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Optimize Medical Machines Through DC Motor Selection: Selecting the right DC motor is an important aspect for optimizing medical machine performance.

There are so many motor choices available for the medical market today, from heavy-duty brushed motors to tiny brushless DC (BLDC) motors. To implement any motor, users must have a full understanding of their specific application parameters, including power, speed, torque, physical size, efficiency, lifetime expectations, and other requirements.

We will focus on the medical market by spelling out the primary differences between DC brushed and brushless motors most often used in the industry. What can they do, where do they fit best, and how do you decide which to use in your application? DC motor selection is one of the most important steps in providing motion control for a medical machine, whether for prosthetics, medical tools, infusion pumps, robotics equipment, or medical training aids.

The medical market is unique in that most devices and machines operate in a clean environment and around people. This means that long life and low noise (see Sidebar 1) are key characteristics the drives need to have in order to fit the requirements of many medical machines. Fortunately, DC motors arc noted for having life-spans that can last tens of thousands of hours of continuous operation, and even longer when operated intermittently. This is especially true for BLDC (brushless DC) motors.

Motor Selection Criteria

Key criteria for selecting a DC motor for a medical machine application includes finding out what voltage is readily available for the application and what physical size the motor needs to be. Other parameters, such as speed, torque, and efficiency can be determined once these two parameters are determined.

Voltage availability is critical and can determine a lot of other aspects of a design. For example, prosthetics are battery operated, while many rack-mounted devices and surgical tools operate from a 24 V power supply. DC motors are available for use at voltages as low as 1.5 V and as high as 48 V, dependent on the power source.

Similarly, physical size can be a limiting factor in motor selection where a compromise needs to he made between which motor to use and the available space it needs to fit into. Again, the prosthetics application requires a small frame motor, while rack-mounted devices can be designed to accommodate larger devices.

The next consideration is motor efficiency. This becomes a primary concern when your application needs to control power consumption to maximize battery life, whether in a prosthetic hand or in a portable surgical tool like a saw or drill. This choice is not as critical when designing a mains-powered device, such as a surgical robot.

At this point, you can consider your application's torque and speed requirements, and the effect they have on the size of the motor frame. For example, it takes a larger motor to rotate the magnets in an MRI machine than it does to run the infusion pump for drug delivery (see Sidebar 2). This means that high torque motors are typically larger in size than their low-torque counterparts, which also means that larger housings and, therefore, larger mounting hardware will be required.

Motor duty cycle can have an effect on your motor choice as well. A motor that requires only intermittent operation reduces the wear and tear on the motor, increasing its life, which means a smaller motor size can be used without depleting the positive characteristics of the machine itself.

Another marked feature that will determine your motor choices is if they need to be sterilized repeatedly. Although there are autoclavable motors available, Maxon has produced the first whole package (motor and encoder) that can go through multiple-up to 2000--cycles of sterilization.

Brushed vs. Brushless

Brushless motors will last much longer than brushed motors, which rely on a mechanical connection to deliver power to the motor's rotor. Further, brushless motors operate at faster speeds than brushed motors. If you've chosen to use a brushless motor for reliability, it is best not to add a gearhead to the mix because the mechanical nature of a gearhead automatically shortens the life cycle of the motor. This only negates the longevity of the combined system. On the other hand, if the environment is such that noise is a concern or that a higher torque is needed, a gearhead will do the job very well.

If your medical application requires high speeds and you've chosen to use a brushed motor, you'll need to add a mechanical gearhead. In this case, using a gearhead won't change the life cycle of the combination to any great extent-both are mechanical components that are subject to wear and tear. For medical equipment, though, you don't have to be concerned with dirt or grime getting into the system. The relative cleanliness of most hospital environments allows you to get the most out of your mechanical components.

The most important issue to be concerned with in selecting between a brushed and brushless motor is the expertise of the machine builder. Brushless motors either come with built in electronics or with external electronics to operate the motor. It takes some experience to provide the custom electronics many machine builders choose to provide. But for high sales volumes, the costs are easily regained.

Brushed motors, on the other hand, don't need electronics to run the motor, which provides a plug-and-play option to the design engineer. This means that if the machines are expected to sell in very low quantities, a brushed motor could save on the overall cost of the system design. On other applications where high efficiency and low noise at low cost is what's needed (such as in drug pumps), a DC bnished motor is a good choice. (Figure 1).

A final concern is the power needed for the motors. For instance, Maxon motors are available in power ratings up to 250 Watts for brushed motors and 400 Watts for brushless motors. Since power is a function of voltage and current, the battery or power supply output you have available is very important. The details of the transformation into mechanical power can be influenced and modified during the selection process. For example, if the power is made available primarily in the form of a high voltage, this may later be adjusted by the selection of a motor winding with a correspondingly low speed constant.

As requirements for medical equipment and device development are more demanding, and with more functionality becoming integrated, brushless DC motors become a viable motor to use. In fact, many medical machine builders continue to select brushless motors for their applications whenever possible. Long life and high speeds make these motors applicable to a broader array of applications. In addition, brushless motors' high efficiency and ability to run at very low speeds without needing an encoder make them an excellent choice for most power tools and prosthetic applications.

By Carsten Horn, Business Development Engineer - Medical, Maxon Motor

Sidebar 1:

There are a number of methods that can lead to low noise in a motor and/or gearhead design. Since it is the characteristics of speed versus mass that creates noise in the first place, using lower-mass materials can have a large effect on noise output. Further, adjusting the tooth angle on gears so that they glide together easier can also reduce noise. Be sure to ask your supplier for their low noise versions if you are designing for infusion pumps, prosthetics, or power tools. [c] 2017 maxon precision motors, inc.

Sidebar 2:

Although speed and torque are independent requirements in many applications, typically speaking when the torque increases the speed will decrease--if the voltage stays the same. This connection is based on the slope of the speed/torque curve (called the speed/torque gradient), calculated using the formula below and shown in the sample curve shown on left. Torque = {power [kw] * 30,000}/ * rotational speed [rpm]
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Title Annotation:Applying Tech Selecting DC motors
Author:Horn, Carsten
Publication:Medical Design Technology
Date:Feb 1, 2018
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