible, and reduce stiction as much as possible, especially for low-speed process applications. Eliminate mechanism backlash, and reduce mechanism compliance as much as possible. Use a rigid (compression style, etc.) or equivalent coupling when applicable for the mechanism, and minimize the number of moving bodies between the load and motor.
Increase feedback resolution to maximum (knee of the cost curve vs. capability). Controls: if possible, run the drive in a position mode for the appropriate time and displacement range. Typically, a better constant velocity tolerance can be achieved at the servomotor when run inside a position loop. Ensure the proposed control method(s) can achieve safety protocols and any other specific requirements. Consider basic maintenance procedure requirements in harmony with the process and safety protocols up front.
Does the axis in question require a continuous force be applied against some load (typically Torque Mode)?
Reduce friction as much as possible because stiction can easily become an issue. If an external force is applied for some time in a locked-rotor state, the motor must be sized accordingly. This is not a typical servo application. Many, if not most servomotors are rated at a low RPM (stalled rotor state), with just enough speed to ensure even heat distribution. Contact the motor manufacturer when applicable. Ensure proposed control method(s) can achieve safety protocols and any other specific requirements. Consider basic maintenance procedure requirements in harmony with the process and safety protocols up front.
Does the axis in question require extremely low speed ( ≤1 rpm)?
Reduce friction and stiction as much as possible; stiction can easily become an issue. Eliminate mechanism backlash. Reduce mechanism compliance as much as possible; use a rigid (compression style, etc.) or equivalent coupling when applicable for the mechanism, and minimize the number of moving bodies between the load and motor. Increase feedback resolution to maximum or at minimum, use knee of the cost curve for higher resolution. Control: if it is a velocity application versus positioning, then if possible, run drive in a position mode for the appropriate time and displacement range. Typically, a better constant velocity tolerance can be achieved at the servomotor when run inside a position loop. Ensure proposed control method(s) can achieve safety protocols and any other specific requirements. Consider basic maintenance procedure requirements in harmony with the process and safety protocols up front.
Is the specific axis in question vertical?
Utilize a failsafe-brake (internal to the motor or external axis brake) and/or counterbalance load. If a failsafe-brake is utilized, ensure its physical engagement and disengagement is timed with the drive commands, with proper delays for the subject brake’s engagement and disengagement. If counterbalancing load, take into consideration the additional load inertia and its effect on acceleration and deceleration torque requirements. If counterbalancing load, there are typically tradeoffs due to actual process cycle times, resulting in only a percentage of the load being counterbalanced. For partially unbalanced loads, use a current offset when applicable to offset the imbalanced load and to minimize control-loop integration requirements (typically reduces phase shift and lowers risk). Ensure proposed control method(s) can achieve safety protocols and any other specific requirements. Consider basic maintenance procedure requirements in harmony with the process and safety protocols up front. Refer to suggestions above for typical mode of axis operation: position, velocity, etc.
Summary of the Design Approach for Each Axis
In order to enhance the bandwidth response capability and controllability of any servomotor-controlled axis, a combination of factors must be considered in relation to the machine function, chosen process, and work to be performed by each subject axis. They are friction and stiction, external loading, backlash and compliance, load and mechanism inertia at the motor, feedback resolution, and finally, when applicable, the motor’s drive, PWM/SVM, and update rates (separate controller update rates, if applicable). Furthermore, the total (but desirably minimized) number of moving bodies between the load and motor along with the natural frequencies of the design may also need to be considered as the mechatronic design comes to completion.
One cannot reasonably apply cost reductions to a machine whose manufacturing process doesn’t yet work. This is why the needs of the chosen operating process should take decision priority over the machine’s initial performance tradeoffs and cost reductions. It is often best, for the initial machine build, to design for the highest capability at the lowest cost. If the new machine meets the production requirements utilizing the knee of the cost curve for its components, there may still be room for some cost reductions. On the other hand, if any specific component of the machine requires additional capability, the additional cost can be more easily justified.
Machine builders are continually faced with challenges in areas of technology complexity and knowledge barriers related to the scope of work to be performed, whether it involves a new design, re-design, or conversion implementation. By utilizing the latest servo system technology with a simplified axis design approach, and identifying the action items for a number of typical mechanical configurations, they can effectively manage design risk and achieve optimal machine performance while reducing development time.
This article was contributed by Kollmorgen Corporation, Radford, VA. For more information, Click Here .