MINIATURIZED MOTOR CONTROLLER FOR SPACE ROBOTIC AND ROVER

APPLICATIONS

Henrik Löfgren(1)

, Fredrik Bruhn(1)

, Marcus Gunnarsson(1)

, Maria Hagström(1)

, Anders Ljunggren(1)

, Karin

Häll(1)

, Robert Lindegren(1)

(1) ÅAC Microtec AB, Dag Hammarskjölds väg 54B, 751 83 Uppsala, Sweden Email: henrik.lofgren@aacmicrotec.com

ABSTRACT

ÅAC Microtec was the leader of an international consortium developing a Motion Control Chip (MCC) for the European Space Agency (ESA) under a TRP contract. The team consisted of the prime ÅAC (Sweden), Aeroflex Gaisler (Sweden), Centre Suisse d'Electronique et de Microtechnique (CSEM, Switzerland), Selex Galileo (Italy), Astrium (UK), and DLR Institute of Space Systems (Germany).

In order to improve the motor performance of rovers and robotic arms, one solution is to place the controller physically as close as possible to the motors. This reduces the harness and hence saves weight, decreases thermal leakage from the main system body and simplifies the final assembly. Nevertheless, with this approach the constraints on the electronics become more stringent: the assembly has to survive a very wide temperature range as well as vibrations and possibly dust, and at the same time it should be as small and light as possible. To cope with these design constraints, the Motion Control Chip (MCC) is based on stacked ceramic substrate technology in a Multi Chip Module (MCM), on which active components are assembled as bare dies. This approach was preferred before a large ASIC development to reduce cost and make the design more flexible. By choosing a MCM solution, the design will allow both FPGA and ASICs to be used. FPGAs can be used initially to lower the prototyping cost and later be replaced with ASICs as the packaging technology is qualified for the extreme environments of ISS, Mars and Moon.

The manufacturing of the first iteration of miniaturized MCC modules is finished, and the full test campaign including environmental tests have been performed.

As part of the MCC design, a miniaturized fly-back isolated Power Supply Unit (PSU) was developed in order to convert the incoming +28V bus to the needed

internal voltages and maintain galvanic isolation for the communication interfaces. The PSU design is generic and can be reused for other projects where a highly miniaturized and environmentally robust power supply is needed.

Aeroflex Gaisler is the official ESA maintainer of the RTEMS port for the LEON3 processor and has been providing support to several developments. CSEM provided the software for the MCC, which includes PID position, velocity, and torque control for brushed and brushless DC motors, as well as telecommand, telemetry and housekeeping through SpaceWire and CAN bus. Astrium UK was in charge of the definition of requirements for rover locomotion applications of the MCC. Astrium UK has experience from the Beagle project and is responsible for the Exomars rover development. Selex Galileo was in charge of the definition of requirements for three major applications of the MCC: robotic arms, complex motorized payloads (as drills and sample distribution systems) and exoskeletons. The DLR Institute of Space Systems contributed to the definition of requirements related to rover locomotion drives and was furthermore in charge of environmental testing of the MCC prototype.

1. DESCRIPTION OF THE MCC

The complete miniaturized MCC-module has a volume of 100x60x11 mm3 for a weight of 300g, including metallic casing and connectors (the height of the module is 30 mm with the auxiliary Power Supply Unit). Excluding the metallic casing, harnesses and connectors, the weight is only 30g. MCC is built to survive the Martian winter with the surrounding temperature going down to -120°C while the operating temperature is -55°C to 75°C. The current version is assembled using commercial-quality parts but all closest high reliability equivalent devices have been identified, making a direct path for a 20 krad (Si)

radiation tolerant MCC. The power consumption (excluding the motors) is around 4W in standby and around 9W when controlling and driving 3 brushed motors in parallel.

The present MCC embeds all electronic functions for the following features:

• Motor driving (3 brushed or 1 brushless motors) in 4-quadrant mode for a total power up to 150 W

• Motor heater driving for a total power up to 90 W

• Sensor interfaces (see Table 1 below) • Two isolated CAN-bus interfaces • Embedded auxiliary power unit for

generating all necessary voltages from a 28V bus

• Isolated temperature measurement • Computing power to run a variety of

control algorithms (e.g. position, velocity, and torque)

Table 1. List of sensors supported by the MCC

Sensor Number

of

interfaces

Data Description

of accepted

device

Resolver 2 (synchronized)

Axis position

Single pole, 4kHz

Strain Gauge 1 Axis torque 350ohm to 1 kohm Wheatstone bridge

Digital encoder

3 Axis position counter

3open-collector signals

Potentiometer 3 Axis position

5 to 10 kohm potentiometer

Thermistor 3 Motor temperature

PT1000

Hall sensor 1 Axis position

3open-collector signals

End switches 6 Position 6switches referenced to ground

Inside the MCC, motion-control functions and CAN communications are managed by an Actel flash-based FPGA embedding a LEON3 architecture equipped with a 20 Mbit SRAM (with EDAC) and a 16 Mbit flash memory running RTEMS. The LEON3 is

a 32-bit SPARC compatible processor, clocked at 25 MHz in this case. The flash-based FPGA allows easy reprogramming during the development phase since no boot sequence is required. All control algorithms (current as well as position, velocity, and torque) are implemented in software.

The MCC also includes several protections and housekeeping functions such as a protection for permanent short circuit on sensor supply, overvoltage protection for the interfaces to the system, voltage-level monitoring and independent over-temperature protection.

A PCB version has been produced for development purposes and electrical verification (electrical functions, algorithm performance, etc.). Physically, the MCC is composed of 3 modules:

• The MCC-C (Computer) embedding the FPGA, SRAM, Flash, ADC, oscillator, CAN transceiver and isolators, and low-level voltage regulators

• The MCC-M&D (Main & Driver) embedding H-bridge driver, analog electronics for sensor interfacing, heater driver, and thermal protections

• The MCC-PSU (Power Supply Unit) is a flyback converter used to generate the various voltages required in the module

Figure 1. The principle of the MCC architecture

The MCC has a wide range of interfaces for connections to external devices. The architecture will allow future expandability and evolution as listed below.

� Modularity: to allow reuse for future re-makes of the design as well as in future designs.

� Scalability: to be usable in both low-end and high-end applications with minimum hardware overhead.

� Portability: to guarantee long-term availability, the design is portable across a wide range of FPGA and ASIC technologies with minimum cost and effort, while maintaining functionality and performance.

The MCC can cover a wide range of potential applications such as rover locomotion, exoskeletons, robotic joints, and other applications. The MCC is implemented through a flexible combination of chips that can be upgraded relatively easy and work around obsolescence problems.

�

2. ALGORITHM

The control software is written in C++ and runs on RTEMS. The present version proposes a digital control with an inner loop for current (PI) and an outer loop for position (PID), velocity (PI) or torque (PID) as shown in fig.2.

Figure 2. MCC control algorithm block diagram

This control structure is available for both brushed and brushless motors. The nominal current-loop frequency is 10 kHz while the outer loop frequency is 1 kHz. The PWM frequency can be set from 20 kHz to 100 kHz and is implemented in the FPGA.

3. POWER SUPPLY UNIT

The PSU (Power Supply Unit) of the MCC is a self-resonant flyback converter, powered from the incoming +28V bus. It generates the voltages that are used inside

the MCC: +12V, -12V, 7V and an isolated 5V. The total maximum output power is 10W.

The PSU also features the following protections:

� Input under-voltage lock-out (non-latching) � Output over-voltage protection (latching) � Output short-circuit protection (latching) � Output current limiter

In its miniaturized form, the footprint of the MCC-PSU is 34x28 mm2. It has a maximum total height of 16 mm when the flyback transformer is mounted.

Because of the flexibility of the fly-back regulator, the PSU design from the MCC can easily be adapted for use in other applications where size and environmental robustness are strong requirements.

4. MINIATURIZATION AND ASSEMBLY TECHNIQUES

After tests and validation of the PCB version, the design was ported to multi-layer Low Temperature Cofired Ceramic (LTCC) substrates, leading to 3 different substrates which are procured from a sub-supplier. The LTCC substrates contain up to 9 thick film layers (silver) and one top thin film layer of copper done by ÅAC. This last layer is added to increase the density of assembly because the thin film technology allows much smaller distances between traces (10 to 20um) than the thick film process (75 to 100um). LTCC was chosen as the primary substrate carrier for the MCC due to the possibility of achieving good thermal conductivity with thermal vias and its Coefficient of Thermal Expansion (CTE) match with Silicon-die chips. The high thermal conductivity of LTCC makes it fairly easy to keep the junction temperature of the power transistors well below 110 ºC. Hence, degradation at End-of-Life (EOL) for the current provided to each motor is not expected to drop below 75% of the Beginning-of-Life (BOL) value. Using LTCC, it is possible to reach almost identical miniaturization as with Silicon substrates and even lower cost.

Complex active components (SRAM, Flash, ADC, OPA, etc.) are mounted as flip chip on each substrate. Before assembly each bare die component is prepared with UBM (Under Bump Metallization) and SST

(Solder Sphere Transfer). Discrete actives (diodes, BJT, MOSFET, etc.) are mounted through wire bonding because as the geometry of these components is not suited for flip chip. However, flip chip mounting is preferred, because it offers better mechanical, volume and thermal performances compared to wire bonding. The fully assembled sub-modules are shown in Fig. 3-5 below.

The FPGA used for the MCC required one additional step based on the construction of a silicon interposer: the FPGA is first chemically prepared and assembled on a silicon interposer which is used to spread electrical signals from the very high density component pads to a standard BGA interface. This operation was necessary to improve the global yield and simplify the assembly of the MCC. To avoid CTE problems the interposer substrate is silicon. In this way the die/substrate interface becomes the same material (same CTE) and the CTE problems are transferred to the interposer/carrier interface. Here the solder spheres with their greater stand-off height are far more resistant to movements and strains induced by the thermal expansion.

Figure 3. Fully assembled MCC-C module (sub-module of the MCC)

Figure 4. Fully assembled MCC-M&D module (sub-module of the MCC)

Figure 5. Fully assembled MCC-PSU module (sub-module of the MCC), bottom side shown

Also, the interposer technology simplifies the assembly onto the substrate since very fine-pitch components no longer needs to be attached directly. If a component fails, rework of that component is facilitated by replacing the whole interposer/component-package. The BGA interface also grants flexibility for the metal composition and size of the BGA spheres. The interposer solution also allows future upgrades of the component without a need to change the interface design of the substrate. Fig 6 shows an Actel/Microsemi A3PE3000 FPGA mounted on an interposer.

Figure 6. Actel A3PE3000 3MGate FPGA assembled on a silicon interposer

After assembly of the components on each substrate, the MCC-C module is stacked onto the MCC-M&D module using solder spheres. The MCC-PSU module is kept above the MCC-M&D, held by a metallic bracket. The MCC-C module is 34x34x3 mm3 and the MCC-M&D is 80x40x6 mm3. Fig. 7 shows the assembly principle of the MCC-C stacked onto the MCC-M&D.

Figure 7. Stacking principle of the MCC-C -module on the MCC-M&D module

After stacking the module is assembled in the casing using glue. Fig 8-10 shows a MCC stack in the casing (without the MCC-PSU assembled).

Components MCC-C

MCC-M&D

Solder Sphere

Power Transistors

Figure 8. Assembled MCC-stack in casing (without MCC-PSU module, cabling and connectors and lid).

After assembly of the MCC stack in the casing the MCC-PSU and the connectors are soldered to the MCC stack.

Figure 9. Assembled MCC-stack in casing with PSU, cabling and connectors (without lid).

Figure 10. Complete MCC in casing

The manufacturing flow described above summarized in Fig. 11 below.

stack in casing (without PSU module, cabling and connectors and lid).

After assembly of the MCC stack in the casing the assembled and

stack in casing with PSU,

manufacturing flow described above is

Figure 11. Overview of ÅAC (SIP) manufacturing process flow

5. ENVIRONMENTAL TESTS

The environmental tests of the MCC DLR in Bremen. The test campaign consisted of the following tests, in the following order:

� Dry Heat Microbial Reduction (DHMR) simulation (125°C, 3 x 6h

� Vibration (Sine andout of plane)

� Thermal cycling (-135100 cycles)

� Power load cycling55°C base temperature

Figure 12. Two MCCs mounted for thermal cycling tests

All tests were passed without failure except for the thermal cycling in which one SRAM circuit failed after 55 thermal cycles. The failure was related to problems in the assembly processes of the specific SRAM die.ÅAC Microtec’s assembly

Substrate manufacturing

Metallization (Thin film)

Components assembly

ÅAC Factory

Sub-suppliers

Overview of ÅAC System In a Package manufacturing process flow

ENVIRONMENTAL TESTS

The environmental tests of the MCC were performed at DLR in Bremen. The test campaign consisted of the following tests, in the following order:

ry Heat Microbial Reduction (DHMR) , 3 x 6h)

(Sine and random vibration, in and

135°C to +60°C, 3 °C /min,

load cycling (1000 cycles, 0.1 Hz, temperature)

Figure 12. Two MCCs mounted for thermal cycling

All tests were passed without failure except for the thermal cycling in which one SRAM circuit failed after 55 thermal cycles. The failure was related to problems

of the specific SRAM die. ÅAC Microtec’s assembly processes are currently

Substrate stacking (SiP)

Casing, integration and verification

Delivery to customer

ÅAC Factory

being validated in an ongoing ESA GSTP project in order to avoid similar issues in future designs.

6. CONCLUSION

Even if this was mainly a R&D project, the MCC can already be considered a major technology development on many levels with respect to space business.

For the MCC product:

• A robust, low weight and low volume rover motor control chip that is compatible with harsh environment e.g. motion control chip outside a warm box in subzero temperatures storage and operation.

• Robotic arms, drills, and exoskeletons are other major characteristic applications of the MCC. In brief, the most demanding requirements for these three classes of applications are: - High performance - Small dimensions and very low mass - Compatibility with extremely low temperatures

For assembly:

• The packaging technology applied to the Motion Control Chip is not limited to this application. It is general in nature and can enable extremely miniaturized modules for harsh environments. For the MCC it enables a design that can be upgraded with ASICs

• Use of a silicon interposer equipped with through-silicon-via (TSV)

• Stacking of ceramic substrates (MCM) • Systematic use of flip-chip mounting for

increased volume efficiency • Use the “subsystem on ceramic” approach (no

PCB technology is used in the MCC) in order to cope with environment and volume constraints

For electronics:

• The MCC is built around a flash based FPGA in which communication layers, a memory manager, aLEON-3 32bit processor and various interfaces to control ADC and PWM controller are implemented.

• Fully digital control loops for motor

7. DISCUSSION

As the MCC is very versatile, one drawback is its complexity coming from its support for different motor driving applications as well as sensor interfaces. For the next generation of the MCC it will be considered to divide the substrates into easily reusable blocks to make it more scalable for different motor control applications. With this approach the total complexity for a given motor control application will be considerably lower which will give a higher yield, improve the reliability and most probably improve the performance. With a modular approach the number of substrates will increase and more substrates will be stacked on top of each other. This will lead to a more cubical form factor of the MCC which will reduce the volume and weight of the complete MCC even further.

8. ACKNOWLEDGMENT

The Motion Control Chip team acknowledges ESA for the support of the development of Motion Control Chip. Dr Johan Köhler, former technical officer at ESA, is acknowledged for his most valuable contribution on project management, design and requirements. Olivier Mourra at ESA is acknowledged for his technical inputs to the project. Pantelis Poulakis at ESA is acknowledged for his support in the final stages of the project. The Swedish National Space Board (SNSB) is acknowledged for their long term commitment of supporting advanced packaging for space applications.

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