So you want or need to add some motion to your system, and you think a stepper motor is the right solution for your application? Great! But how do you start? Well, let’s start from the beginning: define the needs of your mechanism, a.k.a. your spec.
Step 1: Define your requirements
This may sound obvious to some, but for the rest of you: KNOW THY SPEC. You want to look at your mechanism from a high level perspective and determine what you want or need your mechanism to do, in detail. Bend what wire? How far? What alloy? At what speed? With what precision?
Generating a list of specs is a fundamental step in the design process that many times gets ignored or skipped for the sake of the schedule, but doing so only brings headaches, tears, painful meetings and sad puppies later on. Dedicate all the time you need to really think through what your requirements need to be and to determine if they are realistic and achievable or not. Be a good engineer and stand your ground. If you are asked to comply with a spec that sounds crazy, it probably is.
Reviewing your specs will help you determine if a stepper motor is the right fit for your application or not. You may find that you’re better suited with a servomotor, or a brushless motor, or even an AC motor. It all depends on what the needs of your system are.
When we looked at the machine and what new capabilities we wanted it to have (based on customer input), we looked at the following specs:
- Axis torque – Max wire diameter to bend and max feed pull force
- Axis accuracy – Part repeatability
- Axis rotational speed – Max bend and feed speed and part cycle time
- Overall package size and envelope availability – Fits within machine envelope
- Power consumption – Operates with outboarded power supply
- Weight – We are designing a desktop machine
- Cost – Fits within desired budget
- Additional peripheral components needed – motor driver, shaft couplers, belts, pulleys, structure, software modifications, etc (effects on cost, complexity and or reliability)
- Usability and safety – Mind the use and the user. Knowing this is a desktop machine, how difficult should the machine be to use? How can we ensure it fails safely?
Your application is most likely different, but I’m sure you already have a set of requirements to start with. We discussed them as a team and documented them in what is called PRD, or Product Requirements Document, which proved to be very helpful later on in the design and detail engineering process. I’d recommend you to do the same. Just keep it high level but specific.
In the PRD document you wouldn’t be saying things like “the machine needs to operate with Nema 23 hybrid stepper motors with 3Nm of holding torque” but rather “the machine needs to be able to bend 304, half hard stainless steel wire with an OD of 0.188 at 5000 deg/min”. ” You see the difference? the first one is locking you on a solution without knowing if that’s what you actually need, when the later one is giving you a torque requirement. When you finish up your list of specs and requirements you’ll have a very clear idea of what needs to be done. Also, as a byproduct, you’ll have a bullet point list that is also usable by the sales and marketing department, how about that!
At the end of our process, we chose to use stepper motors since they are easy to drive and to control (simplified electronics), can provide enough torque for the available size and weight budgets, run at low speeds with smooth-enough motion, and allow us to stay under 200 Watt of total machine power (external power supply compatible). Also, if an axis gets stuck or hits a hard stop, the failure mode is just a loss of position that can be easily recovered with a homing sequence, instead of some part breaking or the machine “cutting itself in half”. This makes the machine less intimidating for CNC first-timers.
Remember, all the decisions you make later on during the design and engineering process will be based on this very first thought process, so think it right.
Once you have defined your high level specification, it’s time to look at it in more detail. Does a stepper motor still make sense? How can you ensure the torque and accuracy requirements are met? Well, it’s time to go to the next step: Define your actual torque spec.