A COMMUNICATION INTERFACE BETWEEN A CONTROL UNIT AND A HIGH VOLTAGE UNIT

FIELD OF THE INVENTION

The present invention relates to a communication interface between a control unit and a high voltage unit comprising an isolation device arranged as an isolation barrier between the control unit and the high voltage unit to ensure personal safety. The present invention is useful in any systems including a control unit sending control signals to a high voltage unit, which impose a demand on the isolation between the high and low voltage parts in order to ensure personal safety, such as motor drives, high voltage power supply systems, systems for monitoring high voltage processes, and control systems of industrial robots.

PRIOR ART

Parallel signal interfaces are often used for transferring control signals between a control unit and a high voltage unit. For ex- ample, a traditional multi-axis robot drive for an industrial robot is designed with a parallel interface between a robot controller and drive units. In many applications, the signal interface has to be galvanically isolated to comply with electrical safety regulations in order to avoid personal injury. The isolation of the paral- IeI signal interface is achieved by means of optocouplers connected between the control unit and the power unit. However, there are some drawbacks with such a solution, such as high costs for optocouplers due to the large number of signals in the parallel interface. The large number of optocouplers leads to a large number of pins, which requires complex and expensive connectors and limits the cable length between the control unit and the power unit, and thereby limits the possibility of distribut-

ing the units. The reliability of the high voltage unit is affected in a negative way due to the large number of components and connector pins. In the multi-axis robot driveexample, the number of additional axis in the system is limited by the parallel inter- face.

In some applications, there is a desire to use a serial interface between the control unit and the high voltage unit. However, there are some drawbacks with using optocouplers for achieving the required isolation between the control unit and the high voltage unit when a serial interface is used. One drawback with optocouplers is the poor bandwidth. Due to the poor bandwidth, optocouplers can only be used in applications with low demand on communication speed. Optocouplers can handle communica- tion speeds up to 10Mbps. Another drawback with optocouplers is that they age faster and thus have a limited lifetime. Thus, optocouplers cannot be used in applications with high demand on the communication speed and reliability, such as a robot control system.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to provide an improved communication interface between a control unit and a high volt- age unit, which comply with electrical safety regulations with regard to personal safety.

This object is achieved by the initially defined communication interface characterized in that the interface comprises a serial communication link between the control unit and the high voltage unit and a signal transformer adapted to transfer serial communication signals arranged as an isolation barrier between the control unit and the high voltage unit to ensure personal safety.

According to the invention, the galvanic isolation is achieved by means of a single signal transformer. The signal transformer provides a significantly improved bandwidth compared to the op- tocouplers, and enables a communication speed of up to 1 Gbps. This makes it possible to use serial communication in applications with a high demand on the bandwidth, as well as demands on the isolation between the high and low voltage parts in order to comply with electrical safety regulations, which today use a parallel interface. The invention makes it possible to omit the optocouplers, which leads to a significant reduction of the costs for the communication interface. A further advantage of using a signal transformer instead of optocouplers is the increased lifetime.

The transformer is designed so that it provides the required demand on the isolation between the high and low voltage parts. Since the demands vary between different applications, the design of the transformer varies in different applications. The design of the transformer must be such that it provides sufficient isolation to comply with applicable electrical safety regulations.

By control unit it is meant a low voltage unit including processing means, such as a CPU, adapted to execute a control program producing control signals. The control signals are trans- ferred to the high voltage unit on the serial communication link. By a high voltage unit it is meant a unit comprising power electronics adapted to handle voltages above 48V.

According to an embodiment of the invention, the serial commu- nication is Ethernet-based. This is advantageous since it makes it possible to use cheap standard Ethernet components, such as a standard Ethernet cable.

According to an embodiment of the invention, the serial commu- nication link comprises a cable and a connector between the cable and the high voltage unit, and the transformer is arranged

between the connector and the power electronics of the high voltage unit. In this embodiment, the transformer is used to protect the signal interface connector of the high voltage unit. Thereby, it is safe for a user to touch the connector between the high voltage unit and the cable.

According to a further embodiment of the invention, the high voltage unit comprises a printed circuit board provided with power electronics, and the transformer is arranged on the board. It is practical to arrange the transformer on the circuit board provided with power electronics.

In applications in which safety isolation is required, the transformer must have a reinforced isolation. With reinforced isola- tion means that the safety distance of the transformer has been increased according to safety regulations; preferably the safety distance has been doubled. This embodiment further improves the personal safety of the communication interface between the control unit and the high voltage unit.

According to an embodiment of the invention, the control unit and the high voltage unit are parts of a drive system of an industrial robot. Preferably, the control unit and the high voltage unit are adapted to control a plurality of motor axes of the robot, a so-called multi-axis robot drive. The invention is particularly useful in a multi-axis robot drive. Instead of using a parallel interface between the control unit and the high voltage unit, a serial communication is used. The transformer replaces the opto- couplers used in the prior art. By using a serial communication link between the control unit and the high voltage unit, it is possible to use only one transformer for achieving the required isolation barrier between the control unit and the high voltage 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 high voltage unit is reduced. By using a serial communication link between the control unit and

the high voltage unit it is possible to distribute the high voltage unit.

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 a communication interface between a control unit and a high voltage unit according to an embodiment of the invention.

Fig. 2 shows a multi-axis robot drive including a communication interface according to an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 shows a control unit 1 and a high voltage unit 2. For example, the control unit is a supervisory computer and the high voltage unit is a power-feeding device, which transfers status signals to the supervisory computer on the serial link. In another example, the control unit and the high voltage unit are parts of process equipment having a high power part that sends status signals to the control unit.

The control unit and the high voltage unit are connected via a communication interface having a serial communication link 3. 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 connecting the cable 6, and a second connector 8 arranged in the high voltage unit for connecting the cable 6 with the high voltage unit 2. Further, the serial communication link comprises an isolation device in the form of a signal transformer 10 arranged as an isolation barrier 13 between the con-

trol unit 1 and the high voltage unit 2 to ensure personal safety. The transformer 10 is physically located in the high voltage unit.

The signal transformer 10 is adapted to transfer serial communi- cation signals and has at least a primary and a secondary winding. The transformer can have multiple windings depending on whether it is a full or a half duplex communication. The cable 6 includes a plurality of wires. The connectors 7 and 8 include corresponding wires. The transformer 10 is connected to the wires of the cable 6 via wires routed from the connector 8. A transformer with reinforced isolation should preferably be used to comply with electrical safety regulations.

The high voltage unit 2 further comprises a logic part 12, includ- ing circuits for communication, which is a low voltage part, and power electronics 14, which is a high voltage part. The logic part 12 is connected to the power electronics 14. The signal transformer 10 is arranged between the connector 8 and the logic part 12.

The signal transformer 10 is selected to provide enough isolation to comply with safety regulations for personal safety. For example, the safety distance d 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 with- stand 4 kV for one minute without causing breakdown.

The high voltage unit 2 comprises a Printed Circuit Board 18, in the following called a PCB 18. The power electronics 14 and the logic part 12 are arranged on the PCB 18. The signal trans- former 10 is also arranged on the PCB 18. The second connector 8 is arranged on the edge of the PCB 18 and is connected to

the transformer 10 with wires arranged on the board. The transformer 10 is electrically connected to the logic part 12 via wires arranged on the PCB 18. In another embodiment, the PCB 18 may consist of more than one board, for example, the connector 8, the transformer 10, and the logic part 12 can be located on one board and the power electronics 14 can be located on another PCB.

The signal transformer 10 forms an isolation barrier 13 between the connector 8 and the logic part 12 and power electronics 14, thereby making it safe for a user to touch the connector 8, the cable 6, and the control unit 1 .

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, which comprises a control unit generating control signals to the motors. The control signals determine motor torque, motor speed, and drive currents for the axes. The control system also com- prises one or more drive units, which control the motors by converting DC current to a variable alternating current in dependence on the control signals from the control unit. The drive units are supplied with AC power. The drive unit includes a rectifier converting the supplied AC power into DC power, and a switch- ing unit, denoted an inverter, converting the DC power to AC power in response to the control signals from the control unit. The motors are equipped with angle measuring devices to provide position feedback signals. The feedback signals from the angle measuring devices are transferred from the motors to the control unit.

Figure 2 shows an example of a communication interface between a control unit and a high voltage unit in a multi-axis robot drive. Like or corresponding parts as in figure 1 are indicated with the same reference numbers. The multi-axis robot drive includes a control unit 20 including software for current control of

the motors driving the axes of the robot. The control unit 20 comprises hardware and software for generating voltage control signals. The multi-axis robot drive further includes a drive unit 22 includi ng power electronics 14 for generati ng control signals to a plurality of motors, in this example three motors 23, 24, 25 which control the axes of the robot. Typically, the number of axes varies between three and seven . The control unit 20 and the drive unit 22 communicate with each other via an interface includi ng a serial communication link 3 including two connectors 7, 8 and an Ethernet cable 6. Feedback signals of motor currents are transferred from the drive unit 22 to the control unit 20 via the serial communication link 3. The voltage control signals are transferred from the control unit 20 to the drive unit 22 via the serial communication link 3.

The signal transformer 10 forms an isolation barrier 13 between the connector 8 and the logic part 12 and the power electronics 14, thereby making it safe for a user to touch the connector 8, the cable 6, and the control unit 20.