A CONTROL SYSTEM FOR CONTROLLING AT LEAST ONE INDUSTRIAL ROBOT

FIELD OF THE INVENTION

The present invention relates to a control system for controlling at least one industrial robot having a plurality of motors, the control system comprising: a control unit including a plurality of current controllers adapted to calculate control signals for the currents of said plurality of motors based on current feedback signals, a drive arrangement adapted to control the motors by generating variable alternating currents for the motors in dependence on said control signals from the control unit, and an interface unit between the control unit and the drive arrangement adapted to transfer the control signals from the control unit to the drive arrangement and to transfer current feedback signals from the drive arrangement to the control unit.

PRIOR ART

The movements of the axes of an industrial robot are driven by motors mounted on each axis. The speeds and accelerations of the axes are controlled by the control system of the robot. The control system comprises a control unit and drive arrangement including one or more drive units. The control unit comprises a main computer that is adapted to execute a program with i nstructions for the movements and that supplies one or more current control computers with control instructions. These control instructions are then transformed by the current control computers into control signals for the drive unit(s). The control sig- nals to the drive unit(s) determine motor torque, motor speed, and drive currents for the axes. The drive arrangement controls the motors by converting direct current (DC) to variable alternat-

ing currents (AC) in dependence on the control signals from the control unit. The drive unit(s) is(are) supplied with AC power. A drive unit includes a rectifier converting the supplied AC power into DC power, and a switching unit, denoted an inverter, con- verting the DC power to AC power in response to the control signals from the control unit.

The inverters measure the current of the motors and provide current feedback signals. The current feedback signals are transferred from the drive unit(s) to the current control computers).

The current control computer includes one or more current loops and is adapted to generate control signals to the drive units based on the current feedback signals. The current loops are very time critical, and delays in the transfer of signals between the control unit and the drive arrangement should be kept as low as possible. The current loop generates a voltage reference that is fed into a modulation algorithm that calculates the pulse dura- tions (switching times) needed to cause the desired voltage to be applied to the motor. A timer-compare unit then transforms these pulse durations into Pulse Width Modulated (PWM) signals. A PWM signal is an ON /OFF command for power switches of the inverter. As the PWM signals are on/off commands they need to change state at a specific time with an accuracy of around 10ns.

A traditional control system for a multi-axis industrial robot is designed with a parallel interface between the control unit and the drive arrangement. The PWM signals and the current feedback signals are transferred via the parallel interface. An advantage with the parallel interface is that it transfers the signals at a very high speed. However, there are some drawbacks with the parallel interface, such as high costs for hardware components of the interface, for example optocouplers for achieving galvanic isolation of the interface, due to the large number of signals in

the parallel interface. For example, a nine-axis drive arrangement requires at a minimum 27 digital inputs for the control signals and 18 analogue outputs for current feedback. In addition, both analogue and digital outputs for status information and digital inputs, for example for shutdown commands, are needed. The large number of components leads to a large number of pins, which requires complex and expensive connectors and limits the cable length between the control unit and the drive unit(s), and thereby limits the possibility of distributing the drive unit(s). Accordingly, the reliability of the robot drive is affected in a negative way due to the large number of components and connector pins.

There is a trend today that the number of axes to be controlled by the control system is increased. Besides controlling the axes of the robot, external axes are controlled by the same control system. Also, the control system may control more than one robot. Therefore, it is desired to be able to add additional axes to the control system in a simple way.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to provide a low cost control system for an industrial robot.

This object is achieved by a control system as defined in claim 1 .

According to the invention, the control system comprises a sin- gle current control unit for providing all motors, controlled by the system, with control signals, and the interface between the current control unit and the drive arrangement is a high-speed serial communication link. The high-speed serial communication link comprises a high-speed serial bus, and the drive arrange- ment comprises one or more physically separated drive units including power electronics. The invention proposes a central-

ized current control unit and separate power electronics, with the parallel interface replaced by a high-speed serial link. In this context, by a high-speed serial communication link is meant that the communication speed of the link is equal to or greater than 50Mb/s, and preferably equal to or greater than 100 UbIs. The high communication speed is necessary in order to comply with the demand on the timing of the current loop.

The invention makes it possible to use a standard Ethernet link as the communication link between the current control unit and the high power electronics. Standard Ethernet cables are more cost-effective than customized parallel cables. By using a serial communication link between the current control unit and the drive arrangement, it is possible to distribute the drive units.

Each control cycle, the current control unit receives digital current feedback over the serial link from the drive arrangement. The current control unit then calculates a new set of digital control signals, which are sent back over the serial link to the drive arrangement allowing it to actuate the motors. The advantageous of this is that the current control unit needs only a single output port for communication . New drive units for controlling additional axes can easily be added to the system by connecti ng the new drive unit to the previous drive unit via an new high- speed serial communication link. This is a cost-effective solution for addi ng additional process axes.

Accordi ng to an embodiment of the invention, the current control unit is adapted to generate pulse duration signals for controlling the motors, and the current control unit comprises a converter unit adapted to convert the pulse duration signals into data packages suitable for being transferred on the serial communication link. 4. Preferably, the drive arrangement comprises a converter unit adapted to convert the data packages to pulse duration signals and a timer compare unit adapted to transform the pulse duration signals into Pulse Width Modulated signals.

The current control unit is adapted to generate voltage references to the motors, and to calculate the pulse durations needed to cause the desired voltage to be applied to the motors based on the voltage references. Pulse duration signals are switching times for power switches of the inverter of the drive unit. According to prior art, the pulse duration signals (switching times) are transferred into PWM signals that are sent over the parallel interface. However, it is not possible to send raw PWM signals over a serial link, since they are on/off commands, which need to change state at a specific time with an accuracy of around 10ns. This would require that we can send new data packets every 10ns, which is beyond today's technology. To avoid this the invention proposes to send the pulse durations only, and then have a timer compare circuit placed in the drive unit. Switching times typically only need to be updated in the range of 100 - 500us, which is possible with a high-speed serial link.

According to an embodiment of the invention the drive arrangement comprises a first and a second power unit including power electronics, and a second serial communication link arranged between the first and second power units, the first power unit is connected to the current control unit via said first mentioned se- rial communication link, the second power unit comprises power electronics for converting direct current to a variable alternating current for at least one motor in dependence on one of said control signals from the current control unit, and the second serial communication link is adapted to transfer said control signal from the first power unit to the second power unit and to transfer at least one current feedback signal from the second power unit to the first power unit. At least one of the power units is a drive unit comprising electronics for converting direct current to a variable alternating current for at least one motor in dependence on one of said control signals from the control unit.

The drive arrangement comprises a plurality of power units connected in series, such that each power unit is connected to the previous power unit via a serial communication link, and the first power unit in the series is connected to the current control unit via the first serial communication link. A power unit can be either a rectifier unit or a drive unit. The advantage of this is that the current control unit needs only a single output port for communication, as each new power electronics node is added to the network by connecting to the previous node. This system is very flexible as it can be built up of any number of multi-axis and single-axis drive units. This makes the system more cost-effective and scalable.

According to an embodiment of the invention, one of the power units comprises a rectifier adapted to convert alternating current to direct current and to supply the other power unit(s) with direct current. The drive arrangement comprises a single rectifier, which can supply DC power to the other power units. In this way it is not necessary to have a separate rectifier in every power unit.

According to an embodiment of the invention, at least one of the power units comprises electronics for converting direct current to a variable alternating current for a plurality of motors in de- pendence on the control signals from the current control unit. The invention also makes it possible to arrange one of the power units, for example the first power unit, as a drive unit including power electronics for all axes of the robot, and then (for instance at some later stage) to add an additional drive unit for each additional axis.

According to a preferred embodiment, the at least one of power unit comprises electronics for converting direct current to a variable alternating current for a plurality of motors and a rectifier adapted to convert alternating current to direct current and to supply other power unit(s) with direct current. This is a preferred

embodiment since it provides an integrated drive unit with a rectifier and a plurality of inverters for controlling all of the robot motors and then (for instance at some later stage) it is possible to add additional drive units without rectifiers for running addi- tional axes.

At least one of power unit is a drive unit comprising electronics for converting direct current to a variable alternating current for at least one motor in dependence on one of said control signals from the control unit.

According to an embodiment of the invention, the drive arrangement comprises: a third power unit including electronics for converting direct current to a variable alternating current for at least one motor in dependence on one of said control signals from the current control unit, and a third serial communication link arranged between the second and third power unit and adapted to transfer said control signal from the second power unit to the third power unit and to transfer at least one current feedback signal from the third power unit to the second power unit. This embodiment makes it possible to connect more than one drive unit together, which enables the control system to have physically separate drive units. Thus, it is easy to expand the control system if additional axes are added to the control system.

According to an embodiment of the invention, said serial communication link(s) comprises a signal transformer adapted to transfer serial communication signals arranged as an isolation barrier between the current control unit and the drive arrangement to ensure personal safety. The transformer replaces the optocouplers used in the prior art. By using a serial communication link between the control unit and the power unit, it is possible to use only one transformer for achieving the required isola- tion barrier between the control unit and the power unit. The cost for the signal transformer is much less than the cost for the

optocouplers. Thus, the cost for the communication interface between the control unit and the power unit is reduced. Further advantages with using a signal transformer instead of optocouplers are the improved bandwidth and the increased lifetime.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

Fig. 1 shows an example of a robot control system according to the invention.

Fig. 2 shows the robot control system of figure 1 in more detail.

Fig. 3 shows another example of a robot control system according to the invention.

Fig. 4 shows yet another example of a robot control system according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 shows a part of a robot control system, including a current control unit 1 and a drive arrangement comprises a drive unit 2 including power electronics. The power electronics controls one or more motors M of the robot. The current control unit and the drive unit are connected via a communication interface having a high-speed serial communication link 3, including a serial bus, for example, of Ethernet type. The serial communication link 3 comprises a serial cable 6, for example, an Ethernet cable, a first connector 7 arranged in the control unit for con- necting the cable 6, and a second connector 8 arranged at the drive unit for connecting the cable 6 with the drive unit 2. Fur-

ther, the serial communication link 3 comprises an isolation device in the form of a signal transformer 10 arranged as an isolation barrier between the control unit 1 and the drive unit 2 to ensure personal safety. The signal transformer 10 is adapted to transfer serial communication signals. The transformer 10 is physically located in the drive unit.

The signal transformer 10 is adapted to transfer serial communication signals and has at least a primary and a secondary wind- ing. The transformer can have multiple windings depending on whether it is a full or a half duplex. The cable 6 includes a plurality of wires. The connector 8 includes corresponding wires. The transformer 10 is connected to the wires of the cable 6 via the wires of the connector 8. A transformer with reinforced isola- tion, for example 4 kV should preferably be used to comply with electrical safety regulations.

The transformer 10 is selected to provide enough isolation to comply with safety regulations for personal safety. For example, the safety distance between the coils of the transformer is provided according to safety regulations. The safety distance required depends on the main voltage, pollution degree, and regulation standard. Another requirement is that the transformer must be able to withstand a certain voltage over a certain time without causing electrical breakdown of the isolation barrier. For example, the transformer should be able to withstand 3.5 kV during one minute without causing breakdown.

The drive unit 2 comprises a printed circuit board (PCB) 14. The drive unit 2 comprises logic electronics 17, including circuits for providing communication on the serial link, such as a converter for converting digital signals into data packages, and power electronics 18. The power electronics 18 and the logic electronics 14 are arranged on the circuit board 14. The transformer 10 is also arranged on the board 14. The second connector 8 is arranged on the edge of the board 14 and is connected to the

transformer 10 with wires arranged on the board. The transformer 10 is electrically connected to the logic electronics via wires arranged on the board 14. The signal transformer 10 is arranged between the connector 8 and the logic electronics 17. The transformer 10 form an isolation barrier between the connector 8 and logic electronics 17 and the power electronics 18, thereby making it safe for a user to come into contact with the connector 8, the cable 6, and the current control unit 1 .

The drive unit 2 is provided with a further connector 16 adapted to be connected to another drive unit via a further serial communication link. The further connector 16 is connected to the first communication link 3 via a second signal transformer.

Figure 2 shows in more detail the function of the current control unit 1 and the drive unit 2 and how they are connected. According to the invention, it is the signals in connection with the current control of the motors of the robot, which are transferred by means of the serial communication link 3. The serial link, for ex- ample, includes an Ethernet cable having two pairs of wires 3a and 3b, one for transmitting data and one for receiving data. The current control unit 1 comprises a current control computer. The current control unit 1 receives reference values lref for the currents of the motors from a main computer (not shown in the fig- ure) of the control system.

The drive unit 2 includes a sensor 20 that measures the current of the motor M. The drive arrangement comprises an A/D converter 22 adapted to convert the current feedback signals, i.e. the measured motor current values, from analogue signals into digital signals before transferring them over the serial communication link, and communication electronics adapted to transform the digital current feedback signals into data packages suitable to be transferred on the communication link. The measured mo- tor current lmotor is transformed into data packages, which are sent via the pair of wires 3a of the serial communication link 3.

The measured motor current lmotor is compared with the calculated reference value lref for the current in the current control unit 1 . A current controller 24 generates voltage references to the motors. The voltage references are fed to a modulation unit 25, which calculates the pulse durations needed to cause the desired voltage to be applied to the motors based on the voltage references. The current control unit 1 further comprises a converter unit 26 adapted to convert the pulse duration signals into data packages suitable to be transferred on the serial communication link. The pulse durations are then sent as data packages via the serial communication link to the drive unit 2. Thus, the control signals from the current control unit are serially transferred to the drive unit, and the current feedback signals from the drive unit are serially transferred to the control unit.

The drive unit 2 comprises a converter unit 27 adapted to convert the received data packages to pulse duration signals and a timer compare unit 28 adapted to generate Pulse Width Modu- lated PWM signals based on the pulse duration signals. The drive unit 2 controls the motors by converting direct current (DC) to variable alternating currents (AC) in dependence on the generated PWM signals. The drive unit 2 is supplied with AC power and includes a rectifier for converting the supplied AC power into DC power. The drive unit 2 further comprises power electronic 18 including an inverter converting the DC power to AC power in response to the generated PWM signals. The AC power generated by the inverter is supplied to the motor M.

In an alternative embodiment, the modulation unit 25 can be arranged in the drive unit and the voltage references to the motors, generated by the current controller 24, can be sent over the serial communication link, instead of the pulse durations. However, since the modulation unit needs to run a modulation algorithm and therefore requires processing means, it is more

advantageous to arrange the modulation unit in the current control unit 1 .

Figure 3 shows a control system according to an embodiment of the invention including a centralized current control unit 1 and a plurality of separate drive units 30-32. The current control unit 1 is connected to the first drive unit 30 via a first serial communication link 3. The first drive unit 30 is connected to the second drive unit 31 via a second serial communication link 34, and the second drive unit 31 is connected to the third drive unit 32 via a third serial communication link 36. The serial communication links form an internal network and the current control unit 1 and the drive units 30-32 form nodes in the network. Each drive unit is provided with a transformer 10, 10b-c arranged as an isolation barrier between the current control unit and the drive unit to ensure personal safety. The transformer is arranged between the connector 8, 8b-c connecting to the serial communication link 3,34,36 to the drive unit and the power electronics 40, 40b-c including the inverters.

In this embodiment, the first drive unit 30 comprises a rectifier 38 receiving alternating current (AC) and rectifying it into direct current (DC) and a plurality of inverters 40 for controlling a plurality of motor of the robot. The other drive units are provided with DC power from the rectifier 38 of the first drive unit. Each drive unit 31 ,32 includes an inverter 40 for controlling a motor. If the robot is a six-axes robot, the power electronics includes six inverters 40 for controlling the six motors. All six inverters receive control signals from the current control unit 1 . Accordingly, in this embodiment the first drive unit 30 is adapted to control all motors of the robot. The other drive units 31 , 32 are adapted for controlling one motor each, for example, for controlling the movements of external axes. If the current control unit is controlling more than one robot, each robot have its own drive unit. Each external axis controlled by the current control unit can be provided with a drive unit of its own .

The current control unit is arranged as a centralized current control unit and includes a current loop, including a current controller 24, for each axis to be controlled by the control system. Con- trol signals to all motors are transferred to the drive arrangement via the serial communication link 3. The advantage of this is that the current control unit needs only a single output port 7 for communication , as each new drive unit node is added to the network by connecti ng to the node of the previous drive unit. This makes the system more cost-effective and scalable.

Figure 4 shows a control system according to an embodiment of the invention including a centralized current control unit 1 , and a drive arrangement comprising a rectifier unit 45 and a plurality of separate drive units 46-48. In this embodiment, the current control unit 1 is connected to a rectifier unit 45 including a rectifier 38 via a first serial communication link 3. The rectifier unit 45 receives alternating current and is adapted to rectify it into direct current and to supply the drive units 46-48 with DC power. Each of the drive units 31 ,32 includes an inverter 40 for controlling one motor. Thus, each drive unit controls one axis of the robot. If the robot has six axes, the drive arrangement includes at least six drive units. The rectifier unit 45 is connected to the first drive unit 46 via a second serial communication link 50, the first drive unit 46 is connected to the second drive unit 47 via a third serial communication link 51 , and the second drive unit 47 is connected to the third drive unit 48 via a fourth serial communication link 52. The serial communication links 3, 50-52 form an internal network, and the current control unit 1 , the rectifier unit 45, and the drive units 46-48 form nodes in the network. All communication has to pass through the previous power units in the network.

The present invention is not limited to the embodiments dis- closed but may be varied and modified within the scope of the following claims. For example, the number of drive units may

vary in dependence on the number of axes controlled by the control system. Also, the number of inverters included in the drive units can vary in dependence on the number of axes controlled by the drive unit. It is possible to have more than one drive unit controlling a plurality of motors if the control system controls more one robot.