CIRCUS

Overview

At the time in which this activity started, CIRCUS was supposed to be one candidate to be the controller of the PAT (PAyload Tutor) arm, a robotic system proposed by the Italian Space Agency devoted to perform payload servicing inside the ISS pressurized laboratories.

The main contraint in the CIRCUS development has been the compactness: the robotic system must be easy to be transported and relocated by ISS Crew. Therefore the control electronics must be light and distributed on the arm structure itself.

While CESAR and SPEAR, the hardware developed in the frame of the "Space Robotics Components Development", has been designed to operate in open space (imposing the use of bulky and expensive space compatible components), the benign pressurized ISS environment allows for the use of industrial grade components with high integration factor, small packaging dimensions and lower cost.

Aim of CIRCUS has been to realize and produce in several specimens, at rather low cost, a compact, integrated robot controller and servo amplifier unit, to support internal ISS robotics.

CIRCUS architecture

The architecture of CIRCUS is basically the same as CESAR. It is composed of a Robot Control Unit (RCU), which performs the most computation intensive high level tasks, and a set of more or less intelligent slave modules, named Servo Control Units (SCU), which control the robotics hardware (servo drives, sensors).

In CIRCUS, the functions allocated to the RCU are the same as the one in CESAR. For what regards the SCUs, the functions of Servo Amplifier and Current control (present in SPEAR) have been added to the ones present in the CESAR SCUs.

The major components of CIRCUS are:

• one Robot Control Unit (RCU) - to govern and interface the arm
• 8 Servo Control Units (SCUs) - to command and control the single arm joint
• one Power Supply Unit (PSU) - to supply the whole system

The RCU and SCUs host the Internal Bus I/F which is in charge to carry out the data and power interfacing of all units. (The Internal Bus I/F and the PSU realise together the Electrical Distribution Unit).

CIRCUS RCU

The RCU sustains the command interface with the operator between an external System Controller Device, either the EXPRESS Rack and/or the Crew PC for the space application.

The RCU receives commands and sends telemetry from/to the System Controller Device via a RS-422 serial line, interprets and executes the commands, performs robot motion planning and interpolation, and sends servo level commands to the SCUs.

The communication between RCU and SCUs is realised via a dedicated bus (SERCOS) implemented as a part of the Electrical Distribution / Power Supply Unit.

The RCU is mainly composed by two parts:

• the RCU Base Board;
• the Internal BUS I/F board.

Both boards are according to PC104 standard and realise a stacked board system.
The RCU base board hosts the CPU (EPSON Card-PC) on it bottom face and so it has to be at the bottom of the stack. This architecture allows any expansion of system functionality, adding to the stack others PC104 standard expansion boards.

CIRCUS SCU

The SCU (Servo Control Unit) is interfaced to the internal bus to receive main and auxiliary power signals and exchange data with the RCU, receiving position and velocity set-points and operation commands.

The SCU is implemented around an industrial DSP (TMS320F240) addressed to motor control. It is a nearly single-chip based solution to realise the best function integration with the minimum layout implementation.
Each SCU drives one robot joint equipped with Brushless Motors (BM), motor shaft resolver, output shaft position resolvers (coarse and fine), failsafe brake and force/torque sensor. Each SCU implements the functions of µ-interpolator, position/velocity/current/torque control, motor commutation (based on resolver reading), PWM power amplifier and on/off brake control.

The SCU consists mainly of:

• the DSP;
• the Power and Data I/F (Power switch, Latch-up protection, SERCOS controller);
• the Arm Joint I/F (power bridges and signala mplifiers);
• the Servicing Circuitry (PROM, oscillator, hardware watchdog, current watchdog, digital I/O, latch-up protection and safety chain driver).

CIRCUS adaptation for ground demonstration

In order to implement a ground demonstrator, The CIRCUS hardware has been modified to make it able to cotrol any robot arm and, in particular, the Mitsubishi PA-10 robot arm. This has been done also in view of upgrading, in the frame of the CONTEXT project, the prototype of the EUROBOT testbed, based on three PA-10 arms. At the same time, CIRCUS is developed to control a low/medium sized space robot for internal operation on the ISS.

This project orientation choice heavily impacts the design of the servo power controllers and the power supplies in order to cover specific electrical and safety requirements addressed to PA-10 arm.
For example, the +100Vdc needed to power the PA-10 motors had obliged the use of motor drivers with optoisolated stages, increasing the complexity and the size of the related electronic board.
The same +100Vdc required the introduction of a new power supply unit (APSU), as well the real control of the robot arm (PA-10) requested the implementation of all needed safety devices (extendible safety chain, emergency push button, etc.) to assure safe operations in a ground stand alone configuration.

The resulting new system is a compromise between a small and highly integrated design addressed to space operation and a design of an industrial ground equipment.
Less lightweight and compactness have been compensated by more adaptability and flexibility in general use (for space and ground environments).
In any case the resulting CIRCUS system is miniaturised and installable inside a low/medium size space robot arm, also with motor power which exceeds the usual low power requirements of typical space application and is a candidate for controlling the robotic arm being developed in the frame of the DEXARM activity.