Final Report: Arachnae II

Introduction

Arachnae II is a relatively complex robot. At the time of this writing it reads 11 sensor signals and actuates 6 legs comprising 18 servo motors which gives the machine a total of 21 degree of freedoms. Keeping this mechanical complexity in mind it is obvious that the control system has to be designed so, that it makes the the control of the robot manageable. That is why we divided the controller system into smart subsystems each responsible for a specific part of the computing and linked them together with a low speed serial network (simple two-wire 2400 baud rs232c links). Following the suggestions in [24, Ferrell] we made each leg and the body an individual subsystems with its own controllers and sensory. As shown in figure 6.2, Arachnae II is organized with a central coordination processor (MCP) to which the two subsystems motion control and sensory processing are attached (the final design will have 7 subsystems). Calculation for step sequences, gait control and reading sensor data is then handled locally in the according processor nodes with only high level communication on the serial network.

Overview and Architecture

After we have chosen a proper leg design, the remaining question is, how to actuate and control them.
For Arachnae II we decided to use and implement a so-called distributed system which has a lot of advantages over a single-processor solution. A good introduction and discussion of distributed system can be found in [19, George Coulouris et al].

The fundamental arcitecture designed for the control of Arachnae is as follows:

Figure 6.1: Distributed Control System

The Master Controller is meant as supervisor of all the events on the distributed network. It does no relevant computing for step sequences or sensor readings.

The actual computation for proper gaits and step sequences is performed in the Master Motion Controller which itself is the master for its underlying 'Legs Network'. Results are sent to the Leg Control Processors which themselves calculate a trajectory corresponding to the required step sequence.

The Sensory Processing Unit on the other hand is both responsible for requesting sensor readings from an adequate sensor and testing the returned data for validity.

See a more detailed overview of the currently implemented controllers and their internetworkings in figure 6.2:

Figure 6.2: Detailed overview of Control System

In the following paragraphs are brief descriptions of the functionality and types of messages sent to and received from each processor nodes in the network.

MCP (Master Control Processor)

Type: level 0 node,       Address: 0x01,        Status: necessary

The MCP is a coordinationprocessor that is responsible for initializing and message scheduling. After a systemstart (turn on or reset) the MCP resets the connected processors (MMC and SPU. These processors are reseting the other processors). The main purpose is message scheduling. The MCP contacts the SPU (Sensor Processing Unit). After that, the SPU sends sensor-data and the MCP has to do something reasonable with it, that means that the message to the MMC (Master Motion Controller) depends on the SPU-data.

SPU (Sensor Processing Unit)

Type: level 0 node,       Address: 0x04,        Status: necessary

The SPU is initialized by a resetprocedure from the MCP. The SPU is responsible for all connected sensors. The SPU receives data from the sensors and sends it to the MCP. The MCP "knows" what it is to do with this sensor-data.

Messages from MCP -> SPU

Message Description 0x04 SENSOR SENSOR_NR The variable SENSOR means which sensor will be needed (0x27 is the ultrasonic-sensor, 0x28 is the whisker-sensor, 0x2B is the temperature-sensor and 0x2C is the light-sensor). The variable SENSOR_NR is implemented to distinguish between two sensors of the same type, e.g. our robot has two temperature-sensors.

Messages from SPU -> MCP

Message Description SENSOR DATA The SPU sends a message that contains which SENSOR was activated and the DATA from the sensor. The values of the variable SENSOR are the same from above. The DATA will be worked up from the MCP.

MMC (Master Motion Controller)

Type: level 0 node,       Address: 0x03,        Status: necessary

The MMC is initialized by a resetprocedure from the MCP. The MMC gets some movecommands from the MCP. After that, the MMC sends commands to the BIU's. Every leg has a BIU and a SCP. These two processors (BIU and SCP) are responsible for the moves (lift, back, ahead and down) of one leg. The MMC is responsible for the right sequence of one step.

Messages from MCP -> MMC

Message Description 0x03 MOVE The message is depending on the result of the SPU (Sensor Processing Unit). MOVE is a variable. If MOVE is 0x01 then the robot will walk one step forward. 0x05 means one step backward, 0x04 means turn left and 0x03 turn right.

Messages from MMC -> MCP

Message Description 0x03 The MMC sends an acknowledge-signal that everything went right. And so the MCP can do some other work.

BIU (Bus Interface Unit)

Type: level 0 node,       Address: 0x00 - 0x06,        Status: necessary

The BIU's (each leg has one BIU) is initialized by a resetprocedure from the MMC. The BIU's are connected with the MMC and are responsible for message scheduling. The MMC sends a command to a BIU and the BIU sends a message to the connected SCP. A SCP of a leg is connected to the servos of a leg.

Messages from MMC -> BIU

Message Description LEG_NR MOVE The LEG_NR variable means which leg is selected(0x01 is the first leg, 0x02 is the second leg, and so on. 0x00 means broadcasting, but broadcasting is not needed by our robot, because the serial bus(one bus for all stamps) does not work). The MOVE variable has the purpose to send a message from the BIU to the SCP that contains information about which servo has to move from one position to another(0x00 means that the leg has to be lifted up, 0x01 means that the leg has to go ahead, 0x02 is the value that tells the robot to put it's leg down and 0x03 tells the robot to put it's leg back).

Messages from BIU -> MMC

Message Description 0x31 The BIU of a leg sends an acknowledge-signal that everything went right and that this leg is ready. And so the MMC can do some other work.

SCP (Servo Control Processor)

Type: level 0 node,       Address: -,        Status: necessary

Each leg has a SCP, that is connected with the servos and the BIU of a leg. The SCP is responsible for controlling the servos. The SCP gets a command from the BIU and depending on the message from the BIU it realizes the command with help of the servos. An address is not necessary, because a SCP is only connected with a BIU from one leg.

Messages from BIU -> SCP

Message Description MOVE The MOVE variable has the purpose to send a message from the BIU to the SCP that contains information about which servo has to move from one position to another(0x03 means that the leg has to be lifted up, 0x05 means that the leg has to go ahead, 0x04 is the value that tells the robot to put it's leg down and 0x06 tells the robot to put it's leg back. 0x01 and 0x02 which are responsible for the small servo (black servo) are not needed yet).

Messages from SCP -> BIU

Message Description 0x00 The SCP of a leg sends an acknowledge-signal that everything went right. And so the BIU can signal to the MMC that this leg is ready and the MMC can do some other work.

Last updated: 26.6.1999