Guide to selecting motors and actuators for Arduino

When we are about to start a robotics project, a vehicle, or any other mobile device, one of the first decisions will be choosing the types of actuators that we are going to use.

An actuator is the generic name we use for any device capable of performing actions in the physical world and that we can control from an automaton or processor like Arduino. In particular, we use the name actuator for devices that are capable of generating movement.

Selecting the correct actuator for our project is a complicated task as we have a wide range of options (DC motors, servos, stepper motors, brushless motors, etc.), each of which has its advantages and disadvantages.

::: Explaining all the actuators in a single entry would be too long, so in this entry we will look at the factors and criteria that condition the choice of one or another

In the next entry, we will see the characteristics and functioning of the main types of rotary motors, and in the last entry of the series, other types of actuators available. :::

Factors for Choosing a Motor or Actuator

In general, when selecting an actuator, we should consider the mechanical and electrical characteristics, the control we will have, and of course, the price.

However, it is important to be clear that the characteristics of one motor are not better or worse than those of another. For example, having a small speed or torque may seem bad at first, but it does not have to be necessarily so.

Imagine that we are making a project to spin a billboard. Surely spinning it at 12000 rpm will not achieve the effect we are looking for. Or imagine that we want a motor with low torque so that if someone accidentally puts a finger in, the motor stops without causing damage.

The same goes for robots and vehicles. It doesn’t matter if the wheels can spin at high speed if they stop as soon as they touch the ground. Or if the robot has enough torque to climb a wall, but in return moves at turtle speed.

Once this is refreshed, let’s finally break down each of the factors.

Mechanical Characteristics

The mechanical characteristics include the speed, the force or torque it can exert, the precision, and the maximum load it can support.

Speed

Speed is the relationship between the displacement performed by the actuator and the time, that is,

In the case of rotary actuators, we will use angular speed, which is the relationship between the angle turned and time

Force / Torque

The force exerted by an actuator applied to a certain mass is used to accelerate it, that is, to modify its speed. An actuator with a certain force will be able to quickly accelerate small masses and slowly accelerate large masses.

The expression for force is,

Where,

However, in the real world, there are friction forces, so for practical purposes, an actuator will not be able to move any mass. Above the available force, the load simply will not move at all and we can even damage the actuator.

In the case of rotary actuators, the equivalent of force is torque (often referred to as torque in some places).

The expression for torque is as follows,

Where angular acceleration is the change per unit of time of angular speed, and rotational inertia is a characteristic parameter of the load we are going to rotate that is given by the geometric distribution of its mass.

Alternatively, torque can be rewritten as follows,

For example, a motor with a maximum torque of 10 Kgf·cm to which we attach a pulley with a radius of 1 cm will be able to lift a mass of 10 Kg vertically, while if the radius of the pulley is 2 cm, it will only be able to lift 5 Kg.

Mechanical Power

Mechanical power is the amount of energy per unit of time that the actuator is capable of delivering to the load.

Mechanical power is given by the following expression,

While in the case of rotary actuators,

Maximum Load

The maximum load is the weight or stress that the actuator can withstand without breaking, which is not the same as the maximum load it can move.

Consider a vehicle with wheels; the axle may support, for example, 50 Kg, but it does not mean that the motor has to exert a force of 50 Kg, as the rolling load is much lower.

Precision

Not all actuators have the same precision in movements. Regardless of the quality and control we use, certain actuators are more prone to higher levels of precision.

For example, with a stepper motor, it is easy to obtain precisions of tenths of degrees, something that will be much harder to achieve with a DC motor.

Electrical Characteristics

The electrical characteristics include power, voltage, and nominal current. This will condition the size of the components, the cross-section of the conductors, and the capacity of the batteries.

Nominal Voltage

It is the voltage at which we must power the motor for correct operation, measured in Volts. It is often also the maximum voltage at which we can power the device without damaging it.

Sometimes the nominal voltage is a range, rather than a single value. The operating point will vary depending on the voltage we apply.

Common values for motors and actuators are 6V, 12V, and 24V.

Nominal Current

It is the current intensity that we must supply to the motor for correct operation, measured in Amperes. Similarly, the nominal current often coincides with the maximum current at which we can power the device without damaging it.

Some actuators have a nominal intensity lower than what would circulate according to Ohm’s law based on their nominal voltage and resistance. For this reason, the controller must have a current limiting device.

Electrical Power

Electrical power is the energy per unit of time absorbed by the actuator from the power source, measured in Watts (W), has the following expression,

At the same time, mechanical power is related to electrical power as follows,

Where μ is the overall efficiency of the actuator, which encompasses mechanical losses due to friction, and electrical losses due to losses in copper from the Joule effect, losses from induced currents, losses from magnetic hysteresis, and losses from magnetic flux dispersion.

On the other hand, the electrical load consumed by the motor is

The unit for energy is Joules (J), with common units being Ampere-hours (Ah) and milliampere-hours (mAh).

Other Factors

There are other factors to consider when selecting an actuator for our project. We will not go into details because they are quite obvious, but it is worth at least mentioning them briefly.

• Shape and dimensions.
• Mountings and supports both for the actuator to the robot, and for the load to the actuator (for example, diameter of a rotor shaft).
• Weight of both the actuator itself and the additional components required to operate it (batteries, controllers).
• Degree of protection (IP) which includes resistance to water and dust, and conditions whether the actuator can be used in outdoor environments.
• Temperature range that we must respect if we do not want to damage the actuator.
• Lifespan understood as the average time we can expect the actuator to function.

Control Characteristics

In addition to mechanical and electrical criteria, a factor that greatly conditions the choice of the actuator (and is often the most forgotten) is the control we will have. In general, in a robot or vehicle, we are interested in three types of control:

• Speed control, knowing at what speed the vehicle is moving.
• Position control, knowing where our vehicle is.
• Orientation control, knowing the direction the vehicle is pointing.

In principle, forget about having total control over any of these variables. The real world is not perfect; wheels slip, motors have nonlinear responses, loads are unbalanced, components are not identical and perfect, and sensors have drift… All these defects mean that, in general, we cannot have total and absolute precision.

Speed control is generally the easiest to measure or at least estimate. In many actuators, we act directly on their speed. On the other hand, if we know the position of the actuator, we can obtain its speed simply by deriving (dividing) with respect to time, and calculate its average speed.

The position of the vehicle is the hardest to know. Some actuators allow for good position control, but this does not guarantee knowing the real position of the robot. In general, we will have to install additional sensors, such as photointerrupters or limit switches, that allow us to position the actuator absolutely.

We have other types of sensors that help us determine the position of a robot, such as ultrasonic or infrared distance sensors. GPS allows us to obtain the real position, but it has an accuracy of 0.5 meters, which is too high for most vehicles. Other options include vision systems or triangulation of radiofrequency beacons.

On the other hand, forget about obtaining the position of the vehicle by integrating the speed (multiplying speed x time). Errors in measuring speed accumulate, and in the end, you will always have a drift in position. Use it only as a last resort or as interpolation between positions given by sensors.

Finally, the robot’s orientation is almost impossible to know through the control of the actuators, for the same reasons as the position. Fortunately, in most cases, we will be able to use magnetic compasses and gyroscopes to determine the orientation of the vehicle with a high degree of precision.

Adding an Encoder

The most widespread solution to improve the control of our actuators is to add an encoder. An encoder is a device that allows recording the position of the actuator, which means having total control over position and speed.

The most common encoders are optical or magnetic. In optical encoders, a grooved or transparent disc with opaque areas is coupled to the shaft. A photointerrupter detects the interruption of a light beam as the disc passes. In magnetic sensors, one or more magnets are coupled to the actuator, and a Hall sensor is used to detect the passage of the magnet by the sensor.

In both types of sensors, in the intervals where we do not detect, we are “blind,” so the finer the step (finer grooves or more magnets), the better the precision we will have.

However, although greatly improving the control capacity, they do not make us immune to, for example, wheel slippage. Remember, knowing how much a motor has turned does not mean knowing the position of the vehicle.

Conclusion

All these factors are related and the design must be approached globally. For example, increasing the size of a robot means greater weight, which requires larger motors and components, which require higher consumption, which means larger batteries, which in turn implies an increase in weight.

That is, as we increase the power of a robot or vehicle, all components must grow accordingly, and the price skyrockets quickly.

Selecting the motor or actuator that best fits the particular conditions of our design and, of course, keeping the price as low as possible is not an easy task and there is no single possible solution.