Motion Control & Controlling of Elements

Motion Control with Mechatronics

Motion Control is the integration of mechanical mechanisms with electronics or electrical components, along with a control system. The mechanism being controlled is purely mechanical, while the control system typically consists of electronic or electrical elements. In simpler terms, motion control in a mechatronics system involves managing the supply voltage provided to an electrical or electronic device to control a particular mechanism. These mechanisms can be operated or driven by hydraulic or pneumatic systems, as well as servo drives and motors. In some cases, a controller is used, and occasionally a PLC (Programmable Logic Controller) is employed for control. This article will discuss various movements and support functions associated with mechatronics systems.

Different Types of Movements

Comprehensive control over various movements is required in any automated system. Mechatronics systems primarily involve three types of movements: controlled movement, preset movement, and continuous rotary movement.

Controlled Movement Controlled movement involves precise position control, such as the movement of a robotic arm or the axes movements of a CNC machine. Stepper motors or servo motors are commonly used to achieve controlled movement in mechatronics systems. These motors ensure accurate and controlled movement within fine tolerances.

Preset Movement Preset movement refers to a mechanism that moves or shifts a certain distance along a predefined path. Examples of preset movements include automatic door open/close systems in machines or the clamp/de-clamp mechanism of a material-collecting robotic arm. These movements are typically executed using hydraulic or pneumatic cylinders.

Continuous Rotary Movement Continuous rotary movement is observed in mechanisms like the spindle and tool magazine movements of CNC machines. This type of movement is usually achieved using AC or DC motors, sometimes assisted by pneumatic or hydraulic systems.

The image below illustrates multiple movements of a robotic arm, where servo motors are employed for different controlled movements, and a hydraulic cylinder is used for the material clamping system.

Types of Movements in Mechatronics Systems:

1. Controlled Movement

• Linear Controlled Movement
• Rotary Controlled Movement

2. Limited Movement

• Limited Linear Movement
• Limited Rotary Movement

3. Continuous rotary movement

1. Controlled Movement

Controlled movements are commonly found in mechatronics systems, such as the axes movements in CNC machines, arm movements in robotic systems, and material handling systems. These movements are often accomplished using stepper motors or servo motors. The controlled movement aims to establish precise movement paths with accurate measurements and the ability to stop at specific distances as required. In mechatronics systems, controlled movement can be categorized into two types: linear controlled movement and rotary controlled movement.

Linear-controlled movement involves the straight-line movement of an element or equipment, while rotary-controlled movement positions the element along a circular path. Ball-screw and nut systems are commonly used for controlled linear movements, while worm and worm wheel systems are used for controlled rotary movements. Refer to the images below for illustrations of controlled linear and rotary movements commonly found in mechatronics systems. 

In the image on the left, a ball screw is coupled with a motor shaft to achieve linear movement. When the motor shaft (servo or stepper motor) rotates, the ball screw also revolves, causing the connected "Bed" to move in a linear or straight line. Precise control of the motor shaft displacement enables accurate linear movement of the bed. This means that the linear controlled movement in a mechatronics system is achieved by controlling the servo or stepper motor shafts. The image on the right shows a similar setup, but with a worm shaft connected to the motor shaft, resulting in the rotary motion of the worm wheel. By controlling the position of the motor shaft precisely, the rotary movement of the wheel can also be controlled accurately. Chapter VII of this document discusses in detail the use of ball-screw and worm and worm wheel systems for linear and rotary movements.

Controlled movement in mechatronics systems can be further classified into two groups: open-loop systems and closed-loop systems. Open-loop systems are rarely used because they lack feedback devices. In an open-loop system, the controller sends commands for movement without verifying whether the actual movement has been executed. Stepper motors are commonly used in open-loop systems, driven by special driver modules that receive digital pulse signals. Since there is no real-time information about the movement and position of the device, open-loop systems cannot account for obstructions or resistance during movement. These systems are suitable for mechatronics systems with small and uniform loading torque requirements that involve repetitive work. The image below illustrates the architecture of an open-loop system with linear controlled movement. 

 In line with the earlier image, the controller sends a reference or command voltage to the "Motor Driver" unit, which provides the necessary supply voltage to the stepper motor. The driver unit controls the voltage for motor shaft movement according to the command voltage. The driver unit also establishes the displacement of the machine bed and the speed of the motor. Once the movement command is conveyed to the motor, the controller does not verify the movement of the motor shaft or bed position. Chapter IV provides detailed information on the working principle of stepper motors.

On the other hand, most advanced mechatronics systems operate using closed-loop systems, which incorporate various feedback components. In a closed-loop system, the controller generates movement instructions and continuously monitors the results using different feedback devices. This ensures that the controller always has accurate and definite position information of a moving element. A closed-loop control system typically utilizes two types of feedback arrangements: position feedback and velocity feedback. Continuous feedback devices such as encoders and linear scales are used to obtain this feedback. A separate Chapter comprehensively discusses these feedback devices, known as continuous measuring sensors. The image below demonstrates the design of a closed-loop movement system using a ball screw and servo motor for movement on the axis of a CNC machine. 

In the closed-loop system shown in the previous image, the command or reference voltage from the controller is sent to the servo amplifier unit, which generates the necessary supply voltage for the servo motor. The servo amplifier unit ensures that the servo motor receives the exact voltage required for precise control of the motor's movement, based on the specific movement command for the ball screw. The basic working principle of a servomotor is discussed in Chapter IV. In a closed-loop control system, the controller continuously monitors whether the movement of the motor shaft or the machine bed corresponds to the command. Feedback devices such as encoders and linear scales are connected to the servo motor or directly coupled with the moving element to provide real-time position information to the controller and velocity information to the servo amplifier. A detailed discussion of different feedback elements used in mechatronics systems is presented in a separate chapter.

2. Limited Movement

Limited movement refers to the movement of an element within a specific distance, usually in a bidirectional manner. In a limited movement process, the moving element cannot stop at any position within its movement path. Once the starting command is given to a moving device, it will shift to one end of the movement track, and there is no way to stop it at an intermediate position. For example, the open/close mechanism in a machine door demonstrates limited movement. The stroke length of an actuator is restricted to open or close the door as required.

Mechatronics systems employ two types of limited movements: linear and rotary. Linear limited movements in mechatronics systems typically utilize different cylinders, which are selected based on the required movement stroke and the type of load to be overcome. Hydraulic cylinders are commonly used to overcome greater barriers or transport heavy loads, while pneumatic cylinders are suitable for lighter loads. For instance, pneumatic cylinders are used for the open/close mechanism of a machine door, while hydraulic cylinders are employed for work-piece clamping mechanisms in robotic arms that carry heavy loads. Rotary-limited movements are achieved using hydraulic and pneumatic rotary actuators, which have restricted movement paths. CNC machine pallet changing systems often utilize this type of limited rotary movement.

In an advanced mechatronics system, limited movement is usually controlled by a PLC (Programmable Logic Controller). The movement command is sent from the controller to the PLC, which verifies the entire situation and generates the required signal. The PLC then sends this signal to an actuator for the movement of the element. Advanced mechatronics systems often interface with various sensors (feedback elements), and the signals from these sensors are directly sent to the PLC to ensure proper completion of the element's movement. The working principles of different sensors and actuators employed in mechatronics systems are discussed in a separate chapter. The following picture shows a basic configuration of a limited linear movement, where a pneumatic cylinder's piston is controlled by the PLC.

If you examine the previous picture, you will notice that after issuing the movement command (in this case, throwing out or retracting the pneumatic cylinder's piston), the controller sends it directly to the PLC for decisive action. The PLC generates the desired voltage to energize the solenoid valve coil according to the command. Both ends of the double-acting pneumatic cylinder are connected to an air pressure line through solenoid valves, allowing extension or retraction of the cylinder piston. By activating the solenoid coil on the right side, air pressure reaches the rear side port of the cylinder through a solenoid valve, moving the piston to an extended position. Similarly, activating the solenoid coil on the left side allows air pressure to reach the front port of the cylinder through the left-side solenoid valve, retracting the piston. Two magnetic sensors are fitted on the cylinder to identify the piston's front and rear positions, and these feedback signals are sent to the PLC. The PLC can determine whether the actual movement of the cylinder piston (extension and retraction) is correct based on these signals.

For limited rotary movement, a similar mechanism is applied in mechatronics systems, but rotary actuators are used instead of cylinders. The electrical connections between the PLC, solenoid valve coil, and the controller remain the same, but the output pressure line of the solenoid valve is connected to both ends of the rotary actuator. As mentioned before, the selection of a rotary actuator depends on the desired movement path. For example, if a robotic arm requires a rotary movement from 0 to 90 degrees, the selected rotary actuator's movement will be restricted to this range, and it will not be possible to stop the actuator at any intermediate position. Feedback systems can also interface with limited-movement rotary actuators to ensure the correct rotary movement between the two ends. The following picture shows a 315-degree limited movement hydraulic rotary actuator and its control mechanisms (no feedback system is shown). 

3. Continuous rotary movement

Continuous rotary movement refers to the uninterrupted rotation of a moving element, which can be both controlled and uncontrolled. Controlled continuous rotary movement involves speed control and is commonly achieved using servomotors and servo amplifiers, eliminating the need for gear mechanisms. Servomotors offer different RPM options without requiring additional attachments (the operation and working principle of a servomotor are discussed in a separate chapter).

In mechatronics systems where controlled continuous rotary movement is not necessary, fixed RPM DC or AC motors are used to achieve continuous rotary movements at specific speeds. Hydro-motors and pneumatic motors are also utilized for continuous rotary movement in mechatronics systems. The advantage of using hydro-motors or pneumatic motors is that they provide a large starting torque compared to traditional electric motors. For example, a hydro-motor may be used for the tool arm movement of a CNC machine when continuous rotation with a high starting torque is required. In certain situations where using electricity is inconvenient, such as in the mining industry, pneumatic motors are employed to achieve high rotational speeds. PLCs typically control the continuous rotary movement of hydro-motors or pneumatic motors in advanced mechatronics systems, using a control method similar to that of limited movement systems discussed earlier. Instead of cylinders, hydro-motors or pneumatic motors are used. The working processes of hydro-motors and pneumatic motors are addressed in a separate chapter. The following picture shows the control mechanism of a hydro-motor with a PLC unit.  

Control of elements

In addition to controlled and preset movements in mechatronics systems, various electro-mechanical elements need to be controlled. For example, starting a cooling system, operating a three-phase induction motor, or controlling a lighting system. The PLC supplies voltage to devices like relays and contactors to make them operational within automated systems. Feedback systems are sometimes employed as well. The following picture represents a three-phase induction motor-driven pump controlled by a PLC. Upon receiving a command from the controller, the PLC sends the necessary voltage to activate the motor contactor. A three-phase AC voltage is then supplied to start the induction motor, drawing water through the attached pump unit and delivering it through the outlet. A flow switch is connected to the outlet line, providing a feedback signal to the PLC, allowing the controller to immediately detect any interrupted flow of water.

Most PLCs used in mechatronics systems operate on 24V DC voltage. Sensors and actuators with different voltage ratings connected to the mechatronics system are supplied through relays to obtain the desired voltages for those devices. In the previous example, if a 220V AC coil voltage contactor is used instead of a 24V DC coil voltage, it would not be possible to control that voltage directly from the PLC. However, by using a 24V DC relay, the required voltage (220V AC) can be obtained to activate the motor contactor. The following image explains how a 220V AC voltage is obtained using a 24V DC relay controlled by an output signal from the PLC.

A relay is an electromagnetic switch commonly used to turn an electric circuit on or off. It controls a large amount of current in a circuit by regulating a relatively small current. There are two main types of relays: electro-mechanical and solid-state relays. Electro-mechanical relays use magnetic force to turn electrical contacts on or off, while solid-state relays employ electronic circuits. A relay typically has multiple switching elements, each with three terminals: normally closed (NC), normally open (NO), and common (C). When a relay is deactivated, the NC and common terminals are connected, whereas in the activated state, the common terminal is connected to the NO terminal. This enables the relay to switch an electrical circuit on or off by connecting or disconnecting the NC, NO, and common terminals. The active and deactivate states of a relay are shown in the following pictures.

Multiple types of relays are used in mechatronics systems for controlling various electrical circuits and creating logical circuits. In simple mechatronics systems without a PLC, relays are used to create simple logical operation circuits. The following picture illustrates the control of different-rated electrical elements. A 24V DC voltage is supplied from a PLC to activate or deactivate each relay, and the common terminals of the relays are connected to 24V DC, 110V AC, and 220V AC, respectively. As a result, different-rated lights connected to the NO terminals of the relays will illuminate according to the activation of the respective relay.