IE83729B1 - Method and apparatus for controlling servo drives - Google Patents

Abstract

ABSTRACT A method of controlling a series of slave devices using a Programmable Logic Controller (PLC) communicating in synchronous transmission through a device level network to each of the slave devices. The slave devices power-up and a timer controls the power—up sequence of the PLC. Control instructions held in data stores in the PLC are transferred to a buffer in the device level network when the power—up sequnce of the PLC is complete. Instructions are then transmitted through the device level network in synchronous transmission to the slave devices.

Description

METHOD AND APPARATUS FOR CONTROLLING SERVO DRIVES The present invention relates to a method and apparatus for controlling a plurality of servo drives in a manufacturing or process environment. In particular the invention relates to a method and apparatus for controlling a plurality of servo drives using a Programmable Logic Controller (PLC) communicating through a control module such as a DeviceNet'M module to each of the servo drives in synchronous transmission.

Industrial automation processes involve using motion control systems to control servo drives of an industrial machine, thereby providing accurate and recursive positioning of machine parts and tools. Servo drives reduce the need for fixed tools in industrial machines as they are adaptable and easily support any machine tool. Multi—axis motion control systems are common place in automation and allow two or more axes of motion on a single machine. In a multi—axis system, synchronization between the axes is important as it allows simultaneous movement of the axes relative to each other and can prevent collision of moving parts. The position and velocity synchronization of the axes determines how accurately a machine part is oriented as it moves.

A typical motion control system consists of a motor, motor drive and a motion controller.

The controller receives motion commands from a host computer or an internally stored program. These commands are interpreted by the controller which continuously communicates updated position commands to the motor drive in the form of distance, speed and time coordinates. The motor drive controls the current and frequency to the motor. which results in a commanded position. In a multi—axis system, one controller can control a number of motor and drive combinations. Alternatively, separate controllers may be controlled by a central system.

Traditionally in multi—axis systems the motion controller consists of a dedicated Computer Numerical Controller (CNC) for each axis. CNC controllers are quite expensive and use their own dedicated software to control a process. As a result, control processes are limited by their implementation in the dedicated software of the CNC. If it is required to change the automated process on a particular axis, it is necessary to re-program the CNC controller for that axis. It is desirable to have a solution in which motion control in a multi—axis system is tagged onto the Input/Output control of the overall machine, whereby a single controller is used for both process I/O and motion control for each axis obviating the need for a dedicated controller for each axis. It is the object of the present invention to provide such a controller in the form of a PLC. It is a further object of the present invention to provide tagged motion control of the type described for multiple servo axis operating in synchronous transmission of DeviceNetTM.

The present invention involves using a PLC to communicate in synchronous transmission through a control module such as a DeviceNetTM module to a plurality of servo drives.

DeviceNetTM is a standardized device—level network for industrial automation and is widely used in factory automation. It has multi-vendor interoperability in addition to supporting multiple network models eg. master/slave, peer to peer and multi—master. DeviceNetTM allows for simple devices to be networked replacing hardwiring of such devices. It also provides device diagnostics, and controls, configures and collects data. A DeviceNetTM module acts as an interface for connecting DeviceNetTM to a programmable controller.

The DeviceNetTM module functions as the master device on DeviceNetTM and interfaces between the programmable controller and the DeviceNetTM slave devices.

Accordingly, the present invention provides a method of controlling a series of slave devices using a Programmable Logic Controller (PLC) communicating in synchronous transmission through a device level network to each of the slaves, the method comprising: powering up the slave devices; using a timer to control the power—up sequence of the PLC; transferring instructions held in data stores in the PLC into a buffer in the device level network; and transmitting the instructions through the device level network in synchronous transmission to the slave devices.

For the purposes of the present invention, a ‘class 1’ slave is a term used to refer to the named description of the functionality of the slave device connected via the device level network.

Preferably, the device level network is DeviceNetTM.

Ideally, the slave devices are servo drives.

Preferably, the instructions are in a 32-bit stream format or alternatively a 16-bit stream format.

Preferably, the 32-bit stream instruction comprises the bit sequence, distance 24, move type 4 and speed 4. Alternatively, a 16-bit stream instruction can be used comprising the bit sequence, distance 12, move type 2 and speed 2.

Ideally, transmission of instructions is at 500 kbps.

Conveniently, the slave devices power-up before the power-up sequence of the PLC is complete. In this way, the servo drives are ready to receive control instructions. The power-up of the device level network is dependent on the PLC. If the device level network powers-up before the slave devices power-up and are ready to receive instructions, one of a number of bus errors are displayed on the device level network.

Advantageously, the PLC notifies itself when an instruction has been transferred into a buffer in the device level network.

Conveniently, no transmission an instruction to the slave devices occurs until the complete -bit stream is in a buffer in the device level network.

The protocol for verification of the bit stream is thus carried out by the PLC. Traditionally, verification of the bit stream is performed by the servo drives. By removing the protocol for verification from the servo drives, faster, more accurate and adaptable communication with the slave devices is achieved.

The present invention also provides an apparatus for controlling a series of slave devices according to the method of the invention, comprising: a Programmable Logic Controller (PLC); a base rack; a DeviceNetTM module; a power supply unit; two Local Area Networks (LAN); and one or more servo drives.

Preferably, the PLC and the DeviceNetTM module are located on the base rack of the apparatus.

Conveniently, power is supplied to the DeviceNetTM module from the power supply unit (PSU) of the PLC. Power is supplied directly to the servo drives from the PSU of the apparatus.

Advantageously, the PLC has its own software in which any number of control processes can be implemented.

Conveniently, the PLC provides both motion control and Input/Output control.

Ideally, the PLC provides tagged motion control onto the Input/Output of a process allowing a plurality of servo axis or servo drives to be controlled synchronously.

Advantageously, dials located on the front of the servo drives can be used to set MAC and CORE identifications for the servo drives.

Preferably, the PLC communicates through the DeviceNetTM module to the servo drives via a Local Area Network (LAN). The internal clocks of the servo drives are linked to each other by the LAN.

Conveniently, up to three servo drives can be linked by a second LAN allowing the drives to follow a programmed move in gearbox mode across the LAN.

The apparatus of the present invention can be utilised in wire forming and spring and strip metal manufacturing processes, where the servo drives are operable to run tools which produce wire-formed and spring components. The apparatus of the invention is ideal in this regard as discrete sub-components can be produced simultaneously by tools running on different servo-drives of the apparatus before a joining/final forming operation on the sub-components is performed by a tool operating on another servo-drive to produce a finished component. Alternatively, each servo drive can run a tool which performs a different forming operation on the same component.

Advantageously, a plurality of servo drives offers greater flexibility to the manufacturing process in terms of production and process time.

The invention will now be more particularly described with reference to the accompanying drawings which, show by way of example only, one embodiment of an apparatus for controlling a series of class 1 slaves according to the invention. In the drawings: Figure 1 is a schematic view of the topography of the apparatus; Figure 2 is an information flow chart illustrating the method of the invention; Figure 3 is a chart illustrating decoding of information in the slave devices; Figure 4 is a How chart illustrating the control topography of the system control; Figure 5 is a front view of the DeviceNetTM module; Figure 6 is a front View of the servo drives; and Figure 7 is a block diagram illustrating the general method and apparatus of the invention.

Referring initially to Figure 7, a block diagram illustrating the general method and apparatus of the invention is shown. A timer block A is used to control the power-up sequence of the PLC block B. The PLC block communicates in synchronous mode and at 500 kbps through a DevieeNetTM block C to a block D of servo drives. The method and apparatus of the invention can be utilised in machine tool control where the servo drives are operable to run machine tools. A Human Machine Interface (HMI) is used to configure the apparatus of the invention and input control instructions. The HMI may be a computer. a touch screen display or any suitable interface means known in the art.

Referring now to Figure l, with the mains isolator (not shown) of the apparatus switched on, a 24V DC input voltage is applied to contact A1 of a timer relay 1. With the input voltage applied the timer relay 1 begins to time out. While this is taking place, a series of class 1 slave devices (not shown) are powering up. Intemal diagnostics are carried out in the slave devices during power—up and any fault conditions are indicated.

After the period of time set by the timer has elapsed, the relay 1 switches on closing the contact. The line voltage L1 is supplied to the power supply unit (PSU) 2 of a Programmable Logic Controller (PLC) 5 located in a base rack 3 of the apparatus. With the line voltage Ll supplied, the PSU 2 powers—up and outputs a 24V DC voltage via wire no"s 140 and N2. The wires connect to a DeviceNetTM module 4 also located in the base rack 3. The DeviceNetTM module 4 in turn powers up. The slave devices are already powered-up and ready to receive control instructions by the time PSU 2 has powered up.

Referring now to Figures 2 and 3, the transmission of control instructions from the PLC to the slave devices using the DeviceNetTM protocol is described. Control instructions are pre-programmed and held in a series of data stores 6 in the PLC 5. Upon power-up of both the PLC and the DeviceNetTM module 4, an instruction held in a data store 6 is transferred into a buffer 7 in the DeviceNetTM module 4. Instructions appear as a 32-bit number to registers in the DeviceNetTM module. When the complete 32-bit instruction has been transferred into the buffer 7 the DeviceNetTM module notifies the PLC 5. Upon notification, the instruction is communicated in synchronous mode and at 500 kbps through the DeviceNetTM module to the slave devices 8. The individual slave devices process the received instruction which results in a controlled action. In the example of Figure 2, the transmitted instruction results in a box moving a required distance ‘X’ at a speed of ‘Y’ r.p.m.. When a received instruction has been processed by the slave devices and a controlled action executed, the PLC is notified that the action is complete and the next instruction can proceed.

Figure 3 shows the format of an instruction sent to the slave devices and a decoding key for command numbers. Instructions are 32-bit numbers comprising the bit sequence, distance 24, move type 4 and speed 4. Using DeviceNetTM it is possible to establish a filter for activation of the slave devices in accordance with instructions received. For example by setting register R248 in DeviceNetTM to l (R248=l), activation triggers if the previous command was ‘0’. By setting R248=2, activation triggers on any change in command. By setting R248=2 synchronous transmission is obtained and instructions can be compressed to 16-bit numbers comprising the bit sequence, distance 12, move type 2 and speed 2. In figure 3, move type corresponds to the command number and speed corresponds to the profile number.

Referring to Figure 4, a flow chart illustrating the structure of the DeviceNetTM configuration software is shown. The PLC contains bespoke software which controls all aspects of machine management and sequence control. By communicating through DeviceNetTM the PLC acts as the controller in a multi-axis system. The bespoke software can manipulate the number in the quantity of slaves register in the DeviceNetTM module. thereby allowing configuration of the DeviceNetTM network for multiple axis. It is possible to configure the network for any number of slave devices up to but not limited to In Figure 4, the PLC 5 communicates through the DeviceNetTM module 4 to servo drives numbered 1 to 6 via a Local Area Network LAN2. The PLC is the master control for both sequence and motion control. LANl can link a number of the servo drives together allowing programmed moves in different gearbox modes to be followed by the servo drives. This is particularly useful where the servo drives run machines tools in a multi- form tooling process. Individual drives can perform separate tooling operations before combining their outputs through a tooling operation on another drive in a timed and sequence controlled manner.

Figure 5 is a front view of the DeviceNetTM module and illustrates the settings used for communication at 500 kbps. Referring now to Figure 6, dials S1 and S2 located on the front of the servo drives are shown. With the power to the servo drive switched off and by adjusting dial S1, it is possible to set the core identification for that drive. The CORE I.D. identifies the individual devices in a multi-axis system i.e. master, slave l, slave 2, slave 3 etc. By adjusting dial S2 it is possible to set the MAC identification. The MAC I.D. is the DeviceNetTM bus identification. By utilising the dials on the servo drives it is possible to set the drives by just turning the dials to the required setting.

Conventional ly, identifications of drives are implemented by code in the motion controller.

It is to be understood that the invention is not limited to the specific details described above which are given by way of example only and that various modifications and alterations are possible without departing from the scope of the invention as defined by the appended claims.

Claims (1)

1. CLAIMS: A method of controlling a series of slave devices using a Programmable Logic Controller (PLC) communicating in synchronous transmission through a device level network to each of the slaves, the method comprising: powering up the slave devices; using a timer to control the power-up sequence of the PLC; transferring instructions held in data stores in the PLC into a buffer in the device level network; and transmitting the instructions through the device level network in synchronous transmission to the slave devices. A method according to claim 1, in which the device level network is DeviceNetTM. A method according to claim 1 or claim 2, in which the slave devices are servo drives. A method according to any one of the preceding claims, in which the instructions are in a 32-bit stream format comprising the bit sequence, distance 24, move type 4 and speed 4. A method according to any one of claim 1 to 3, in which the instructions are in a 16-bit stream format comprising the bit sequence, distance 12. move type 2 and speed 2. A method according to any one of the preceding claims, in which transmission of instructions is at a speed of 500 kbps. A method as claimed in any one of the preceding claims, in which the slave devices power-up before the power—up sequence of the PLC is complete. A method as claimed in any one of the preceding claims, in which the PLC notifies itself when an instruction has been transferred into a buffer in the device level network. A method as claimed in any one of the preceding claims, in which no transmission of an instruction to the slave devices occurs until the complete bit stream is in a buffer in the device level network. A method as claimed in any preceding claim, in which the protocol for verification ofa bit stream instruction is carried out by the PLC. A method for controlling a series of slave devices, substantially in accordance with any of the embodiments herein described with reference to and as shown in the accompanying drawings. An apparatus for controlling a series of slave devices using a Programmable Logic Controller (PLC) communicating in synchronous transmission through a device level network to each of the slaves in accordance with the method of claim 1, the apparatus comprising: a Programmable Logic Controller (PLC); a base rack; a DeviceNetTM module; a power supply unit; two Local Area Networks (LAN); and one or more servo drives. An apparatus for controlling a series of slave devices as claimed in claim 12, in which the PLC and the DeviceNetTM module are located on the base rack of the apparatus. An apparatus for controlling a series of slave devices as claimed in claim 12 or claim 13, in which power is supplied to the DeviceNetTM module from the power supply unit (PSU) of the PLC. An apparatus for controlling a series of slave devices as claimed in any one of claim 12 to 14, in which power is supplied directly to the servo drives from the PSU of the apparatus. An apparatus for controlling a series of slave devices as claimed in any one of claim 12 to 15, in which the PLC has its own software in which any number of control processes can be implemented. An apparatus for controlling a series of slave devices as claimed in any one of claim 12 to 16, in which the PLC provides both motion control and Input/()utput Comm 1. An apparatus for controlling a series of slave devices as claimed in claim 17, in which the PLC provides tagged motion control onto the Input/Output of a process. An apparatus for controlling a series of slave devices as claimed in any one of claim 12 to 18, in which dials located on the front of the servo drives can be used to set MAC and CORE identifications for the servo drives. An apparatus for controlling a series of slave devices as claimed in any one of claim 12 to 19, in which the PLC communicates through the DeviceNetTM module to the servo drives via a Local Area Network (LAN). An apparatus for controlling a series of slave devices as claimed in claim 20, in which the internal clocks of the servo drives are linked to each other by the LAN. An apparatus for controlling a series of slave devices as claimed in any one of claim 12 to 21, in which up to three servo drives can be linked by a second LAN allowing the drives to follow a programmed move in gearbox mode across the LAN. An apparatus for controlling a series of slave devices as claimed in claim 12, in which the servo drives are operable to run tools which produce wire—formed and spring components. An apparatus for controlling a series of slave devices as claimed in claim l2, substantially in accordance with any of the embodiments herein described with reference to and as shown in the accompanying drawings. MACLACHLAN & DONALDSON, Applicants’ Agents,