Technical Field

The present disclosure generally relates to a drive unit for at least one electric motor of a robot. In particular, a drive unit for at least one electric motor of a robot, where the drive unit comprises a DC/DC converter and a DC bank energy storage, a robot comprising at least one electric motor and a drive unit, and a method for handling electric energy in a drive unit for at least one electric motor of a robot, are provided.

Background

A robot drive system may include one or more drive units. A robot drive unit may include a rectifier for converting alternating current (AC) into direct current (DC), a frequency inverter, and a DC bus connected between the rectifier and the inverter. The inverter converts the DC current to a variable alternating current in dependence on reference values generated based on a robot control program. The variable alternating current from the inverter is then supplied to an electric motor.

A drive unit for a robot typically comprises a power resistor, for example a large bleeder resistor, arranged in the DC bus in order to take care of energy generated in the electric motor when the speed of the electric motor is reduced. Further, the DC bus is also normally provided with a capacitor with a high capacitance, which must be charged with a limited current upon start up of the drive unit. When the electric motors of the robot are electrically braked, energy from the electric motors are recovered and fed back to the capacitor. If this energy is not consumed in another motor, the voltage across the capacitor is increased. In order to protect the capacitor and other components, it is necessary to provide an alternative path for the respective electric current in order to prevent the voltage across the capacitor from becoming too high. Therefore, the voltage across the capacitor is supervised. If the voltage rises above a limit value, the bleeder resistor is connected to the capacitor so that the resistor discharges energy from the capacitor. When the voltage across the capacitor is below a limit value, the bleeder resistor is disconnected. A bleeder resistor requires a very reliable cooling flow to cool down the heat generated by the bleeder resistor. The regenerative electric energy has to be burned out to heat on the bleeder resistor and a cooling device (e.g. a cooling fan) has to consume more power to cool the heat from the bleeder resistor. Bleeder resistors can also be very warm, e.g. up to 200° C. It will therefore become a safety issue if the cooling of the bleeder resistor does not work properly. The heat from the bleeder resistor may also reduce the reliability of the drive unit since the bleeder resistor often has to be put inside a controller near the drive unit in order to function properly.

JP 2010226875 A discloses a power supply device including a power source part, which receives power supply from a primary-side power source; a

DC/DC converter, which supplies the power supplied from the power source part to a servo motor and receives regenerative power generated at the servo motor; a power storage part; and a control circuit.

Summary One object of the present disclosure is to provide a drive unit for at least one electric motor of a robot, which drive unit is energy efficient.

A further object of the present disclosure is to provide a drive unit for at least one electric motor of a robot, which drive unit reduces cooling costs.

A still further object of the present disclosure is to provide a drive unit for at least one electric motor of a robot, which drive unit has a small size.

A still further object of the present disclosure is to provide a drive unit for at least one electric motor of a robot, which drive unit is reliable. A still further object of the present disclosure is to provide a drive unit for at least one electric motor of a robot, which drive unit has reduced costs over its lifetime.

A still further object of the present disclosure is to provide a drive unit for at least one electric motor of a robot, which drive unit has a safe operation.

A still further object of the present disclosure is to provide a drive unit for at least one electric motor of a robot, which drive unit solves several or all of the foregoing objects.

A still further object of the present disclosure is to provide a robot comprising at least one electric motor and a drive unit, which robot solves one, several or all of the foregoing objects.

A still further object of the present disclosure is to provide a method for handling electric energy in a drive unit for at least one electric motor of a robot, which method solves one, several or all of the foregoing objects. According to one aspect, there is provided a drive unit for at least one electric motor of a robot, the drive unit comprising at least one inverter for producing current to the at least one electric motor; a power source for producing direct current to the at least one inverter; a DC bus for transferring the direct current from the power source to the at least one inverter; a DC bus energy storage arranged on the DC bus for smoothing the direct current and for storing electric energy recovered during braking of the at least one electric motor; a DC bank energy storage for storing electric energy from the DC bus; a DC/DC converter arranged between the DC bus and the DC bank energy storage, the DC/DC converter being operable in a buck mode, a boost mode and an idle mode; and a control unit configured to control the modes of the DC/DC converter based on a DC bus voltage across the DC bus energy storage.

The DC bank energy storage is thus connected to the DC bus via the DC/DC converter. When the DC/DC converter adopts the buck mode, energy is transferred from the DC bus and eventually from the DC bus energy storage to the DC bank energy storage. The buck mode may alternatively be referred to as a generic mode. When the DC/DC converter adopts the boost mode, energy is transferred from the DC bank energy storage to the DC bus energy storage.

The DC bank energy storage may alternatively be referred to as an extra energy storage or an extra energy bank. The power source may for example be constituted by a rectifier.

The control unit may control the modes of the DC/DC converter to carry out at least one bi-directional energy transfer function between the DC bus energy storage and the DC bank energy storage. When the DC bus voltage level rises during energy regeneration from the at least one electric motor, the regenerative electric energy may be stored in the DC bus energy storage and in the DC bank energy storage. When the DC bus voltage level becomes low, the DC/DC converter may convert energy from the DC bank energy storage to the DC bus. The control unit may additionally be configured to control the modes of the DC/DC converter based on a DC bank voltage across the DC bank energy storage.

The drive unit according to the present disclosure provides a new way of handling regenerative electric energy in a robot. Instead of consuming regenerative electric energy by a bleeder resistor, regenerative electric energy can flow between the DC bus energy storage and the DC bank energy storage in both directions. The drive unit according to the present disclosure therefore has reduced cooling costs, increased reliability and increased efficiency. Also the costs for the drive unit over time is reduced.

The control unit may be configured to switch the DC/DC converter from the idle mode to the boost mode when the DC bus voltage is equal to, or smaller than, a lower DC bus voltage threshold value. The lower DC bus voltage threshold value may be constituted by a nominal voltage. The control unit may be configured to switch the DC/DC converter from the boost mode to the idle mode when the DC bus voltage is equal to, or larger than, an intermediate DC bus voltage threshold value, or when the DC bank voltage is equal to, or smaller than, a lower DC bank voltage threshold value. The control unit may be configured to switch the DC/DC converter from the idle mode to the buck mode when the DC bus voltage is equal to, or larger than, an upper DC bus voltage threshold value, and when the DC bank voltage is smaller than an upper DC bank voltage threshold value.

The control unit may be configured to switch the DC/DC converter from the buck mode to the idle mode when the DC bank voltage is equal to, or larger than, an upper DC bank voltage threshold value, or when the DC bus voltage is equal to, or smaller than, an intermediate DC bus voltage threshold value.

The intermediate DC bus voltage threshold value may be a value between the upper DC bus voltage threshold value and the lower DC bus voltage threshold value. The lower DC bus voltage threshold value may be a value that is smaller than the upper DC bank voltage threshold value.

The control unit may be configured to control the DC/DC converter such that a substantially stable, or stable, DC bus voltage is provided on the DC bus. By controlling the DC/DC converter such that a stable DC voltage is applied on the DC bus, a larger capacity of the DC bank energy storage can be used. As used herein, a substantially stable voltage may deviate ±10%, such as ±5%, such as ±2%, from a perfectly stable voltage.

The DC/DC converter may comprise a buck converter and a boost converter. The DC/DC converter thereby constitutes one type of buck-boost converter. The DC/DC converter according to the present disclosure may however be constituted by alternative types of buck-boost converters. The DC/DC converter may additionally comprise the control unit.

The DC bank energy storage may comprise at least one DC bank capacitor. The at least one DC bank capacitor may for example be constituted by an aluminum electrolytic capacitor or a film capacitor. Alternatively, the DC bank energy storage may be constituted by a rechargeable battery for energy storage.

The DC bus energy storage may comprise at least one DC bus capacitor. The at least one DC bus capacitor may for example be constituted by an aluminum electrolytic capacitor or a film capacitor.

According to a further aspect, there is provided a robot comprising at least one electric motor and at least one drive unit according to the present disclosure. The robot may comprise a plurality of electric motors arranged in a drive train.

According to a further aspect, there is provided a method for handling electric energy, the method comprising providing at least one inverter for producing current to the at least one electric motor; providing a power source for producing direct current to the at least one inverter; providing a DC bus for transferring the direct current from the power source to the at least one inverter; providing a DC bus energy storage arranged on the DC bus for smoothing the direct current and for storing regenerative electric energy recovered during braking of the at least one electric motor; providing a DC bank energy storage for storing electric energy from the DC bus; providing a DC/DC converter arranged between the DC bus and the DC bank energy storage, the DC/DC converter being operable in a buck mode, a boost mode and an idle mode; and controlling the modes of the DC/DC converter based on a DC bus voltage across the DC bus energy storage.

The method may further comprise controlling the modes of the DC/DC converter based on a DC bank voltage across the DC bank energy storage.

The method may further comprise switching the DC/DC converter from the idle mode to the boost mode when the DC bus voltage is equal to, or smaller than, a lower DC bus voltage threshold value. The method may further comprise switching the DC/DC converter from the boost mode to the idle mode when the DC bus voltage is equal to, or larger than, an intermediate DC bus voltage threshold value, or when the DC bank voltage is equal to, or smaller than, a lower DC bank voltage threshold value. The method may further comprise switching the DC/DC converter from the idle mode to the buck mode when the DC bus voltage is equal to, or larger than, an upper DC bus voltage threshold value, and when the DC bank voltage is smaller than an upper DC bank voltage threshold value.

The method may further comprise switching the DC/DC converter from the buck mode to the idle mode when the DC bank voltage is equal to, or larger than, an upper DC bank voltage threshold value, or when the DC bus voltage is equal to, or smaller than, an intermediate DC bus voltage threshold value.

The method may further comprise controlling the DC/DC converter such that a substantially stable, or stable, DC bus voltage is provided on the DC bus. Brief Description of the Drawings

Further details, advantages and aspects of the present disclosure will become apparent from the following embodiments taken in conjunction with the drawings, wherein:

Fig. 1: schematically represents one example of a drive unit; and

Fig. 2: schematically represents one example of a method for handling electric energy in the drive unit.

Detailed Description

In the following, a drive unit for at least one electric motor of a robot, where the drive unit comprises a DC/DC converter and a DC bank energy storage, a robot comprising at least one electric motor and a drive unit, and a method for handling electric energy in a drive unit for at least one electric motor of a robot, will be described. The same reference numerals will be used to denote the same or similar structural features. Fig. l schematically represents one example of a drive unit 10. Fig. l further shows a robot 12 comprising a plurality of electric motors 14 (six in Fig. 1) and the drive unit 10. The robot 12 may be constituted by any type of manipulator programmable in three or more axes, for example a four axis, six axis or seven axis industrial robot. The drive unit 10 according to the present disclosure is however not limited to robots. For example, the drive unit 10 according to the present disclosure can be used for a wide range of alternative actuators.

The drive unit 10 further comprises a plurality of inverters 16 (six in Fig. 1). Each inverter 16 is configured to convert energy from DC to AC for each electric motor 14.

The drive unit 10 further comprises a power source 18. The power source 18 is configured to produce direct current to the inverters 16. In this example, the power source 18 is constituted by a rectifier configured to convert an AC input 20 to direct current. The power source 18 may alternatively be constituted by a DC power source.

The drive unit 10 further comprises a DC bus 22 for transferring the direct current from the power source 18 to the inverters 16. The DC bus 22 is provided with a DC bus energy storage 24 arranged at the output of the power source 18, and accordingly at the input of the inverters 16, for storing energy recovered during braking of the electric motors 14.

The DC bus energy storage 24 of the example in Fig. 1 comprises three DC bus capacitors 26. Each DC bus capacitor 26 of the DC bus energy storage 24 is in this example constituted by an aluminum electrolytic capacitor. Each DC bus capacitor 26 may however be constituted by an alternative type of capacitor, for example a film capacitor. The DC bus energy storage 24 also provides a smooth DC voltage between the power source 18 and the inverters 16, or between the inverters 16.

The DC bus 22 comprises a first electrical conductor 28 and a second electrical conductor 30. One node of each DC bus capacitor 26 is connected to the first electrical conductor 28 and one node of each DC bus capacitor 26 is connected to the second electrical conductor 30.

The drive unit 10 further comprises a DC bank energy storage 32. In the example of Fig. 1, the DC bank energy storage 32 comprises three DC bank capacitors 34. Also the DC bank capacitors 34 are each constituted by an aluminum electrolytic capacitor in this example. Each DC bank capacitor 34 may however be constituted by an alternative type of capacitor, for example a film capacitor. As a further alternative, the DC bank energy storage 32 may be constituted by a rechargeable battery.

The drive unit 10 further comprises a DC/DC converter 36 arranged between the DC bus 22 and the DC bank energy storage 32. The DC/DC converter 36 is operable in a buck mode, a boost mode and in an idle mode.

The DC/DC converter 36 of this example comprises a buck converter 38, a boost converter 40 and a control unit 42. Other types of DC/DC converters for operation in a buck mode, a boost mode and in an idle mode exist.

The buck converter 38 is configured to convert energy from the DC bus 22 (high DC voltage side) to the DC bank energy storage 32 (low DC voltage side). In Fig. 1, the voltage across the DC bus energy storage 24 is denoted as Vbus and the voltage across the DC bank energy storage 32 is denoted as Vbank. The boost converter 40 is configured to convert energy from the DC bank energy storage 32 (low DC voltage side) to the DC bus 22 (high DC voltage side). The buck converter 38 and the boost converter 40 are arranged in parallel.

The control unit 42 is configured to control the modes of the DC/DC converter 36, e.g. via a state machine (not shown). When the DC/DC converter 36 adopts the buck mode, energy is transferred from the DC bus 22 to the DC bank energy storage 32 via the buck converter 38. When the DC/DC converter 36 adopts the boost mode, energy is transferred from the DC bank energy storage 32 to the DC bus 22 via the boost converter 40.

When the DC/DC converter 36 adopts the idle mode, no energy transfer takes place. The DC bus energy storage 24 thus provides an energy storage for energy exchange with the DC bank energy storage 32.

The control unit 42 is configured to control the DC/DC converter 36 such that a substantially stable DC bus voltage Vbus is provided on the DC bus 22. Fig. 2 schematically represents one example of a method for handling electric energy in the drive unit 10 by control of the DC/DC converter 36.

If the DC bus voltage Vbus is equal to, or larger than, an upper DC bus voltage threshold value Vbusup, and if the DC bank voltage Vbank is smaller than an upper DC bank voltage threshold value Vbankup, the DC/DC converter 36 is switched from the idle mode to the buck mode. The DC/DC converter 36 remains in the buck mode as long as the DC bus voltage Vbus is larger than an intermediate DC bus voltage threshold value Vbusint and the DC bank voltage Vbank is smaller than the upper DC bank voltage threshold value Vbankup. If the DC bank voltage Vbank becomes equal to, or larger than, the upper DC bank voltage threshold value Vbankup, or if the DC bus voltage Vbus becomes equal to, or smaller than, an intermediate DC bus voltage threshold value Vbusint, the DC/DC converter 36 is switched from the buck mode back to the idle mode.

If the DC bus voltage Vbus becomes equal to, or smaller than, a lower DC bus voltage threshold value Vbuslow, the DC/DC converter 36 is switched from the idle mode to the boost mode. The DC/DC converter 36 remains in the boost mode as long as the DC bus voltage Vbus is smaller than the

intermediate DC bus voltage threshold value Vbusint, and the DC bank voltage Vbank is larger than the lower DC bank voltage threshold value Vbanklow. If the DC bus voltage Vbus becomes equal to, or larger than, the intermediate DC bus voltage threshold value Vbusint, or if the DC bank voltage Vbank becomes equal to, or smaller than, the lower DC bank voltage threshold value Vbanklow, the DC/DC converter 36 is switched from the boost mode back to the idle mode. With the control of the DC/DC converter 36 according to Fig. 2, regenerative electric energy will be stored in the DC bank energy storage 32 and in the DC bus energy storage 24 with their respective maximum usable capacity. Thus, the drive unit 10 does not have to burn the regenerative electric energy to heat during operation. The drive unit 10 according to the present disclosure may however optionally comprise a power resistor, such as a bleeder resistor, as a backup. The power resistor may for example be used for discharging electric energy when the DC bus voltage Vbus is larger than the upper DC bus voltage threshold value Vbusup and the DC bank voltage Vbank is larger than the upper DC bank voltage threshold value Vbankup. The drive unit 10 according to the present disclosure is particularly feasible for small and midsize robots.

The heat generated by the DC bank energy storage 32 may be less than 5% of the heat generated by a prior art drive unit comprising a bleeder resistor. Thanks to the intelligent control method according to the present disclosure, the capacitance of the DC bank capacitors 34 may be two to three times better used than a prior art drive unit comprising only one energy storage. Thus, the DC bus capacitors 26 and the DC bank capacitors 34 can be made smaller, up to a third of the size in prior art drive units, while being capable of handling the same amount of regenerative electric energy as prior art drive unit energy storages.

According to one non-limiting example, the lower DC bus voltage threshold value Vbuslow is 400 V, the intermediate DC bus voltage threshold value Vbusint is 410 V, the upper DC bus voltage threshold value Vbusup is 420 V, the lower DC bank voltage threshold value Vbanklow is 75 V, and the upper DC bank voltage threshold value Vbankup is 410 V. These threshold values may however be set as desired and depending on implementation.

While the present disclosure has been described with reference to exemplary embodiments, it will be appreciated that the present invention is not limited to what has been described above. For example, it will be appreciated that the dimensions of the parts may be varied as needed.