Servodrives synchronize rotary and linear movements.
An increasingly common control challenge is managing complex, multiaxis motion paths that include both rotary and linear motors. A case in point involves an Italian manufacturer of coated-wire twist-ties for closing bags used in a range of industries. The application requires unwinding wire from a large base drum and winding it onto smaller target spools. Twist-tie wire on the smaller spools is subsequently cut to length by the consumer.
To fully and efficiently control the overall winding process, Elmo Motion Control designed a master-slave electronic-cam system in which the target-spool rotary motor serves as the master, and a linear motor acts as a slave. The linear motor controls the unwinding/winding process between the base drum and target spool. Three SAXophone-Series digital servodrives control and synchronize the speed and position of each motor. An encoder attached to the base drum monitors the amount of wire being wound onto the target spool, using signals output from the target-spool servodrive. The servodrive attached to the base drum senses these signals and adjusts the base-drum rotational speed as needed. To ensure that the target spool consistently receives the amount of wire required for accurate winding, the base-drum speed must increase as more wire unwinds.
The back-and-forth movements of the linear motor control the actual winding of the twist-tie wire up and down on the spool. Facilitating this process is an electronic cam (ECAM), in essence a table listing position reference values that define a complete motion path. The values subsequently specify the slave commands for a single motion cycle. The ECAM table includes the start position of the slave motor, distances between consecutive points of the master motor, and the end position of the slave motor. The table can contain up to 1,024 positions.
In the twist-tie application, the linear-motor slave controls the ECAM trajectory. The twist-ties are wound on the target-spool (rotary motor) master, so the ECAM table lists the set of rotary motor positions which, in turn, causes the linear motor to change position. The entries in the ECAM table are interpreted relative to the previous positions of the winding sequence. The target spool (master) and linear motor (slave) operate in a follower mode so that when the target spool changes speed or stops, the slave follows accordingly. This relationship remains throughout the entire sequence, from the instant the slave is initialized in home position through the final winding of the target spool.
For this specific application, Elmo implemented a feature called "cyclic ECAM," which defines the slave commands for a single ECAM cycle as the linear motor moves through a full back-and-forth motion path. During this period, the target spool rotates 1.3 turns. An additional entry in the ECAM table serves as a delay of one cycle to ensure smooth winding when the wire reaches the top or bottom of the spool and begins winding back again. With cyclical ECAM, once the last entry of the ECAM table is reached, the sequence automatically begins again from the home position of the slave linear motor while the target spool continues winding without stopping.
The entire twist-tie winding process proceeds as follows:
1. At power up, the linear motor resets to a home (base) position. In this application, the home position was offset a bit from one end.
2. The target-spool motor begins rotating and winding the twist-tie wire at a constant speed. The linear motor moves back and forth, drawing wire up and down onto the target spool. Each time the linear motor completes a back-forth motion path, the target-spool completes 1.3 rotations.
3. As the process proceeds and the target spool fills, the encoder that monitors the amount of wire being wound signals the servodrive attached to the base drum to increase the drum rotation speed. This guarantees that the wire unwound from the drum continues to be wound onto the target spool at a consistent pace.
4. When the target spool is full, the same encoder senses that the winding is complete, and the operator temporarily stops the system.
5. The operator removes the full spool and replaces it with a new, empty one.To restart the process, the linear motor moves back to the home position and the winding sequence resumes.
As can be seen in the "Motion Profile" graph, rotary motor position has no effect on the continuous trajectory of the linear motor. A full ECAM cycle is indicated as the distance between points 1 and 2.
To run the entire sequence automatically, a computer program executes the ECAM table program at system startup. An interrupt routine added to the application temporarily halts the sequence if the system detects a motor fault. This lets the operator determine what consequential steps to take.
The user program was downloaded to the slave linear motor's servodrive flash memory, so that when the drive is powered on, the program begins to execute automatically. The slave drive has digital inputs and outputs for initiating the system, setting the homing mode for the motor, and for indicating motor faults.
As a result of implementing this ECAM-based sequence, the twist-tie winding application is now fully automatic, significantly more accurate, and much faster than previous methods. Cyclic ECAM proved to efficiently coordinate this multiaxis system of mixed rotary and linear motors. Now, the twist-tie manufacturer can guarantee that each spool includes the same amount of wire. In addition, the actual winding pattern of the wire on the target spool is evenly spaced and consistent from one spool to the next.
Compact drives, powerful performance
Elmo's SAXophone and MINI-SAXophone digital drives feature servoperformance aligned with sinusoidal vector control. The drives are flexible, operating in a range of modes that include current, velocity, position, and advanced position (PVT, PT, ECAM, and Dual Loop). RS-232, RS-485, and CANopen ensure efficient networking. Both drives are compact and connect directly to the main power source (up to 480 Vac). The SAXophone delivers up to 10kW of power; the MINI-SAXophone up to 2.5 kW.
The drives can be manually or automatically tuned using Elmo Motion Control's proprietary application for drive setup, configuration, tuning, logging, and analysis. The drives are suited for a range of applications, including semiconductor and electronics manufacturing, packaging, material handling, wood processing, machine tool, and robotics.