Motors and motion control are a critical piece of many medical devices. However, making the right selection can depend on a number of factors. This article reviews the motor windings of stepper motors and highlights the most important considerations that medical device manufacturers need to understand.
When attempting to design or select the correct stepper motor for devices in today’s medical industry, two main factors can be reviewed: electrical and mechanical. Although most step motors look quite similar on the outside, the internal mechanics and electrical dynamics can and will vary significantly.
When it comes to the electrical aspect of stepper motors, the first thing to evaluate is the motor windings. How the motor is wound has a direct relationship between its performance, acceleration and deceleration capabilities, output torque, accuracy/vibration, and efficiency. Understanding the proper method behind calculating the correct winding is more often than not the decisive factor in whether a specific stepper motor is able to achieve the application’s objectives. Since torque in a small package has come to be such a great need in medical devices, selecting a winding with optimal efficiency at the application’s motion profile is crucial.
The next logical question would then be how the correct winding is chosen. The first step should always be to calculate the approximate torque required and the motor’s intended operating speed. Once that has been done, a step motor can be wound with the right amount of coils in order to output the best performance while using the minimum amount of power. An efficiency curve exists for every motor, and an excellent step motor manufacturer’s job is to select the best motor for the task at hand.
Figure 1: Different
efficiency curves for different performing motors
Depending on the
application’s specific needs, study and selection of the correct step motor
winding can be taken a step further. In many cases, the medical device has
limited space available for the motor and the amount of power input (current
and voltage) is fixed. Often times, a standard or catalog motor will be chosen
and will not be able to do the job because of lack of torque or speed. In these
cases, the term “Maximizing Torque at Desired Speed” can easily be utilized.
The philosophy behind this concept is to ensure that all of the power that is
being inputted into the motor is being used both effectively and efficiently.
Figure 2: Maximizing torque
at the desired speed
In Figure 2, the medical device manufacturer was originally using a motor that is depicted by the red line. This specific motor was able to output approximately 15 oz-in of torque at an operating speed of 600 pulses per second (PPS). Given changes in market conditions and increased pressure from competitors, the device needed to be updated such that both a higher speed and torque output were provided. The caveat was that the overall size of the device could not change, therefore, the motor size and power input could not be changed either. By normal standards, achieving the customer’s goal of higher speed and higher torque output without increasing the motor size and power input would not be possible. However, using the “Maximizing Torque at Desired Speed” philosophy, the problem was solved and a new motor (depicted by the blue line) was provided. This was accomplished by application engineers at Lin Engineering  who were able to utilize the lost energy at the lower end of the motion profile and transfer it to the higher speeds. Doing so allowed the motor to both run faster and provide more torque than before. The end result was a better running, more efficient stepper motor.
Figure 3: The Xtreme Torque Motor
Mindy Cheng is the project manager at Lin Engineering. She is responsible for managing new product releases as well as any custom projects. Cheng can be reached at 408-919-0200 or firstname.lastname@example.org .