Controlling variable speed conveyors
Published: 08 July, 2011
Conveyors are one of the most useful and widespread types of equipment in the manufacturing process. Controlling conveyors brings its own challenges and variable-speed AC drives are ideally suited to meeting them. PWE reports.
There are a number of conveying systems in use and one of the most popular is the belt conveyor. This typically consists of two pulleys, with a continuous loop of material - the conveyor belt - moving between them. Conveyors may be run continuously, intermittently or at variable speeds, with the motors of variable-speed conveyors being controlled by variable-speed drives (VSDs).
In conveyor applications VSDs are mainly used to improve the control by matching the speed to the production needs, either manually or with the use of feedback devices.
The drives also help to save energy and can make a significant contribution in this respect by allowing processes and machines to run at exactly the right speed, saving electricity and cutting the running costs.
The number of motors and drives used on each conveyor depends on its length and the total load. Calculating the motor requirement is always based on the torque required for producing sufficient tractive force. Modern belt conveyors use belts that are lightweight yet have great strength, allowing them to convey materials over great distances without hindrance from the deadweight of the belt. Only about 10% of the total load is accounted for by the tractive forces required to run the conveyor in normal circumstances.
The right motors for the job
Wherever possible, conveyors are driven by squirrel cage induction motors, which have a robust construction and are highly reliability. Motor output typically ranges from 0.15 to 1.5 kW depending on the size of the conveyor and the nature of track. The rated speed of motors in variable-speed operation is based on the speed at which the conveyors are mostly expected to run. Motor speeds can be adjusted both above and below this rated speed using a VSD. Motors should be sized with sufficient safety margin to provide the necessary output both above and below the rated speed.
Long or heavily loaded conveyors can require very high starting torque. Variable-speed conveyors can overcome this problem by ramping up the speed gradually. Large conveyors in fixed speed operation are often driven by slip-ring motors using extra resistors in the rotor circuit to reduce starting currents.
Some drives are capable of a sophisticated level of control, offering more features and higher accuracy in many applications. ABB general machinery drives are designed for this sector. These drives feature onboard algorithms that are specially designed to suit the needs of conveyor applications.
ABB uses a feature known as sequence programming. This provides an easy way to create sequences of operations. These programs are beneficial in applications where the drive controls the motor using signals from sensors and limit switches installed on or near the conveyor. This can be used for synchronisation, for instance stopping or changing the speed of the belt after a set time or a specified number of units. The drive can also control other equipment through it’s I/Os and in this respect acts like a small PLC. This helps to reduce the need for external PLCs, timers and counters, making for less complex motor control systems and reduced costs.
Sequence programs consist of one to eight steps or states, each state comprising eight operating rules. The rules include run, stop and direction commands. They determine acceleration/deceleration ramp times, reference value sources, state durations, and the status of outputs. Triggers can be internal signals, taken from a timer, real-time clock or speed output settings, for example, or external signals supplied via the digital or analog inputs on the drive.
For more advanced functionality, servo drives have traditionally been used. This was often because there was nothing else available. But with the latest generation of variable-speed drives, users today have more choice.
When using a servo motor, feedback from the motor about the shaft position is always used by the drive to calculate the correct speed. The shaft position is compared to a reference value, indicating where the shaft should be and the difference is corrected continuously by the motion controller’s profile generator. This results in much higher accuracy, suitable for systems requiring high precision levels.
Positioning is a good example that highlights this difference. If a standard drive is used for positioning, the motor normally runs at high speed and then decelerates to a lower speed and stops. Alternatively, the drive can follow an analogue signal. Either way, no reference profile is followed, compared for errors or corrected. This results in low accuracy. High accuracy can be vital in, say, food and beverage where it is necessary to predict the precise position of a bottle or jar to allow filling without spillage.
For this application, the drive runs a determined number of revolutions corresponding to the required length of the conveyor belt. When the target position has been reached, the drive signals to the PLC that it is in the required position. The dosing unit then runs the required number of revolutions to execute the filling operation.
Another example of motion control is a machine that corrects angular error of material laid across two parallel conveyors. This could for instance be a large compartmentalised tray for filling with merchandise.
In this case, two drives will be used in a master/follower set-up. The master determines the main line speed. The follower receives a speed reference. Two sensors are connected to digital inputs. The follower calculates the error distance in number of pulses between two sensor signals. This error is corrected by increasing or decreasing the speed of the follower, ensuring the tray is kept at right angles to the direction of motion.
Many other applications are possible with motion control.
VSDs can bring significant additional functionality to conveyors. In its simplest form they provide an easy way to adjust the running speed but the setup can also be highly automated and sophisticated. Whatever you want your conveyor to achieve, you can be sure there will be a drive to help you do it.
For further information please visit: www.abb.co.uk/energy