How to Choose robot joint actuator?

Introduction

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Choosing between actuators can be a difficult process. Without applying physics and math, you may be left wondering how to make the choice between an HD Hex Motor (REV-41-) and a Core Hex Motor (REV-41-)? This section will take you through the process and mathematics to help you make the best decision for your robot!

Before getting too far here are some general guidelines for where REV DUO actuators are best utilized.

Actuator

Uses

Drivetrains, Elevators, and Arms

Lighter duty arms and intakes

Lighter duty intakes, switches, and latches

Though these basic suggestions are here to guide you in the right direction, all robots are different. Follow through the rest of this guide to find the best fit for your mechanisms.

Core Concepts

When designing a robot, selecting the correct actuator for the application is a critical design challenge. Some tools can be used to estimate the performance of a motor in a particular application.

Understanding these basic concepts is required to make optimized design decisions which consider the trade-offs inherent to any design. This section will briefly cover the definition of these concepts and then explain them in relationship to basic powertrain concepts.

Speed is the measure of how fast an object is moving. The speed of an object is how far it will travel in a given amount of time. For rotating parts like gears and wheels, speed is expressed in how many revolutions are made in a given amount of time. Under ideal conditions, the rotation of a wheel is converted into linear speed and can be calculated by multiplying the diameter of the wheel by the rotations for a given time. The SI unit for speed is meters per second, but speed is also commonly expressed in feet per second.

Angular Velocity is how the speed of a rotating object is described. The SI unit for the Angular Velocity is radians per second (rad/s), revolutions per minute is also commonly used.

Torque is roughly the measure of the turning force on an object like a gear or a wheel. Mathematically, torque is defined as the rate of change of the angular momentum of an object. This can also be stated as a force that acts normal (at 90 degrees) to a radial lever arm which causes the object to rotate. A common example of torque is the use of a wrench in order to tighten or loosen a bolt. In that example, using a longer wrench can produce more torque on the bolt than using a shorter wrench. Torque is commonly expressed in N'm or in'lbs.

When torque is turning an object like a spur gear, the gear will create a straight line (linear) force at the point where the teeth contact the other gear. The magnitude of the torque created is the product of the rotational force applied and the length of the lever arm which in the case of a gear, is half of the pitch diameter (the radius).

Work is concept used to describe changes in energy. Path independent work is defined as force times displacement; for example if a 1 kilogram(kg) weight is lifted vertically 1 meter(m) against gravity at a constant velocity the work done is:

Power (P) is the rate of work over time. The concept of power includes both a physical change and a time period in which the change occurs. This is different from the concept of work which only measures a physical change. The difference in these two concepts is that it takes the same amount of work to carry a brick up a mountain whether you walk or run but running takes more power because the work is done in a shorter amount of time. The SI unit for power is the Watt (W) which is equivalent to one joule per second (J/s). Rotational power is calculated by multiplying torque and angular velocity.

In competitive robotics, the total amount of available power is determined by the motors and batteries allowed. The maximum speed at which an arm can lift a certain load is dictated by the maximum system power.

Choosing an Actuator Walkthrough

This section will utilize key metrics and terms from the Motor and Servo pages. Please visit those pages before going through this walk-through.

The first round of analysis to narrow down your actuator is to compare the the maximum output power of the actuator to the power required to run the mechanism.

Lets say you are building an elevator that lifts a game piece. Before you can begin your calculations you have to make some basic assumptions about your design. In this case, assume zero frictional losses and instantaneous acceleration. Consider also, the following information:

• Game piece weight: 0.5 kg

• Lifter weight: 1.5 kg

• Lifter maximum height: 1.5 meters

• Time to full extension: 5 seconds

For extra practice try running the same calculations but with t = 10 seconds instead of t = 5 seconds.

Step 1: Calculate Work

Work is a concept used to describe changes in energy. Path Independent work is defined as force times displacement. Since Force is mass times acceleration the following formula for work can be derived:

As a standard when calculating work, calculate Net Work, or the total amount of forces displacing your mechanism. In the example, the only force to account for is the acceleration against gravity at a constant velocity, as it has been assumed there are no frictional forces at play. Therefore in the example Work is:

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Where the 2(kg) = the combined mass of the elevator and the game element.

The example given is a simplified version assuming that no other forces are at play. For optimal results take time to consider what forces may affect the work done by your mechanism.

Step 2: Calculating Power

In the example, work was calculated to be 29.4. It was also stated that the time period is 5 seconds. With those numbers known power can be calculated as:

What is power if the elevator reached full height in 10 seconds rather than 5?

When you are building a mechanism it is advisable to apply a safety factor (sf) to the power of the mechanism to ensure that your mechanism is able to withstand things like unexpected loads or degradation over time. For this example, apply a safety factor of 2.

Remember these calculations are made with an assumption of no friction. The safety factor helps to account for some of the assumptions made with this example.

After applying the safety factor of 2 and rounding, it is found that 12 watts of power are needed to power the mechanism. Now that the power estimate is known it is time to decide which actuator meets the criteria given.

What do you get for the t = 10 seconds calculation if sf = 2?

Step 3: Choosing an Actuator

Take the known power estimate of 12 watts and compare it against the Maximum Output Power for the various actuators in the key metric table. Choose a motor that has a maximum output power greater than that of the estimated power for the mechanism. With those parameters given, the likely choices for the elevator are any of the HD Hex Motor variants.

What actuator would you choose for the same mechanism if the amount of power you need is 6W?

Step 4: Estimating Performance

A motor performance graph can be used both for selecting a motor and for understanding the motors behavior once installed in the robot. The below prototypical performance graph can be used to estimate the performance of a motor.

The prototypical performance graph exhibits standard curves to help calculate where your motor falls on the performance scale.

Using the Performance Curve to Select an Actuator

When selecting a motor the expected power requirement for the motor is used. In the example estimate power was found to be 12 watts. The graphic below for finding power/torque range has a power estimate similar to that of the example (12 watts is 80% of max power). This will still be a good example of how to use the motor performance graph to estimate your metrics.

Using the Performance Curve to Understand Behavior

If you have already selected an actuator and installed it you may want to check to see what your robots performance is. This process is similar to the estimation to select a motor.

Previously the power estimate was used to find the other metrics, now amperage will be used as it can be reported by the Control Hub (REV-31-) and Expansion Hub (REV-31-). When amperage is known, draw a line horizontally at the known value using the current scale, then at the point your horizontal line intersects the current curve, draw a vertical line. The points at which the vertical line intersects the key metric curves will give you the estimated performance for each metric.

When designing with minimal constraints it is best maximize power and efficiency. The point of maximum efficiency usually occurs around the 25% of maximum torque point.

Step 5: Powertrain Design

Once a motor has been selected, a power train can be designed. The goal of the power train is getting the final torque and angular velocity to the necessary values within the possible range that can be produced by the motor. When designing the power train, the fewer elements present in the power train the more efficient the power train will be. For example, using 30:72 gear box and a 20:20 chain drive will be less efficient than directly using 20:54 chain drive.

As a rule, gears are more efficient than chain drive.

Motor performance curves are useful at this stage of design as well because given an estimated power requirement you can estimate an angular velocity and torque range that the motor will be outputting. When designing the power train, the values used should be at the lower end of the viable rpm and torque range as the motor can be given more power to bring it into performance should the estimates be off.

Robotics is an extremely intricate field, especially when it comes to building a robot to perform specific tasks. Robots are built to sense the environment and perform the task by manipulating the obstacles. Sensing an environment is more of a sensor's task but acting on a specific task is performed by actuators. A robot is fitted with different actuators, which perform different activities on receiving a message from a sensor. Being a field that demands precision, it becomes important to select appropriate actuators for your robot. For this selection, there are two stages- first, you must know the available types of actuators and second, you must know consideration factors. This post focuses exactly on these two stages. The types of actuators are discussed followed by the selection guidelines for actuators.

Types of Actuators Suitable for Robotics

Robotics demand precision because they are designed to simulate human movements. Robots generally perform a linear or rotary movement of their active parts. Therefore, the actuators are selected according to the type of motion required. Let us discuss types of actuators for robots according to motion requirements.

• Actuators for Linear Motion Activity: The actuators that generate linear motion are listed as follows.
  • Linear Actuators: Linear actuators trigger push or pull activity of the robot. Generally, actions like forward or reverse motion of the robot, successive distance travel, and arm extension are the activities triggered using linear actuators. The active end of this actuator is connected to the lever arm of a robot to trigger such motion.
  • Solenoid Actuators: Solenoids are special-purpose linear actuators. These actuators feature a solenoid latch that acts on electromagnetic activity. Solenoid actuators are used for controlled motion. Robot activities like a push button, latch, start and reverse, etc are performed using these solenoid actuators.
• Actuators for Rotational Motion Activity: The actuators that generate rotational motion are listed as follows.
• DC Motor Actuators: These actuators are mostly utilized for rotary robotic motion. DC motor actuators come in varying sizes and torque generation capacity. Therefore, it can be used for varying speed during rotary motions. The activities like robotic drilling, robotic drivetrain motion, are performed by using DC motor actuators.
• Servo Actuators: Servo motor actuators are used for controlled and monitored rotary motion. These are advanced types of DC motors that allow 360° rotation, however, the continuous rotation is not mandatory. It allows halts during a rotational motion. The activities like pick and place are performed by using these actuators.
• Stepper Motor Actuators: Stepper motor actuators contribute to repetitive rotational activities in robots. These actuators are an amalgamation of DC motors and servo motor actuators. For automation robots where repeatability of an activity is required, these stepper motor actuators are used.

After the whole discussion, it is needless to say that type of activity and type of actuators must go hand in hand while selecting actuators for robotics. This certainly becomes of the criteria. However, there are few more factors you need to consider to select actuators for robotics. Let's discuss the same.

Guidelines to Select Actuators for Robotics

Robots are built for various purposes. Therefore, while selecting actuators for robotics, the following factors need to be considered.

1. Purpose of the Robot: The purpose of the robot is the first important consideration while selecting an actuator. For appropriate selection, you must know what activities are to be performed by a robot. For example: If a robot is expected to perform forward and backward motion along with pick and place operation. In this case, a linear actuator for straight-line motion and servo motors for arm articulation can be used.
2. Aesthetic Requirements and Constraints: Appearance of the robot is a big consideration in robotic design. If the robot is designed for delicate activities, it must have lightweight actuators. If the robot has speed constraints, then stepper motor with torque limitation is important to use.
3. Mounting Space: Mounting space and control linkage are to be considered while selecting actuators. If there is mounting space limitation or requirement of an external controlling lever, then the actuator must meet these conditions.
4. Communication/Command Protocol: Robots are operated using virtual control technologies. Mostly, pulse width modulation (PWM) or serial communication protocols are used in the robotic controls. Therefore, the command/communication protocol and type of actuator must go hand in hand.
5. Control Interface:  There are many control or monitoring interfaces available for robotics. These interfaces function based on PWM or serial communication protocols. These are often digital controls that allow virtual command and control operations in robotics. The actuator must be compatible with these control interface software.

Now that all the selection guidelines are stated, your selection of actuators for robotics applications should be quick and easy. Along with these, ensure that you select actuators for robotics from trusted brands that assure quality, customer support, and unlimited technical support.

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