Robots are mechanical devices equipped with software-based intelligence that can perform specific physical tasks. There are many types of robots and robotic applications. Robots are designed to suit their application and their mechanical design, body, electronics and software are designed accordingly. Robotic vehicles or robotic cars are one of many types of robotic projects. Robotic cars are designed to move on flat surfaces where they can perform certain tasks through remote control or autonomously. They are equipped with sensors, control circuits and actuators for their operation.
The very movement of such a robot requires the use of motors. There are many types of motors that can be used in robotic applications. Each type of engine is used for different purposes. Motors assist in the movement of the robot and also serve as actuators in the mechanical design of the robot. A robotic application may involve the following types of movement –
1) Vertical movement – moving a part of the robot up and down, usually through shoulder rotation
2) Radial movement – Moving a part of the robot in and out
3) Rotational motion – Clockwise or counterclockwise rotation around a vertical or horizontal axis or around a plane in a three-dimensional frame
4) Tilting movement – Up and down movement with simultaneous rotational movement
5) Rotational movement – Rotation of a part of the robot in relation to the rest of the robotic body on a parallel axis
6) Yawing movement – Rotational movement to the right or left of a part of the robot
7) Locomotion – movement of the robot on a surface or medium
All these types of movements are carried out with the help of various motors or pumps mounted along with transmission systems and end effectors. In this tutorial, the use of motors to provide primary motion to the robot itself or a part thereof will be discussed. The tutorial will look at different types of motors, their applications, selecting a motor, and designing a robotic car.
Types of engines
There are many types of engines available in the industry. For robotic applications, there are certain types of motors that are typically used. Motors typically used in robotic applications can be classified as follows –
• AC motor
• Brushed DC motor
• Brushless DC motor
• Geared DC motor
• Servo motor
• Stepper motor
AC Motor –
AC motors are driven by AC current. They are typically used in heavy-duty applications where high torque (high load capacity or carrying capacity) is required. This is why these engines are used in robotic assembly lines implemented in manufacturing units. Mobile robots are usually powered by DC sources (batteries or series of batteries), so AC motors are rarely used in such robots.
Brushed DC Motor –
Brushed DC motors use brushes to conduct current between the source and the armature. There are several variations of the brushed DC motor, but in robotics permanent magnet DC motors are used. These motors are known for their high torque/inertia ratio. Brushed DC motors have the ability to deliver torque three to four times their rated torque. Brush DC motors consist of six different components: shaft, commutator, armature, stator, magnets and brushes.
Brush DC motors have two terminals. When voltage is applied between the two terminals, a proportional speed is output to the brush DC motor shaft. A brushed DC motor consists of two parts: the stator, which includes the housing, permanent magnets, and brushes, and the rotor, which consists of the output shaft, windings, and commutator. Its stator remains stationary, while the rotor rotates relative to the stator. The stator generates a stationary magnetic field that surrounds the rotor.
The rotor, also called armature, is composed of one or more windings. When these windings are energized, they produce a magnetic field. The magnetic poles of this rotor field are attracted to the opposite poles generated by the stator, causing the rotor to rotate. As the motor rotates, the windings are constantly energized in a different sequence so that the magnetic poles generated by the rotor do not overtake the poles generated in the stator. This change of field in the rotor windings is called commutation.
Figure 1: Image explaining the construction of a brushed DC motor
Geared DC motors –
Geared DC motors are an advanced variation of brushed DC motors. They have a set of gears coupled to the motor. Engine speed is measured in revolutions per minute (RPM). The speed of the engine is reduced by increasing the torque with the help of the gear set. By using a correct combination of motor gears, the speed of the DC motor can be reduced with an increase in torque. This provides stability in engine rotation and the engine can be stopped or changed speed in a controlled manner.
Fig. 2: Close-up view of the brushed DC motor (right) and the geared DC motor (left)
To control geared DC motors, L293D motor driver IC is typically used in hobby robots. The IC interfaces with a microcontroller to control the direction and speed of the DC motor. The motor controller acts as an intermediate device between the motor, controller and batteries. Although the microcontroller decides the speed and direction of the motor, it cannot directly control the motor due to limited power. Even the motor controller can supply power to the motor but cannot instruct it in which direction it should rotate. Therefore, the motor controller and microcontroller need to work together to control the motor. To control the DC motor, an H-Bridge circuit is required which allows voltage to be applied to a load in any direction. The L293D is a double H-bridge motor driver integrated circuit (IC). Motor drivers act as current amplifiers in that they receive a low current control signal and supply a higher current signal. This higher current signal is used to drive the motors. It has 16 pins with the following pin configuration:
Figure 3: Table showing the PIN configuration on a DC motor
The IC can conduct up to 1 amp of current and operate between 4.5V and 36V. For more information about the motor driver IC, see reference L293D IC.
Fig. 4: PIN diagram of L293D IC used in DC motor of robots
Small motors are designed for applications where compaction is valued over torque. Although small high-torque motors exist, they tend to be expensive because they use rare earth magnets, high-efficiency bearings, and other features that increase their cost. Large motors can produce more torque but also require higher currents. High current motors require larger capacity batteries and larger control circuits that do not overheat and burn out under load. Therefore, to match the size of the motor to the rest of the robot, it is advisable not to overload a small robot with a large motor. When deciding engine size, the torque available after gear reduction must be considered. Gear reduction always increases torque. The increase in torque is proportional to the amount of gear reduction. As if the reduction were 3:1, the torque increases by about three times.Servo motor –
Servomotors are typically used where precise rotational motion is required. They are often used in robotic arms and angle control applications. Learn more about servo motors and their control in the Servo Motor tutorial.Stepper motor –
A stepper motor divides rotation into several steps. Just as a servo motor rotates at a specific angle, a stepper motor rotates at a specific number of angular steps. Learn more about stepper motors in the Stepper Motor Tutorial.Brushless DC Motor –
Brushless DC motors are similar to brushed DC motors in construction, but are driven by closed-loop controllers and require inverters or SMPS for power supply. These motors have permanent magnets that rotate a fixed armature. In contrast to Brush DC motors, they have a closed-loop electronic controller in place of the commutator assembly. These motors are typically used in industrial robotics where precise control over movement and positioning is required. However, these engines are quite expensive and involve complex construction and electronics.Selecting a motor for a robot
To select a suitable electric motor you need to consider many different parameters, such as the load that a specific motor can support, the torque required to move the robot without being overloaded, the revolutions per minute of the motor when it is loaded, etc. Since there are many types of motors, depending on the application, one type of motor must be selected. For example, to operate the robotic arm, servos are typically used. Wheeled robots have a simple design and navigate the ground using motorized wheels. Wheels are also easier to design and build compared to tracks or legs. The use of wheels has disadvantages as navigation over obstacles or areas with low friction is not easy with wheels. The most common electric motors used in such robots are DC motors. DC motors provide high torque and high efficiency. By applying torque in response to load, DC motors can be characterized by the speed and torque curve. Commonly preferred voltage ratings for DC motors used in hobby robots are 3, 6, 12, and 24 Volts. If a motor is applied with a voltage lower than the voltage indicated in the technical data sheet, the torque will not overcome the internal friction – mainly of the brushes. Furthermore, if a voltage greater than that supported is applied to the motor, it may heat up and be damaged.Wheel combinations for a robotic car
According to the laws of physics, a heavy weight requires strong forces to accelerate. This means that a heavier robot requires stronger motors to accelerate the body. In addition to the forces that act due to the weight of the robot, there are the friction forces of the wheels and the internal friction inside the engine. When taking into account that a robot must climb stairs or run on inclined surfaces, other forces must also be considered, such as gravitational force. A motor can maintain a constant speed only if the torque is greater than the combined forces opposing the robot's motion. If the motor torque is less than the opposing torque, the motor will stop and may be damaged, as electrical energy cannot be converted into torque. For movement, the robots use differential steering that drives the wheels separately. The robot can change the direction of rotation of each wheel at different speeds, and by adding additional wheels that are not driven by actuators, the robot can maintain balance. Two wheels plus a caster or four wheels are the most common combinations for wheeled robots. Both wheel combinations can turn in place and are known as differential drive for the two-wheel version, while all four wheels must be driven independently to turn in place. • Two-wheeled and caster robot –
Fig. 5: Image showing two wheels and a caster robot
• Four-wheeled robot –
Fig. 5: A typical four-wheeled robot
The two-wheel and caster method has its advantages, including the ability to measure movement by adding encoders. For the four-wheel method, adding an encoder may generate inaccurate measurements compared to the actual movements of the robot, but at the same time, this system is best for closed-loop control and provides high tire grip.