Innovative Technology for Small Space and Higher Efficiency Motors
By Mindy Cheng
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|
Figure 1 depicts three different motor efficiencies when plotted across a speed range. Some motors, such as #1, perform at their best only at low speeds while others, such as #3, perform better at high speeds. Although one might be able to use motor #3 at a low to mid speed range and generate enough torque to satisfy the application, more power than necessary will be used and the life of the motor will decrease because it is being overdriven.
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 Stepper Motor|
There is only so much that properly configuring a step motor’s windings can do; this is where the mechanics come into play. Mechanically, the amount of torque a stepper motor is able to output has a direct relationship between the length and diameter of the rotor and the amount of windings that can fit into the stator. Changing the manufacturing process to include unique mechanical designs can go a long way in helping to achieve more torque without increasing the physical size of the end stepper motor. The Xtreme Torque (XT) Series from Lin Engineering (Figure 3) is a patented design that was introduced for this purpose. By utilizing a unique manufacturing process, the XT series is able to provide up to 35% more torque while maintaining the same overall size. Similar to the previous case, having a motor with higher output torque without having to increase the overall size or power input can be of great advantage to medical device manufacturers.
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.