When it comes to motion for handheld and miniature medical device designs, a standard electrical mini motor may seem like a good starting point. But while a miniature motor generates rotary movement, applications like medical pipettes or syringes, and mesotherapy devices often move loads linearly instead of rotationally. For these medical devices — as well as pick-and-place machines and fluid regulation valves for industrial applications — engineers must develop their own system to translate rotary motion into linear motion.

As you focus on creating the core product, it pays to delegate miniature motor selection and transmission system design to motion specialists who can save valuable development time. This article provides a detailed look at some of the linear motion options available.

Rotation can be converted into linear motion via a screw-and-nut system assembled on the motor shaft. There are two main types of screw-and-nut systems: ball screw and lead screw.

A ball screw, shown in Figure 1, operates on rolling contact between the nut and a screw. The ball is recirculated along a helical groove. The rolling components keep friction low, while allowing high efficiency greater than 90 percent along with a high load capability.

Fig. 2 - A lead screw system.

A lead screw, shown in Figure 2, is typically composed of a stainless-steel screw and a plastic nut. Both components are in direct contact, generating more friction than the ball screw system. This can be a good, economical option if cost is a concern. The nut material generally affects the life and maximum load capability of the system. However, with two preloaded nuts, axial play can be eliminated. There are typically two types of linear options to consider:

  • Option 1: The lead screw is directly integrated into the motor.

  • Option 2: The lead screw is mounted on the motor shaft.

Option 1: Motor with Integrated Lead Screw

Standard linear actuators, which are often called digital linear actuators (DLAs), are fully integrated linear mechanisms that use a can stack stepper motor. This is a generally cost-effective choice. Inherent with stepper technology, the motor itself is a positioning system, so the control does not need position feedback. The DLA can be driven in full steps, half steps, or micro steps, depending on the required resolution. In addition, the detent torque from the motor allows the DLA to hold its position when the power is removed.

For the linear transmission, a nut is overmolded onto the rotor assembly, as shown in Figure 3, with a special material that optimizes the friction. This creates an efficient system with a long lifetime.

Fig. 3 - Linear transmission system with an overmolded rotor.

Some products even have a special ball bearing assembly in which the ball bearings are preloaded with a wavy washer to reduce axial play, also shown in Figure 3. This improves linear positioning accuracy and motion repeatability. The lead screw extends and retracts during motion, and it can return to the same starting position.

When considering the right linear motion system, be sure to consider some of the options generally available as standard for an optimized transmission system. They include:

  • Stroke length.

  • Lead screw pitches, which typically involve two or three choices or references.

  • A bipolar or unipolar coil.

  • Coil rated voltage.

  • Rated current.

  • Captive lead screw — antirotation integrated — or noncaptive lead screws, shown in Figure 4.

  • Various thread tips — either metric or imperial.

Fig. 4 - Captive versus non-captive lead screws.

Linear actuators can be a very cost-effective option to deliver high linear force and reliability for a device.

Option 2: Custom Motor Assembly

For applications requiring high performance in a limited package, consider a custom assembly. Most custom assemblies are built with either a brush DC, brushless DC, or stepper disc magnet motor. Each technology has its own benefits and advantages over can stack steppers. For example, high-acceleration applications are well-suited for a low inertia motor such as a disc magnet stepper. For higher power in a small package, the best option may be a combination brushless DC, gearbox, and lead screw. And, for high efficiency, a coreless brush DC motor can be especially desirable for battery-powered applications. Some accessories can also be mounted on the motors, such as an encoder for high-resolution positioning feedback.

A custom assembly also offers flexibility when selecting the lead screw. The R&D team can choose whether a ball screw or regular lead screw is preferable, suggest different pitches, adapt the material, or even optimize the dimensions.

To design a motorized assembly, be sure to understand both the power required from the application and the power generated at the motor level. There are some physical relationships to keep in mind when converting the desired output force and linear speed into the required input torque, as well as the rotational speed. Here are some scenarios and solutions for optimizing an assembly to achieve the application’s output requirements:

Example 1: Digital Linear Actuator

Application: A team is developing a laboratory medical device that moves a tiny amount of liquid in test tubes. One motor controls a multi-pipette channel. The motor package is limited to a maximum diameter of 20 mm. The pipette must have good repeatability and accuracy to consistently deliver the same amount of liquid with each operation. The working process can be divided into two main steps:

  • Step 1: Fill the pipettes in one step in less than 4 seconds.

    • Traveled distance of the pipette: 50 mm in 4 seconds. Speed = 12.5 mm per second. Force for a viscous liquid: 20 N.

  • Step 2: Empty the pipettes. The pipette content is divided into tiny amounts for several test tubes.

    • Traveled distance: The pipette must be able to divide the volume into 30 substeps — specifically, 50 mm divided by 30 = 1.6 mm. Force: 15 N.

Solution: Digital linear actuators are typical well-suited for this type of device because:

Table 1. Selection of the 20DBM screw lead.
Table 2. Power requirements for the handheld med- ical device.
  • They are usually available as standard with no development required.

  • Stepper technology makes it easy to divide and deliver liquids in subvolumes.

  • Their preloaded ball bearing assembly ensures no axial play in the DLA, which allows good repeatability.

To select a motor, the following process is recommended. Note that the example provided uses a 20DBM motor supplied by Portescap (refer to the company’s specification sheet).

  • Dimension: Remove solution with diameter of greater than 20 mm.

  • Stroke length: The traveled distance is 50 mm, so the minimum stroke length is 50 mm. The captive version can be removed because the stroke length is smaller than 50 mm.

  • Power: Determine whether the motor can work at the required force. Calculate the frequency necessary to reach the targeted linear speed. The frequency depends on the lead screw pitch. Refer to Columns 3 and 5 in Table 1. Consult the pull-in force graph to select the screw pitch. Refer to Column 4 in Table 1.

  • Coil: Be sure to choose a coil that is adapted to the power supply. A coil with a low number of turns has a low resistance and is appropriate for a power supply type of high current, low voltage. A coil with a high number of turns has a high resistance, and it is appropriate for a power supply type for low current and high voltage.

Table 3. Calculation of the linear speed and torque, depending on the lead screw.

For this scenario, the motor 20DBM-L with the screw 10 would be a good option. Coil selection, as mentioned above, depends on the power supply.

Example 2: A Custom Motor Assembly

Application: Another engineering team is developing a medical device with the following requirements: The tool will be handled by a doctor, and it will be battery powered for better ergonomics. The engineer can only accommodate a maximum diameter of 13 mm, and the tool must be optimized for efficiency. Table 2 shows the typical power requirements.

Power calculation: Since the tool is battery powered, coreless brush DC motors are suitable for achieving high efficiency. The power requested by the motor can be estimated by assuming a gearbox efficiency of 75 percent and a lead screw efficiency of 50 percent, which results in 1.87 W of power.

By calculating the estimated power, the typical motor size can be identified. For this example, a small diameter can do the job.

Conversion of linear speed and force into relative speed and torque: As the motor produces a rotational motion, the linear speed must be converted into rotation speed and the force into torque. The conversion depends on the lead screw, which is specified by its lead.

Fig. 5 - Motor and lead screw.

Physical relations: If the screw, mounted on the shaft, rotates one round (2 π), the nut moves linearly on a distance equal to the lead as shown in Figure 5. Consequently, this presents the following relation, so linear speed can be converted into rotation speed:

Looking at the power relation, the force-torque relationship can be deduced:

The physical conversion formula can be applied in the example. As the torque and speed depend on the lead screw, we can make the calculation with two different leads in order to guide the lead selection. Note: The lead dimension impacts the lead screw efficiency since efficiency is related to the friction of the material and the screw angle. The previous relations are now used in the example (see Table 3). A smaller lead requires higher speed and lower torque than the larger lead. In general, the smaller lead also requires more power due to the lower efficiency.

Gearbox selection: The output torque and the input speed will help determine the gearbox. The Portescap catalog indicates that the R13 gearbox can be a potential option for working with both screws. This gearbox has a maximum output torque of 0.25 N•m — greater than the application requested torque previously calculated. The maximum input speed of this gearbox is 7,500 rpm.

Table 4. Gearbox ratio selection.

Ratio selection: Thanks to the maximum recommended input speed of the gearbox, users can define which maximum ratio to choose. To define the maximum ratio, divide the maximum input speed by the output working speed. Compare the result with the ratio available (see Table 4). Note: In general, choose the closest and smallest ratio available in order to have a working input speed lower than the recommended maximum input speed.

Table 5. Torque calculation at the motor level.

Motor selection: To choose the motor, calculate the application input torque of the gearbox. Since this is a continuous application, the motor must have a maximum continuous torque higher than the application gearbox input torque (see Table 5). For both screws, the 12G88 motor, which has a maximum continuous torque of 3.5 mNm, can be used.

Custom Motor Assembly Selection

Table 6. Power calculations for each custom motor design.

The electrical power, efficiency, and total dimension can now be calculated for each custom motor design (see Table 6). The 12G88 215E has the following specs:

Technically, both strategies can work, but certain application requirements can favor either option 1 or option 2. For example, if battery life carries more importance than the package, option 2 appears to be the better choice. Indeed, the total efficiency is 30 percent compared to option 1 at only 18 percent efficiency. Option 2 presents a size concern because its gearbox has one more stage than option 1, making it 3 g heavier and 3.1 mm longer.

Partner to Define Both Device and Motor Requirements

For linear applications, motor suppliers can support the development team by offering standard linear motors or by creating a linear, custom motorized design. In addition to defining the device’s technical requirements, both the motor supplier and application designer should also determine the requirements at the motor level. With a full understanding of the project’s needs, design teams can find the best compromise between the tool’s technical and commercial requirements.

This article was written by Clémence Muron, Application Engineer for Portescap, West Chester, PA. For more information, visit here .


Medical Design Briefs Magazine

This article first appeared in the July, 2021 issue of Medical Design Briefs Magazine.

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