5 Things to Know Before Buying Robot Joint Actuator

There is a seemingly endless selection of DC, stepper, and servo motor products on the market, each with their own advantages and drawbacks. Going into the selection process having answered a few key questions will vastly simplify the selection process.

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How to Select the Best Motor for a Jointed Arm Robot

Article from | Rozum Robotics

With their many parts and the need to be able to smoothly rotate all of their axes, jointed arm robots require the perfect actuator to power their specialized movement with the right type and amount of force. Robots with jointed arms are often tasked not only with mundane tasks, but also with performing human-like actions in dangerous or high-stakes environments, so the motor must be perfectly matched to these requirements. There is a seemingly endless selection of DC, stepper, and servo motor products on the market, each with their own advantages and drawbacks. Going into the selection process having answered a few key questions will vastly simplify the selection process.

There are several factors to consider when selecting a motor to power a robot with a robotic joint

1. What type of robotic joints are used? There are five types of robotic joint: linear, orthogonal, rotational, twisting, and revolving. Does your application use the simpler linear and orthogonal joints, the more dynamic rotational, twisting, or revolving joints, or a mixture of both? This will determine the types of motions and the related nuances of their requirements.

2. How much noise is tolerable in the application? If your application will be used in a factory largely away from people, noise may not be an issue. But if it will be used alongside humans for more than a brief amount of time, you may favor a quieter motor.

3. How much precision is required? When a robot is being used to move shelves in a warehouse, not much precision is required, whereas there is no room for error when one is filling prescriptions. Different motors provide precision in different ways, some with distinct disadvantages; it’s important to know which of these may be allowable for your product.

4. How much torque is necessary? Torque can be achieved at various speeds and with varying degrees of constancy. If you need high torque only at a particular speed, you may be able to sacrifice unnecessary torque capability for other motor features.

Now let’s review the three types of electric motors most often used to run applications on a typical jointed arm robot—DC, stepper, and servo—against these considerations.

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DC motors come in brushed and brushless varieties. It is commonly thought that brushless DC motors have supplanted brushed ones, but brushed DC motors are still quite popular for some applications. A brushed DC motor is about 75%–80% efficient, achieves high torque at low speeds, and is simple to control, but creates quite a bit of noise due to the brushes used to rotate the machinery. On the other hand, a brushless DC motor is quieter, even more efficient, and can maintain continuous maximum torque, but is more difficult to control and can sometimes require a specialized regulator. Although DC motors usually provide low torque, they can achieve high speeds and are good for washing machines, fans, drills, and other machines that require constant circular motion.

There is always the option of adding a gearbox to the system to create more torque for robotic applications utilizing a robotic joint mechanism. Keep in mind, the motor and gearbox should be designed to work together, so purchasing a motor with an integrated gearhead is a good idea in this case.

Stepper motors can control precise movement, have maximum torque at low speeds, and are easy to control, making them popular in process automation and some other robotics. However, they come with several drawbacks: They are noisy and relatively inefficient, and they run hot since they continuously draw maximum current. Finally, since they have low top speeds, they are known to skip steps at high loads, which can be a critical flaw in some jointed arm applications. Despite these limitations, they have proven effective in medical imaging machines, 3D printers, and security cameras.

Servo motors provide extremely precise movement, thanks to a feedback loop that senses and corrects discrepancies between actual and target speed. They can provide high torque at high speeds, and can even handle dynamic load changes. Additionally, servo motors are lightweight and efficient. Downsides of using servo motors include their possibility for jitter as they respond to feedback and their requirement for sophisticated control logic. Despite these drawbacks, the precision offered by servo motors often make them a good option for a jointed arm robot, the sophisticated movement of which is designed to match that of humans!

Your jointed arm robot may perform sensitive tasks and come with high expectations, so you need a motor that not only powers your system but makes your robot maximally appropriate for the environment in which it operates. When selecting a motor, making sure you know exactly what you’re trying to achieve and ranking your priorities will help you make smart functionality tradeoffs for optimal performance and suitability.

The content & opinions in this article are the author’s and do not necessarily represent the views of RoboticsTomorrow

Robotic arms can be used for all manner of industrial production, processing and manufacturing roles - any task in which extremely precise, fast and repeatable movements are required, in fact.

Robotic arms of all kinds are used today at every scale of manufacturing, from minutely detailed circuit board assembly to large-volume heavy industries such as automotive production lines, as well as in a huge range of ‘pick and place’ (conveyor belt) applications. This means that it’s important to know which types of programmable robotic arms are better suited to which sorts of environments and tasks before you begin planning a purchase.

In every case, selecting the right type of programmable robot arm for a given role or task should involve consideration of the intended application’s precise nature and requirements. These will typically include:

• Load
  • All types of robotic arms have a given load capacity, and this manufacturer-specified number always needs to exceed the total weight of the payload involved in any job you expect the arm to perform (including tools and attachments).
  • Different sorts of robot arms are supported by differently designed frameworks, which can increase or decrease overall load capacity - this must be balanced with consideration of physical placement and footprint.
• Orientation
  • This criterion is generally defined by the footprint and mounting position of the robotic arm, and how well it fits alongside the other equipment in your production line for the range of movements and manipulations it’s expected to perform. This will in turn influence where the arm can physically be positioned relative to the objects it will be moving.
  • Certain types of robotic arms require bulkier pedestals or more physical clearance space to perform their programmed range of movements, and these factors must be considered in terms of other equipment or workers in the vicinity.
• Speed
  • Particularly when choosing robotic arms for picking and placement applications, it’s important to pay attention to manufacturer ratings for speed, and especially in terms of acceleration over longer distances.
  • Changes and upgrades to speed ratings can be achieved in some types of robotic arm through changes made to the choice of belts, motors or actuators used.
• Travel
  • Tolerances and accuracy over wider spans can be reduced in certain types of robot arms, due to arm deflection and differences in support framework design.
  • If the application requires longer travel distances between payloads or work areas, this may dictate which sorts of robotic arms would be suitable or unsuitable for performing the task, depending on the tightness of tolerances required.
• Precision
  • Certain types of programmable robotic arms are inherently designed to be more precise in their range of movements and articulations than others. This may come at higher cost for a more complex machine, and involve a compromise against other factors such as footprint, speed, potential travel distance and orientation.
  • For many industrial applications such as picking and placement, robotic arms capable of extremely precise repeatable movement may be an unnecessary expense. However, for tooling applications, precision will be a key consideration before most other factors. Again, changes and upgrades can be made to improve precision for certain types of robotic arm, but not all.
• Environment
  • Consideration of atmospheric conditions and potential hazards (including dust, dirt and moisture levels) in the immediate working environment will be important when choosing an appropriate type of robotic arm for a specific location.
  • Physical footprint, orientation and range of movement will also influence how suitable a particular model or arm type is for use in a particular environment, with other equipment and workers taken into account.
• Duty cycle
  • This is essentially an evaluation of how intensively the robotic arm will be expected to perform, and for how long between ‘rest’ or maintenance periods. Wear and tear will obviously become a problem sooner for a robot arm that is run continuously, as opposed to one which is only operated during standard shift cycles.
  • Different models or arm types will require different maintenance regimes, such as lubrication intervals and parts replacement - in any environment where minimal downtime is critical, these will be important considerations to bear in mind when buying robotic arms for specific production roles.

Collectively, the criteria above are sometimes referred to as a robot’s LOSTPED parameters.

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