| Rev # | Date | Description |
|---|---|---|
| 1.0 | August 2019 | Updated and revised for use by customers. Reset as document version 1.0. |
| 1.1 | August 2021 | Document verified for current software release. Updated some content. |
| 1.2 | August 2023 | Document verified for current software release. Significant additions to sections 3e, 8a, 8c, and 8f. Minor revisions elsewhere. |
Integrator Type
Advanced Motion Analysis Options
Accuracy
3D Contact Resolution
Cycle settings
Replace redundant mates with bushings
Frames per second
Motion Study Solver Settings
Partner Products
SOLIDWORKS Motion API
Export SOLIDWORKS Motion Position to SOLIDWORKS Simulation
SOLIDWORKS Simulation Import Motion Loads
Stress Analysis for Motion
Motion Optimization Study
Design Study
Event-Based Motion Analysis Study
SOLIDWORKS Motion Analysis Study
SOLIDWORKS Assembly Modeling
Relevant SOLIDWORKS Software Functionality
SOLIDWORKS Analysis Products of Interest
Design Considerations
Typical Analysis Goals for Mechatronics
Mechatronics
Preface
Note
All SolidPractices are written as guidelines. It is a strong recommendation to use these documents only after properly evaluating your requirements. Distribution of this document is limited to Dassault Systèmes SolidWorks employees, VARs and customers that are on active subscription. You may not post this document on blogs or any internal or external forums without prior written authorization from Dassault Systèmes SolidWorks Corporation.
This document was updated using version SOLIDWORKS 2023 SP03. If you have questions or need assistance in understanding the content, please get in touch with your designated reseller.
Preface
The purpose of this SolidPractice is to guide you through different aspects of SOLIDWORKSׅ Motion studies, including:
Features and functionalities
Solver settings
Troubleshooting recommendations
General tips and best practices
SOLIDWORKS Motion allows you to animate or simulate the motion of assembly models by creating motion studies that include movement elements such as motors. In these motion studies, SOLIDWORKS mates govern the movement constraints between bodies.
There are three types of motion studies available when using SOLIDWORKS Motion:
Animation (available in core SOLIDWORKS)
Basic Motion (available in core SOLIDWORKS)
Motion Analysis (available with the SOLIDWORKS Motion add-in to SOLIDWORKS Premium)
The focus of this SolidPractice is Motion Analysis. Motion Analysis studies are used to simulate and analyze the motion of an assembly using a kinematic solver. In addition to accounting for active mates and material properties for the assembly, Motion Analysis studies can also consider forces, springs, dampers, friction, and contact. You can also use Motion Analysis studies to plot analysis results for further study.
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Mechatronics
Mechatronics is a hybrid field that generally studies the control of mechanisms and machinery using electronics. It is unique in the way that it combines multiple aspects of different fields of engineering, mechanics, and electronics. With recent technological advances, it also transcends into the related disciplines of control systems, computer hardware, and software engineering.
In a much broader definition, mechatronics can also mean any product that has a combination of electronic and mechanical components interacting together. Mechatronics can be found in a wide range of applications, from consumer products such as home appliances, to complex robotics in the industrial machinery field. The focus of this SolidPractice is on introducing the latter application.
Figure 1 – SOLIDWORKS model of an industrial packaging machine featuring robotic arms
Typical Analysis Goals for Mechatronics
The most common goals in designing and optimizing such machinery are twofold:
Minimize energy usage by the machine. For industrial machinery, this is primarily due to movements for a specific task or function of interest.
Minimize the cost of the machine itself. This is slightly more complex because it depends on the mass of the machine, and other factors such as material selection, manufacturing costs to produce the machine parts, and the complexity of the moving parts. The number of parts and the cost of machine assembly also come into play.
Design Considerations
There are many considerations when designing complex machinery. Some of these considerations include:
The process objective. In other words, the details of the task or function that the machine needs to perform.
Function and product limitations. This refers to the function or product that the machine performs or manipulates, not the machine itself.
Available space. Not just the “footprint” of the machine on the floor, but also the space that must remain clear around the machine to accommodate movement of robotic arms.
Safety requirements.
Maintenance considerations. This may include ease of accessibility, frequency of maintenance, etc.
Flexibility of use with various products. The objective is to minimize the cost and difficulty of changing machine attachments and programming the machine to accommodate different products.
Frequency of failure and the consequences of failure. For example, will a small part break allowing for an easy fix or part replacement; or will failure occur on a larger scale and require replacement of large or multiple parts.
SOLIDWORKS Analysis Products of Interest
SOLIDWORKS Motion is designed for kinetic as well as kinematic analysis. The SOLIDWORKS Motion add-in combines the ease of modeling and connecting (mating) mechanisms using SOLIDWORKS 3D CAD, the intuitive SOLIDWORKS Motion user interface , and the powerful and well-established Automated Dynamic Analysis of Mechanical Systems (ADAMS) solver behind the scenes. In addition, SOLIDWORKS Simulation can work in conjunction with SOLIDWORKS Motion to perform stress analysis using static studies on specific components.
Although the SOLIDWORKS Premium package includes SOLIDWORKS Motion, for most mechatronics applications, a SOLIDWORKS Simulation Professional or Premium license is recommended to make full use of the software functionality.
Relevant SOLIDWORKS Software Functionality
This section aims to provide an overview of distinct software features that are often used in mechatronics analysis, starting from basic topics to the more advanced ones. As with any other engineering discipline, you should first become familiar with basic Motion Analysis practices and gradually build up skills and knowledge to the point where you feel comfortable with more advanced analyses, tools, and optimization.
SOLIDWORKS Assembly Modeling
Prior to using any analysis product, a high quality model of the geometry is essential. SOLIDWORKS Motion requires assembly files. A part file (even a multibody part) alone cannot be analyzed in SOLIDWORKS Motion. For models that will be analyzed in SOLIDWORKS Motion, there are many considerations to keep in mind when preparing an assembly for use in a Motion study.
For the majority of analysis applications, you should avoid interferences (including multibody interference) in the model. You should create mates systematically and meticulously. It is important to maintain balance between creating mates that are true to life, and minimizing mate redundancies. This is discussed further in the “Tips and Best Practices” section of this SolidPractice.
Some users may prefer to create specific configurations of their assemblies or a separate “for analysis” file set for use with SOLIDWORKS Motion. The advantage of doing this is that it makes it possible for users to suppress non-essential (i.e. cosmetic) components. Limiting the number of parts in a Motion Analysis study makes creating and managing mates, removing mate redundancies, and solving generally faster and easier.
Figure 2 – SOLIDWORKS Assembly of a welding robot
SOLIDWORKS Motion Analysis Study
Each Motion study has a tab in the motion manager bar near the bottom of the SOLIDWORKS user interface. A Motion Analysis study setup typically involves the application of mate constraints through SOLIDWORKS, and inputs in the form of motors and forces, which are defined in the time domain.
For example, a study might include a force of 1000 pounds applied at t=1s, ramped up to 2000 pounds at t=5s. Figure 3 depicts such a force definition within the SOLIDWORKS Motion Analysis study interface within the SOLIDWORKS MotionManager. To view this interface, you must first load the SOLIDWORKS Motion add-in and change the study type to Motion Analysis by using the drop-down menu in the upper left corner of the study pane.
Figure 3 – SOLIDWORKS Motion Analysis interface
Event-Based Motion Analysis Study
In addition to Motion Analysis studies that involve triggers that are purely time-based, SOLIDWORKS Motion also supports analyses involving sets of motion actions that result from different types of triggering events. This event-based motion functionality requires a SOLIDWORKS Simulation Professional or SOLIDWORKS Simulation Premium license. SOLIDWORKS Premium does not include this feature.
The event-based motion functionality is built on top of a standard motion study. In addition to allowing time-based triggers such as turning a motor on or off, event-based actions involving a variety of trigger types are available. For example, a sequence of motor or force events can be set to occur when a component moves across and triggers a specified proximity sensor. This can be very helpful when setting up complex studies involving system feedback, and then making adjustments to shift the exact timing of certain events.
This motion analysis feature is useful and powerful, especially when analyzing complex machinery that is highly dependent on timing. Such machinery is a common component of an automated assembly line.
Users can toggle between the standard shown in Figure 3, and the
shown in Figure 4 by clicking on the view icon in the
upper-right corner of the MotionManager.
When in Event-based Motion View, a task-based table appears in the central portion of the MotionManager. A Gantt flow chart appears at the extreme right side. The Gantt flow chart is a visualization of the task sequence in the time domain.
Figure 4 – Event-Based Motion Analysis study interface
Design Study
Another SOLIDWORKS tool that can be useful in mechatronics design is the Design Study. The Design Study interface is separate from the core setup of a motion study, and appears separately within the MotionManager interface. Each design study appears on a separate tab to the right of the SOLIDWORKS Model tab.
Design studies let you manage variables and constraints to evaluate different scenarios automatically without defining multiple studies or repeatedly running the same study while adjusting the setup. The functionality is available with a SOLIDWORKS Premium license.
With motion design studies, you can set up parameters that link to a motion study in order to examine the effects of such variables. The results from different scenarios can be tracked by using a Monitor Only constraint.
Figure 5 shows the setup forDesign Study 1, which examines the acceleration of a component at a specific time for three different motor speeds.
Figure 5 – Design Study setup (Variable View)
Figure 6 shows the results fromDesign Study 1.
Figure 6 – Design Study Results view
Motion Optimization Study
The Motion Optimization Study further extends the Design Study functionality by running combinations of variables automatically. This is useful for finding the optimal configuration of parameters that fit within the desired constraints while achieving the requested goal.
To switch Design Studies from non-optimization to optimization, you must first activate the Optimization option that is located next to the Run button. Once the Optimization option is active, you are then able to define goals in the Goals section of the Design Study.
You use sensors to define the goals for your optimization study. When you run the optimization study, the software will attempt to meet the specified goals by adjusting the variables as needed while also honoring the defined constraints. After you choose a sensor for a goal definition, you have three options:
Maximize – The software will attempt to maximize the value of the selected sensor.
Minimize – The software will attempt to minimize the value of the selected sensor.
Is close to – The software will attempt to have the selected sensor match a specified value that you input.
When attempting to optimize a mechanism using a Motion Optimization Study, your constraints and goals will often use a specific category of sensors called Motion Data sensors. When you define a Motion Data type of sensor, you select the Motion Analysis study and either an existing or new Motion Result Plot from that study. Since Motion Result Plots include a wide range of data quantities, this provides Motion Data sensors a significant amount of versatility in defining the parameters for optimization.
Motion Optimization studies with Motion Data sensors require a license of either SOLIDWORKS Simulation Professional or SOLIDWORKS Simulation Premium. When using Motion Optimization, the SOLIDWORKS Simulation add-in should be running. If SOLIDWORKS Simulation is not active but the Optimization option is still active, the Run button is inactive.
Stress Analysis for Motion
The stress analysis feature within the SOLIDWORKS Motion user interface allows users to perform stress, factor of safety, and deformation analysis on components without creating a separate Simulation study within the SOLIDWORKS Simulation user interface. To use of this feature, you must load the SOLIDWORKS Simulation add-in. Since the analysis is limited to static studies, a SOLIDWORKS Premium license is sufficient because it includes both the SOLIDWORKS Motion and SOLIDWORKS Simulation Standard software.
This functionality can be helpful in the field of mechatronics, letting engineers analyze the stress on one or more critical parts in the assembly. This can help determine whether the component design and material selection create stress that is in the allowable range when the system model in SOLIDWORKS Motion performs tasks.
Figure 7 – Structural Simulation Analysis for SOLIDWORKS Motion buttons
SOLIDWORKS Simulation Import Motion Loads
In addition to running stress analysis within the SOLIDWORKS Motion UI, you can also use the Simulation > Import Motion Loads option to automatically generate a SOLIDWORKS Simulation static study (outside of the MotionManager interface) based on the loads SOLIDWORKS Motion calculates at a single frame. It is also possible for SOLIDWORKS Simulation to create a design study for multiple frames of SOLIDWORKS Motion results automatically. The advantage of this method over the stress analysis feature for the method described previously is that the full SOLIDWORKS Simulation interface is available. This allows access to tools such as mesh controls, study properties, and full result tools such as Probe and List Selected.
Figure 8 shows the Import Motion Loads dialog box that appears when creating a SOLIDWORKS Simulation study (or Design Study) that is based on one or more motion frames.
Figure 8 – Import Motion Loads into Simulation dialog box
Export SOLIDWORKS Motion Position to SOLIDWORKS Simulation
At specific time steps in a motion analysis, you can export the position and create different simulation studies to analyse the deformation, strength, and stiffness of the product at the critical positions.
The Copying Multiple Components functionality allows you to copy several components at a specific time step, and retain the position and mates between them.
To accomplish this, you need to create a new assembly and copy the components. For example:
Press Ctrl + click or Shift + click to multi-select all bodies in the SOLIDWORKS FeatureManager® design tree.
Press Ctrl + C or use the Copy command in the context menu.
Create a new assembly document.
Select the new assembly in the FeatureManager tree and then use the Paste command in the context menu or press Ctrl + V to insert the previously selected components.
It is possible to select only those bodies will be analyzed afterward in SOLIDWORKS Simulation. In this case, you must create the SOLIDWORKS Simulation study manually and define all constraints and reaction forces based on information from the motion analysis.
You could repeat this process for different motion steps to analyse the critical steps position depending on the motion analysis results. Each step that requires analysis would be saved as a new assembly.
SOLIDWORKS Motion API
For advanced users who are familiar with programming, it is possible to create macros to automate various operations in SOLIDWORKS Motion. The operations can include set up, running, modifying, and extracting results in motion studies. The SOLIDWORKS Motion API is integrated with the MotionManager API. You can find the documentation in the SOLIDWORKS API Help.
User who have interest in learning the SOLIDWORKS Motion API should start with the basic SOLIDWORKS API documentation and examples before attempting to use the SOLIDWORKS Motion API.
Partner Products
In addition to the built-in features of SOLIDWORKS Motion, there are also SOLIDWORKS partner products that are relevant to the field of Mechatronics.
These products are inherently complex and feature their own partner developed UI. They have functionalities such as creating custom inputs for motion studies, extracting result data back into the partner product, and more. These tools can help you design, analyze, and optimize the programming and motion sequences necessary for an actual machine
The details of these partner products is outside the scope of this document. You can visit the SOLIDWORKS website to search for and learn about the partner products that are available:
https://www.solidworks.com/engineering-software-partners-products
Motion Study Solver Settings
Motion Analysis studies use the same general interface as the Animation and Basic Motion type studies, but have many more features and settings available. This section will go over the basics of some of these features.
Frames per second
.
Multiplying this value by the length of the animation returns the total
number of frames that you want to save the results for. This value does
not affect the playback speed.
It is fundamental that you having enough frames per second in the motion study to capture important events. You can think of the Frames per second setting as a sampling rate, where the solver continues taking its own steps in the background while snapshots of the model are saved only at specific frames.
Replace redundant mates with bushings
The Replace redundant mates with bushings option converts redundant mates in the assembly into bushings during the calculation. In most cases, this increases the necessary calculation time. The Bushing Parameters change the stiffness and damping for all bushings that replace redundant mates.
Including bushings in a Motion Analysis study is equivalent to adding a flexible mate. You can think of a bushing as a spring and damper system.
Cycle settings
The Cycle settings specify the cycle rate or period. These settings define the cycle angle for use as an alternative independent variable in custom Motor or Force profiles.
Figure 9 – Motion Study Properties
3D Contact Resolution
The 3D Contact Resolution slider controls the number of facets that SOLIDWORKS uses to represent the geometry. The position of the slider influences the resolution accuracy of geometries involved with contact when the motion analysis runs.
If high contact precision is necessary, always select the Use Precise Contact option, which represents the geometry exactly as modelled.
Figure 10 – 3D Contact Resolution
Accuracy
The Accuracy parameter is independent of the model dimension and units. This parameter controls the relative error allowable in between each solver iteration.
Advanced Motion Analysis Options
In the Motion Analysis study properties, there is another menu for Advanced Options, which allows you to adjust some of the options used when the solver calculates a solution.
Figure 11 – Advanced Motion Analysis Options
Integrator Type
SOLIDWORKS Motion solves a set of coupled differential and algebraic equations (DAE). There are three types of integrators available in SOLIDWORKS Motion:
GSTIFF
The SOLIDWORKS Motion solver uses the GSTIFF method by default. The GSTIFF method uses a variable order and variable step size integration. This is a fast and accurate method for computing displacement for a wide range of motion analysis problems.
WSTIFF
The WSTIFF method is similar to the GSTIFF method in formulation and behavior. The only difference is that the coefficients used internally by GSTIFF are calculated assuming a constant step size. In WSTIFF, these coefficients are functions of the step size. If the step size changes suddenly during the integration, GSTIFF introduces a small error in the solution, whereas WSTIFF can handle the error without any loss of accuracy. Therefore, the problems run more smoothly in WSTIFF.
Sudden step size changes occur whenever there are discontinuous forces, discontinuous motions, or abrupt events such as 3D contacts in the model.
SI2_GSTIFF
The Stabilized Index Two GSTIFF (SI2_GSTIFF) method is a modification of the GSTIFF integration method. The SI2_GSTIFF method provides better error control over velocity and acceleration terms in the motion equations. If the motion is sufficiently smooth, the SI2 velocity and acceleration results are more accurate when compared with GSTIFF or WSTIFF computations, even for motions with high frequency oscillations. SI2 is also more accurate with smaller step sizes, bit it is significantly slower.
Maximum Iterations
The Maximum Iterations option specifies the maximum number of times the numeric integrator iterates in the search for a solution for a given time step. If the program exceeds this limit, a convergence failure is recorded. The default value is 25.
Initial Integrator Step Size
The Initial Integrator Step Size option controls the speed at which the integration method starts, and its initial accuracy. You can run the simulation more quickly in subsequent runs by increasing this value. Enter the desired first integration step size used by the variable step integrator.
Minimum Integrator Step Size
The Minimum Integrator Step Size option makes it possible to decrease the simulation time by increasing this value. Enter the desired lower boundary of the integration time step.
Maximum Integrator Step Size
The Maximum Integrator Step Size option is important if the integration method does not detect short-lived events such as impacts. Enter the desired upper boundary of the integration time step. Set this value to be of the same order as the short-lived events. If this value is too large, the simulation can ignore some events.
The maximum integrator step size controls the value of the largest time step the integrator can take during the solution.
Increasing the value for the Maximum Integration Step Size option speeds up the solution and reduces the amount of time it takes to solve the Motion Analysis study. However, if this value is too large, there is a chance that the solver will take too large a step and enter a region from which it may not recover and subsequently fail to converge.
When using the GSTIFF integrator, velocities and acceleration can experience discontinuities for larger values of integrator time step.
If you confirm that the motion is smooth and does not experience abrupt changes, you can increase this value to speed up the solution.
Jacobian Re-evaluation
Use the Jacobian Re-evaluation slider to set the frequency at which the software re-evaluates the matrix.
The Jacobian matrix is a matrix of partial derivatives that are required to solve the linearized approximation of the original nonlinear equations of motion during the Newton-Raphson iteration process.
Users may find it useful to view this matrix as similar to the stiffness matrix in the finite element analysis.
The most accurate settings for re-evaluating the Jacobian matrix are the default settings. However, the default settings are also the most time-consuming.
While reducing the frequency of re-evaluation speeds up the solution, you should only do this when changes in the assembly motion are slow. Using a setting that is too low may cause the integrator to fail.
Motion Study Properties - Recommendations for Troubleshooting
An important and fundamental setting is the integrator step size. In cases where motion studies fail on the first time step, lowering the initial integrator step size may be helpful. Similarly, decreasing the value of the maximum integrator step size can also help the solver get past sudden discontinuities throughout the duration of the motion study.
Be aware that changing these settings can drastically affect the time that it takes to solve a motion study. However, in some cases, it may be necessary to use a very small initial or maximum integrator time step to solve a study successfully.
Parameters that commonly require adjustment include:
• Accuracy
• Maximum Integrator Step Size
• 3D Contact Resolution
If changing these parameters does not help the study convergence, make sure that your inputs are smooth and differentiable, especially the user input expressions with mathematical functions.
For motion loads, it is a general recommendation to use the STEP function instead of an IF statement when possible.
On occasions, redundant constraint can cause the integrator to fail because the solver is having difficulty satisfying the constraints. The most likely cause for such a failure is an inconsistent model definition or an ill-behaved model. In these situations, try to eliminate the redundancy or the mating relationships in the assembly.
Tips and Best Practices
The general suggestions for mechatronics analysis in SOLIDWORKS Motion are the same as those for motion analysis studies. However, the consequences of a poor motion study set up in a complex mechatronics assembly can be more troublesome than in a simpler model. Therefore, it is important to consider these tips and best practices to help avoid problems and ensure that you achieve accurate and reliable results.
Mate Setup and Redundancies
The proper and systematic setup of mates is important. Often, one or more individuals within a company will create an assembly model in SOLIDWORKS, and then pass the model along to a person or team responsible for analysis tasks. The mates for such models are often created without considering motion analysis. Instead, the mates may have been created primarily for the purpose of component positioning with little consideration given to degrees of freedom (DOF), proper mechanism motion, or minimizing redundancies.
If a motion study setup uses such “low quality” mate setups, problems often occur quickly when performing motion analysis calculations. The SOLIDWORKS Motion solver may lock up and fail almost immediately, or parts may simply fly apart. In cases where the majority of the assembly is mated improperly, the best approach is often to save a copy of the model, delete all of the mates, and recreate them.
Keep in mind that each motion study can have so-called “Local” mates, which are mates created within a motion study that will only appear in that specific study. These “Local” mates have no effect on other studies or on the mates in the model itself. When a “Local” mate is created, its name automatically starts with the word “Local,” as shown in Figure 12.
Figure 12 – “Local” mates in a Motion Analysis study
To understand how to create mates that are suitable for motion analysis, you must keep several things in mind.
First, all parts in SOLIDWORKS Motion behave as rigid bodies, meaning that they cannot deform or change shape in any way.
Second, each body that is not fixed or constrained has six DOF, which include:
Three translations - movement in x, y, and z directions
Three rotations - rotation about the x, y, and z axes
When you add mates between components or between a component and some kind of reference geometry such as a plane or axis, you remove DOF from the system. In the case of a mechanism, there is often a clear number of DOF present in the system, which is evident from the way the mechanism operates. Redundancies occur when the number of DOF removed from the system by imposed constraints is greater than the number of DOF present in the system.
A common and simple example is a door with two hinges that attach it to a fixed frame using a hinge mate is applied to each hinge. Each hinge mate removes five DOF leaving only one remaining DOF – the rotation about the axis of the hinge. Applying this mate to both hinges removes a total of ten DOF from a system that started with six (not twelve - the fixed frame does not contribute any DOF because it is fixed). We know that the “mechanism” should really have one true DOF – rotation about the hinge axes, which in this case are coincident. Therefore, there are five redundancies in this situation.
By default, when there are redundancies present, the SOLIDWORKS Motion solver will attempt to remove the redundant constraints automatically, otherwise it cannot solve the set of motion equations. With more complex mechanisms, this automatic process sometimes does not work out well from an analysis standpoint. Important constraints may be removed and lead to bodies separating or solver lockup. There may also be cases where the study calculates successfully, but the reaction forces seem incorrect.
The best solution for dealing with redundancies is to create mates in a systematic piece-by-piece manner with DOF in mind. You can view the Degrees of Freedom (Gruebler count) by right-clicking a Mates item in the MotionManager interface (see Figure 13). An alternative approach is to use bushings, which you can think of as spring damper elements in the mates, giving the connections between components some slight “wiggle” room. The presence of bushings theoretically allows the solver to find a solution to a motion study despite redundancies and without the need for the solver to remove redundant constraints automatically.
Figure 13 – Degrees of Freedom (aka Gruebler Count) tool
The problem with using bushings is that users will often simply enable the global Replace redundant mates with bushings option in the Motion Study Properties and expect their mechanism to solve. While this may be the case for some models where forces and torques between components are of similar and relatively constant magnitudes, more often than not, problems will occur. This is because applying global bushings in this way will impose a single set of stiffness and damping values for each translational and rotational DOF in every mate. As a result, some may be too stiff to allow the solver to converge, some may be severely imbalanced, and some may simply fly apart due the mate “breaking” as a result of insufficient stiffness.
The best approach when using bushings is to specify the bushing parameters for each mate individually based on your knowledge of the actual assembly. This is possible when editing a mate by clicking on the Analysis tab and selecting the Bushing option, which will allow modification of the bushing parameters. Determining the best parameters is often a trial and error process because it depends on the forces acting on the mate. In general, if you do not have an estimate for the flexibility of a mate, it is often best to select the values so that the stiffness is high relative to the forces acting on the mate, thereby resulting in small displacements.
How Does the Solver Remove Redundancies?
There is a certain logic by which redundancies are automatically removed. In the situation where redundancies are present when running the motion analysis, the solver will remove redundancies based on the following order:
1. Rotational Constraint
2. Translational Constraint
3. Motion Inputs
Before actually running a motion analysis, the solver goes through the process of detecting if the mechanism contains redundancies. If the solver detects redundancies, it will try to remove them, and if successful, calculates the analysis. At each time step, the solver continues to re-evaluate redundancies and removes them if necessary.
This means that the solver looks for rotational constraints that it can remove to eliminate redundancies. If it cannot find any rotation constraints to remove, it will then try to remove translational constraints. If the solver cannot remove any translational constraints, it will try to remove an input motion (as a last resort).
If all of these attempts fail, the solver will end the process with a message stating to check for redundant or inconsistent constraints in the mechanism (or to see if it is in a locked position).
When Is Joint Redundancy Not a Problem?
A joint redundancy is not a problem if you are interested only in kinematic results, such as displacements, velocities, and accelerations. Additionally, joint redundancy can be acceptable in cases where there are no forces or torques that depend upon other joint forces or torques (such as joint-friction).
That being said, it is typically recommended to remove all existing redundancies before running any motion study. This can help avoid other potential issues, and also makes the system easier to understand and troubleshoot.
Rigid Groups
The use of rigid groups is a quick and useful way to reduce the level of complexity in large assemblies. By right-clicking on a component in the MotionManager tree and selecting Add to New Rigid Group, it is possible to create folders of distinct rigid groups containing multiple components that will move together.
The biggest convenience here is that mates between components in a given rigid group will be ignored. Therefore, if the mates are just there to keep components together (and most likely contain redundancies), adding such components to rigid groups helps to avoid issues due to mate redundancies. For additional details, see the topic “Rigid Groups” in the “Motion Studies” online Help.
Subassemblies
Subassemblies can sometimes be troublesome and difficult to work with in a motion analysis. Subassemblies within an assembly can either be treated as:
Rigid, which prevents movement of the subassembly’s components relative to each other. Rigid subassemblies act and move as a single rigid body within the assembly.
Flexible, which allows movement of the subassembly’s components relative to each other. In other words, the components within a flexible subassembly can move within the DOF according to the subassembly mates.
All of the mates and DOF within a flexible subassembly are taken into account within a Motion Analysis study, and thus makes the study more complex. As such, if this is not necessary and you expect a subassembly to behave rigidly within the context of the Motion Analysis study, be sure that it is treated as a rigid subassembly.
If a flexible subassembly is present and other troubleshooting techniques do not resolve an issue, try making the subassembly rigid or even dissolving it and then adding its components to a rigid group to see if that avoids the issue.
Utilizing Alternate Mate Options
As you gain experience with preparing assembly mates for use with Motion Analysis studies, you will become familiar with alternative ways to mate components together. This is an important skill that you will use to create Motion-Analysis-friendly assemblies or to remove redundancies from an already-mated assembly.
As an example, users often will mate components together such that there is only one rotational DOF between them. Although this effectively forms a hinge, many users accomplish this by methods other than using a single hinge mate. A user might use a concentric mate between two cylindrical faces, and then add another coincident mate between planar faces for constraint in the axial direction. A concentric mate between two cylinders removes two translational and two DOF, while a coincident mate between two planar faces removes one translational and two rotational DOF. This is a total of seven removed DOF, even though in practice only five are removed. If you instead simply define a hinge mate, it will remove just the desired five DOF (three translational, two rotational).
Another basic techniques to reduce redundancies is to utilize mate primitives, which are mates that constrain two DOF or less. You can use mate primitives as alternatives for other mates that might constrain more DOF and thus cause redundancies.
The table below has some examples of common mates and mate primitives, as well as their removed DOF. The rows shaded in light grey are examples of mate primitives that remove two DOF or less.
| Mate Type | Translational DOF removed | Rotational DOF removed | Total DOF removed |
|---|---|---|---|
| Hinge mate | 3 | 2 | 5 |
Concentric mate (2 cylinders) | 2 | 2 | 4 |
Concentric mate (2 spheres) | 3 | 0 | 3 |
| Lock mate | 3 | 3 | 6 |
| Universal mate | 3 | 1 | 4 |
| Screw mate | 2 | 2 (+1) | 5 |
Coincident mate (2 points) | 3 | 0 | 3 |
Coincident mate (point and plane) | 1 | 0 | 1 |
Coincident mate (point and line) | 2 | 0 | 2 |
Perpendicular mate (2 lines) | 0 | 1 | 1 |
Parallel mate (2 lines) | 0 | 2 | 2 |
Coincident mate (line and plane) | 1 | 1 | 2 |
How to Resolve Redundancies
While each case is different, Figure 14 provides a very general decision tree covering the basics of resolving redundancies in an assembly. You will notice that many steps in the decision tree involve topics covered within this section, such as rigid subassemblies, rigid groups, and redefining mates.
Figure 14 – Resolve Redundancies Diagram
Tips on the Inverse Kinematics Approach
The Inverse Kinematics method is often used in mechatronics to convert the motion of an object (which is part of a mechanism) in Cartesian linear space, into joint movement to accomplish a specific motion. It is possible to accomplish this by using a simple SOLIDWORKS Motion Analysis study.
By defining a path mate that is based on the path of the object at a given location on the component and using a path mate motor, it is then possible to obtain joint velocity profiles by extracting the angular velocity at a given joint (constrained by one or more mates). Note that in these cases, there are often multiple solutions available. At that point, it is up to the user to apply torques or motors at the joints and then optimize the mechanism’s motion. Another useful tool for subsequently verifying the motion is the Trace Path result plot, which exists in the Displacement/Velocity/Acceleration Result category.
Additional Resources
For more information about common issues, analysis methods, and results interpretation, refer to the SOLIDWORKS Knowledge Base, SOLIDWORKS Motion training manual, and SOLIDWORKS Motion Help.
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