Choose a Motor Based on Technical Requirements

We have discussed in the previous article which types of robots are generally used in Robotics, but how do we actually choose a motor based on technical requirements that change from project to project? Key factors such as speed, torque, power, size, and control method come into play when selecting the appropriate motor. This article will guide you through understanding these considerations, finishing with an example, and help you choose the right motor for your robotics project.

Understanding Motor Specifications
Making an Informed Decision
Example
Conclusion

Understanding Motor Specifications

Speed

Motor speed, measured in revolutions per minute (RPM), defines how fast the motor can spin under no load. The required speed depends on the application. For example, if the motor is driving the wheels of a robot, the desired travel speed will influence the required motor speed. Note that when under load, the motor’s speed will decrease, which should be considered in your selection.

Torque

Torque, measured in Newton-meters (N.m), is the force that the motor can output. It’s a crucial factor when a motor is used for applications like lifting or moving objects. The torque requirement depends on the load the motor will need to move and the mechanism used (for example, a lever arm will increase the torque required). Motors usually have a rated torque (continuous torque the motor can deliver without overheating) and a peak or stall torque (maximum torque motor can provide momentarily).

Power

Power, measured in watts (W), defines the motor’s ability to perform work over time. It’s essentially the product of speed and torque. A higher power rating indicates that the motor can handle a higher workload but usually also means more energy consumption. Be aware that the power supply must match the voltage and current specifications of the motor.

Size and Weight

The size and weight of the motor can also be a crucial factor, especially in mobile robotics where a larger, heavier motor may affect the robot’s mobility and energy efficiency. Smaller motors may have lower speed and torque, so there’s often a trade-off to consider here.

Making an Informed Decision

Determine Requirements

Begin by understanding the requirements of your robotic application. Identify the load that the motor will need to handle, the speed at which it needs to operate, and the power available. Consider whether the motor needs to provide precise positioning (which would favor a stepper or servo motor), or whether speed and torque are more important (which might favor a DC motor).

Consider the Operating Conditions

Consider the environment where the robot will operate. If the robot will be used outdoors or in harsh conditions, the motor should have suitable durability and environmental resistance. Also, consider whether noise is an issue, as some motors (like stepper motors) can be noisy.

Balance Speed and Torque

A crucial step in motor selection is balancing speed and torque. A high-speed motor might have lower torque, while a high-torque motor might operate at lower speeds. Gearboxes can be used to alter the speed-torque balance: a gear-down reduces speed but increases torque, while a gear-up does the opposite.

Account for Control Complexity

Different types of motors require different control methods. For instance, DC motors can be straightforward to control, but managing speed and direction may require additional components like an H-bridge. Servo motors come with built-in control circuits but require precise pulse-width modulation signals. Stepper motors can provide precise control over position but might require more complex driver circuits.

Cost and Availability

Finally, consider the cost and availability of the motor. More specialized or high-performance motors (like brushless DC motors or high-quality servo motors) may be more expensive than more straightforward alternatives. Ensure the motor is readily available for purchase and that replacement parts (like brushes for brushed DC motors) can be easily sourced if necessary.

Example

Let’s create a practical example to illustrate how to select a motor based on a robot’s specific requirements.

Assume a robot is designed to lift a weight of 10 kg to a height of 1 meter in 2 seconds. To determine the specifications of the motor we need, we’ll need to calculate the required speed, torque, and power.

1. Speed

The speed at which the motor needs to operate can be determined by the time it takes to lift the weight to the desired height. In our case, the robot must lift the weight 1 meter in 2 seconds. Therefore, the speed would be 1 meter/2 seconds = 0.5 m/s.

2. Torque

To calculate the torque needed, we first need to calculate the force required to lift the weight, which is simply the mass of the weight (m) times the acceleration due to gravity (g). Assuming standard gravity (g = 9.81 m/s^2), the force (F) would be F = m*g = 10 kg * 9.81 m/s^2 = 98.1 N.

If the lifting mechanism is directly connected to the motor shaft (for simplicity), the torque (τ) needed is the force times the radius of the motor shaft (r). For example, if the radius of the motor shaft is 0.01 m (1 cm), the required torque would be τ = F*r = 98.1 N * 0.01 m = 0.981 N.m.

3. Power

The power (P) needed can be calculated as the work done (W) divided by the time taken (t). The work done in lifting the weight is its weight times the height lifted, W = F*h = 98.1 N * 1 m = 98.1 J (Joules). Therefore, the power required would be P = W/t = 98.1 J / 2 s = 49.05 W.

However, in practical situations, we need to take into account the efficiency (η) of the motor and the gearbox (if any), which is always less than 1. If we assume a conservative overall efficiency of 50% (η = 0.5), the power of the motor should be P/η = 49.05 W / 0.5 = 98.1 W.

Hence, in this example, we need a motor capable of providing a speed sufficient to move the robot at 0.5 m/s, a torque of at least 0.981 N.m, and power around 98.1 W to adequately lift the weight of 10 kg.

Remember that in real scenarios, it’s advisable to choose a motor that exceeds these calculated requirements to allow for unknowns or changes in the task, and to ensure reliable performance.

Conclusion

In conclusion, choosing the right motor for your robot involves a careful consideration of your application’s needs and constraints. By understanding and balancing the factors of speed, torque, power, size, control complexity, and cost, you can select a motor that optimizes your robot’s performance and efficiency.