Industrial conveyors employ a variety of constituent components, primarily motors. For conveyors operating at constant or variable speeds, motors are adept at controlling their velocity. In this case, the main drive motors simply alter the velocity as per requirement.
However, positioning control has different parameters and requires motors that can start or stop the conveyor with high precision. The first thing to consider here is the type of motor. For positioning applications, a step or servo motor is a great choice.
Furthermore, it requires specific features and accessories to accurately start or stop the conveyor at the desired position. Three such ways for positioning conveyors are addressed below.
1. Pulse Control
Pulse control is an excellent and popular option to precisely control a step or servo motor. This is even more valid if the primary controller or PLC in the operation emanates high-frequency outputs, somewhere in the range of 20 kHz or higher.
For proper operation, a machine should have a minimum frequency of 20 kHz, although some controllers can operate at 100 kHz or even in the MHz. However, the increase in frequency will directly translate to additional operational costs. A better option is to go for low-frequency outputs coupled with an integrated stepper motor.
Now coming to the actual operation, the pulse output from the PLC is connected to the input of the integrated motor. Similarly, another non-pulse output from the PLC is also connected to the motor. The first signal is to dictate the travel length and speed of the motor and the second is to determine the direction of travel, that is, forward or reverse.
Therefore, with both signals coming from the PLC, the integrated motor can move the conveyor a specific distance at a particular speed in a desirable direction. Lastly, for smoother operation, the PLC controller is required to significantly ramp up the frequency of the pulses, to ensure that the motion is not jerky.
2. Velocity Control with Analog Input
Using a PLC for controlling the velocity and direction of the conveyor motion is simple with digital inputs. However, this is ideal only if the starting and stopping velocity of the motor are considered constant.
With ever-changing conditions and parameters, the speed of the motor might be required to increase or decrease to meet the target. This variable can be controlled precisely by adding an analog input along with the digital.
Therefore, there will be two digital inputs for the motor coming from the PLC, run/stop and forward/reverse. The first one controls the acceleration and deceleration of the motor and the second controls the direction of motion. Lastly, there is a third analog input between the PLC and the motor.
This analog input sends a specific voltage to the motor to alter its target velocity as required. Thus, apart from accurate acceleration and declaration, the speed of the positioning conveyor can also be increased or decreased as required. Typically, integrated motors have an analog input that can accept a voltage of at least 5 to 10 volts.
Therefore, adding an analog input to the standard digital input offers an extra degree of flexibility during operation, and is also easy to program in the controller.
3. Network Control
In this third method, there is only a single connection between the primary controller and the motor. This is a network connection that replaces both digital and analog signals mentioned in the previous two methods. There are various network interfaces to choose from such as RS-232, RS-485, Ethernet TCP, Ethernet UDP, EtherNet/IP, Modbus and CANopen.
All commands are sent from the primary controller to the motor through this network. As for how the command is sent from the PLC to the motor, there are numerous ways. Here, the focus is on the Serial Command Language for the RS-232 and RS-485 interfaces.
Serial Command Language (SCL) is a simple programming language which converts digital commands accurately into motion. For instance, the command FL30000 has two components, FL and the number 30000. The first is used to denote the feed to length, and the second denotes 30000 increments of motion or movement. In a step motor, where each step is an increment, this means 30000 steps.
Similarly, there are commands for altering the velocity of the motor, the direction of motion and so on. In fact, there are more than 100 commands in the SCL language, ensuring an exceptional degree of precision when it comes to conveyor movement.
Conclusion
Ultimately, choosing a method depends on the application. If there is only a need for precise movement and direction, a PLC programmed with pulse control will suffice. If an operation requires variable operating speeds of the motor, then the analog input is a great choice. Lastly, if a network connection is available, it is best to opt for this as it encompasses all features.
FAQs:
1. What is the control system of a conveyor system?
The control system of a conveyor is a mechanism designed to control the speed, direction and load-bearing capacity of a conveyor. It typically consists of an electrical motor along with a pulley, roller or belt which is all connected to the conveyor. Apart from these basic components, the control system can also contain software components to ensure the smooth and safe operation of the conveyor.
Some prime examples of a control system are PLCs, temperature sensors, speed sensors, proximity sensors, load cells and more. The above-mentioned features and methods are all part of conveyor control systems geared specifically towards positioning.
2. Which motor is a position control device?
There is a wide variety of motors that are paired with conveyor systems such as AC and DC motors, step motors and servo motors. Out of these options, the best choice when it comes to position control is a servo motor. These motors are designed to ensure precise positioning control and speed control. The servo motor is able to do this using a feedback controller.
However, there are some prerequisites when it comes to a DC servo motor. A DSM should have low inertia and high torque to function optimally, as it is required to start and stop continuously on command.
3. What is the function of a conveyor motor?
The most basic function of a conveyor motor is to ensure the motion of a conveyor, with or without the load. Hence, apart from gravity conveyors, all powered conveyors use one form of motor or the other for optimal function. Apart from ensuring smooth motion, the role of a conveyor motor can vary significantly according to applications.
In operations requiring variable speeds, a specific motor should be installed. The same goes for ensuring position control, starting or stopping the conveyor constantly, or even ensuring the motion of the conveyor is not hindered when it is on an incline or decline.