Successful motion control projects are built, first and foremost, on thoroughly understanding your application's machinery requirements and specifications. So the most common mistakes engineers fall into start with engineering details like the exact motion profiles or torque requirements of the application. But other challenges loom as well, including ineffective engineering processes, an ability to solve integration problems, project management and decision-making skills. Avoiding these pitfalls in your motion control applications will require both engineering and management expertise.
Failure to thoroughly understand machine requirements and plan your motion solution properly. This most basic mistake is the failure to understand, in detail, the application and machinery requirements. Not knowing the machine's motion profiles, torque requirements and communication needs can easily result in not selecting the right hardware and software. Not planning for implementation adds time and cost to installation, setup, test, debug and all the steps it takes to get online and fully functional. By the time the engineer discovers he or she put a ballscrew on a system that needed a linear motor to achieve the speeds required by the application, there is a real bind because we need to re-engineer the system in a significant way. This common mistake goes back to the saying, "If you don't have time to do it right, how will you ever find time to do it over?"
Ignoring the scope of integration required to achieve optimal performance. In this situation, engineers discover there is more to integrating the system and making it work than simply picking out good components to do the job. You can pick out a great actuator as a piece of hardware, but then find out that if you run the machine at the top speed the motor can provide, the actuator won't operate effectively. Check all of your components against machine requirements, make certain all of the components will integrate together, and that all of the components will achieve the performance of the machine specifications. Generally it works well if the engineer is dealing with a single supplier and consulting closely with that supplier. If you are using different suppliers, that job falls to the engineer to make sure the application achieves the performance required.
Failure to look at system options before choosing the solution. This is picking a solution without exploring all the options available. You can over-purchase (buying a servo system for an application that really only needed an induction motor/drive system) or under-purchase (buying a stepper system when a servo system is needed). Sometimes engineers pick a solution that limits the machine's capability because they looked at only suppliers that didn't offer a full range of solutions.
An example is picking a conventional motor with a mechanical actuator when a direct drive solution would have provided higher performance. Look at every axis on your machine and ask, "What are the requirements for that axis and what will meet my requirements and do the very best job without going over the top on the solution?" Select multiple suppliers to work with when looking at the application. Get different views on how to move ahead and make sure you aren't locking yourself into one product or solution type for a particular axis on the machine without a thorough analysis.
Not knowing when to select "best in class" components versus. a complete single supplier system. Choosing the best parts ("best in class"), when you need only basic performance and lack the engineering resources to integrate the solution, can be a recipe for disaster. The "best in class" approach says, "I will buy a motor from one supplier, and the amplifier, controller and mechanical components from other suppliers." This can be a very good solution when you know exactly what you want to achieve and are prepared to provide the resources to select and integrate those components. If your application requires absolute performance, this can be a good choice and you would plan that way. On the other hand if you need a basic solution, you may want to go to a single manufacturer because you know that those components will work together. The important thing is to know what kind of result you want to achieve.
Buying on price rather than value. The key here is to follow engineering process. Get your system specifications right, select the best components and then do the hard part of negotiating with suppliers to get the price you need, rather than trying to set the price upfront and select the components based on price alone. If you opt for the latter approach you run the risk that it won't work, and it will cost time and money to get the system fixed. The truth of the matter is that suppliers will work with customers when it comes to getting the business. And of course help is available after the fact, but it's not as attractive when you first select the lowest cost equipment and then say, "I can't get it to do what I want." At this point the suppliers can always say, "You didn't pick the right products. We can help you solve the problem, but at this point price is not really negotiable."
Selecting products that provide more performance than the application needs. If one mistake is buying on price, this says basically don't buy things because everyone says you should use the latest and greatest technology. There is still a place for dc adjustable speed drives, even though some have been projecting the near-term demise of dc drives for nearly 30 years. Applications may be shifting away slowly from certain technologies, but the reality is no one needs to move to the latest and greatest technology unless it provides tangible benefits to the machinery builder and/or customer.
Misunderstanding the effects of machine dynamics on motion control components. Selecting the best motion control components does not guarantee success. The machine that the motion control components are coupled to adds its own dynamics (friction, inertia, compliance, electrical noise, etc.) that can affect the motion control system and often show up as motion system issues. Not understanding these can lead to mistakes that make systems more costly, underperform, or, in the worst of cases, unusable.
One vitally important issue is compliance. I can't tell you the number of times we have seen people design a machine, select the components and get a servo system that has excellent stiffness with great capabilities and can make products incredibly fast with very precise motion profiles. Then they take the motor and couple it to the machine with a compliant coupling that throws the entire motion design out the window. Customers call and say, "We can't tune this system and get it to work."
We'll get one of our engineers involved and we'll find that something isn't adding up. The motor is more capable than what they needed and everything should work. And then you find the coupling between the motor and the load. Attempts to de-tune the system to overcome the compliance problems now make it impossible to achieve the machine speeds necessary for the application. It's important to step back and say, "Do I understand the machine and how each part of the machine integrates together?"
Underestimating the importance of cables. Some engineers think cables are nothing but wires that connect components together rather than an important part of the motion control system. Many times, even when much effort has been expended to understand the machine, select the right controller, drives, motors and actuators, the cables are an afterthought. Not recognizing that cables are an integral part of the motion solution can result in failure to comply with applicable standards (NEC, UL, CE, etc.). Cable bending or flexing in excess of the material rating or design, electrical noise issues from improper materials, shielding or installation, are all potential problems.
Many of the cables in motion systems are custom-made with specialized shielding and insulation properties. If you are moving cables under load with axial or torsional bending, the material used in the cables becomes very important. And of course, shields not terminated properly can create electrical noise problems. These problems can be vexing because the system doesn't set off an indicator that says, "I have an electrical noise problem." The system behaves erratically, unusual things happen and you end up with service calls.
High quality cabling is not necessarily inexpensive, and often in systems with small motors it's not unusual for the cables to be more expensive than the motors. That can surprise people, but it's often the reality. If you look carefully at the application, you may be able to make the cabling less expensive. Too often, customers wait until the last minute to select and order the cables, overlooking them as an integral part of the machine design process.
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