Inspection robot

Installation and use noted for 4/6-legged inspection robot.

Mobotware on a Raspberry PI

Installation - see How-to guide on main page.

Mobotware on a Beaglebone Black

Installation - ask Søren Hansen in 326/014 or Jens Christian Andersen in 326/008.

A small guide has been made, describing how to connect to the Beaglebone Black, how to mount it's file system, change I²C frequency and how to run RHD and RHDtest. The guide can be downloaded here: BBB_guide_101v3.pdf

RHD Plugin

Features

The plugin is configured by modifying the XML-file: rhdconfig.xml.

• Setting of PID-controller gains from rhdconfig.xml

• Setting of position limits from rhdconfig.xml

• Changing of servo I²C address (range: 1-255)

• Keeps working if connection to one or more servomotors are lost

• Supports up to 24 servomotors

• Debug mode

Starting

When Mobotware has been installed, log on to the Beaglebone Black using a terminal with the following command:

ssh root@<BBB-ip>

Then navigate to the appropriate plugin folder. The exact path may vary depending on the installation. Most common paths are:

rhd/plugins/6legged/
rhd/trunk/plugins/6legged/

This folder contains the configuration file rhdconfig.xml which is used for configuring the plugin parameters such as sampling period, declaration of servomotors and their parameters. Then, from the folder containing rhdconfig.xml, the RHD is started and will load the plugin which will initiate I²C communication with the servo motor. To read out/set variables, RHDtest can be started in another terminal window.

NOTE: The RHD plugin needs to be compiled before using it for the first time after installation of Mobotware. This is done by navigating to the appropriate folder above, and writing the command "Make".

Servomotor

Hardware

The Hitec HS646-WP electric servo motor are used for the inspection robot. See Hitec's website for specifications. The servo does not provide feedback by default, and is therefore augmented with the OpenServo project's PCB and software/firmware. The PCB and firmware are modified to fit the purpose of the inspection robot, and utilizes an ATmega168 MCU (datasheet, 34 MB PDF).

Potentiometer

Mounting: Green towards motor, yellow in center and red away from motor, or maybe more likely, motor is turned 180 deg. NOTE: The A/D converter of the ATmega168 MCU is 10-bits, giving potentiometer readouts between 0-1023.

If the potentiometer or motor terminals have been inverted, this can be compensated for by changing the macro SWAP_PWM_DIRECTION_ENABLED in the firmware source file config.h. This should only be a problem if new servomotors are augmented with the OpenServo PCB and the installer are not cautious.

Power

The supply voltage should be at least 6.5V to allow the regulator to properly produce 5V for the MCU. The maximum supported voltage is around 18V (the maximum rating of the weakest components). Operating with 10-12V is considered sufficient.

PCB Layout

The layout of the modified OpenServo PCB with connections can be seen in the pictures below (front and backside). A diagram/schematic of the modified OpenServo PCB can be seen here

The modification to the standard OpenServo PCB consists of moving two capacitors to make room for the motor (visible on side A).

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  Modified OpenServo PCB, side A

• 

  Modified OpenServo PCB, side B

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  Diagram of Modified OpenServo PCB, side B

Firmware

Features

As of August 2015, the servomotor firmware for the modified OpenServo PCB supports the following:

• Sampling of Back-EMF (only up to duty-cycle of 76%, due to discharge time of armature coil).

• Sampling of voltage, position and armature current.

• PID control (The integration part is untested).

• 500 Hz PWM frequency.

• 500 Hz internal sampling rate (once per PWM cycle).

• Up to 400 kHz I²C bus frequency (Max. supported by the ATmega168 MCU).

• NOTE: Default I²C address is 5 (0x05)

• NOTE: Requires the August 2015 version of the RHD plugin to work. Earlier versions of the firmware are not compatible with this version of the RHD plugin, due to the placement and usage of MCU registers. Using earlier versions of the firmware will cause the PWM frequency divider registers to be overwritten with the wrong values!

Bugs and Issues

The February 2015 version of the firmware contains a bug that causes the firmware to crash if the PWM duty cycle becomes larger than 76% (196/255). When the firmware crashes, communication and control are lost and the servomotor will spin out of control. It is highly recommended to test if a servomotor has this bug, before use, as the gear train might be damaged! This is tested by removing a gear from the gear-train of the servomotor and then demand a large PWM duty cycle (a P-gain of 150 and error of about 500 achieves this). If the error is present, update the servo firmware to the August 2015 version (version can be seen in main.c).

Another bug in the servo firmware, is the inclusion of register 0x2F in the register range that is saved to the onboard EEPROM. Because register 0x2F is used as a communication timeout register, the value of this register changes very frequently. This implicates that in the time between a checksum has been created and register values are written to EEPROM, the value of this register has most likely changed. Thus, when restoring register values from EEPROM, the checksum will rarely match the stored data resulting in default register values being loaded. This causes the servomotor's I²C address to be reset to the default value of 10 (0x0A). This bug was fixed in the February 2015 version.

A minor issue is loss of data during communication. In the February 2015 version, when manually setting controller gains and operating with an RHD time period of less than 4.0 ms, loss of I²C communication data would occur. As of the August 2015 version of the firmware, where controller gains are set automatically from rhdconfig.xml this issue seems to have minimized itself. When operating with an RHD time period of 2.0 ms and 1.0 ms, loss of I²C communication data rarely occurs. However, this phenomenon needs further investigation.

Needs Investigation

Some features needs further testing/investigation to validate their functionality or finding errors:

• Effects of high D-gain.

• Integration part of build-in PID-controller.

• Anti-windup limits of integrator.

• On rare occasions, data loss when operating with an RHD time period of less than 4.0 ms.

• Write protection of registers 0x20 through 0x2E does not seem work. Perhaps not a necessary feature?

• Why servomotor moves to maximum position limit (when narrower than full range, 0-1023), when being below the lower position limit on power up (when above the upper position limit on power up, it also moves to the upper position limit).

Programming of Servomotors

In order to update the firmware of the servomotors, the backside is opened by unscrewing the four philips-head screws and removing the back panel. The OpenServo PCB should be visible with side B facing out. The servomotor is then CAREFULLY placed in the created programming stand. There is only one way to orient the servomotor correctly in the programming stand! When oriented correctly, the pins of the programmer will connect to terminals: MISO1, MOSI1, SCK1, RESET-P and GND-P of the modified OpenServo PCB (see diagram).

The servomotor is then programmed by connecting the programming stand to the ISP port of an STK500/600 programmer and using AVRdude.

NOTE: The servomotor needs external power while being programmed.

Programming stand.

Operation

Worth Knowing

• NOTE: Make sure that servomotor firmware is updated to the August 2015 version!

• NOTE: Even though the position range theoretically is 0-1023, in practice it is not! Due to the internal end-stops on the output axle, the full motion range is about 20-980 (one servo had a range of 20-973, another 15-988). Beware of overshooting when operating near the limits.

• The full range of the servo motor is estimated to be 207° ≈ 3.6128 rad.

• A P-gain is needed to make the servo run. A value of about 50 is a good starting point.

• A D-gain does not affect the precision/overshooting. See the "Controller Characteristics" section for test results.

• Internal sampling of Back-EMF, voltage, position and current runs at 500 Hz (once per PWM cycle).

• Obtainable external sampling rates with 400 kHz I²C bus frequency is approx. 259-312 Hz with four servomotors (a single robot leg) and approx. 41-43 Hz with 24 servomotors (six legs of four servos).

• If the servomotor is powered up while outside of the defined position range (if the position range is narrower than full range), it will automatically move to the upper position limit. If the servomotor is powered up while being within the position range, it will remain stationary until a reference is given.

Commands

By writing a command number to the scmd variable in RHDtest, the command is sent to the servomotor which performs special functions. The commands are listed in the table below. When written to scmd the format should be decimal.

Cmd. No.		Action
0x80	128	Restart the microcontroller
0x81	129	Write simple checksum to read/write registers
0x82	130	Enable PWM to motor
0x83	131	Disable PWM to motor
0x84	132	Enable write of read/write protected registers
0x85	133	Disable write of read/write protected registers
0x86	134	Save read/write protected registers to EEPROM
0x87	135	Load read/write protected registers from EEPROM
0x88	136	Restore read/write protected registers from EEPROM
0x89	137	Change servo I2C address to saddr

The registers saved to EEPROM are in the range 0x20 through 0x2E, which are the registers containing the I²C address, controller gains, PWM frequency divider and minimum/maximum position range of the servo (0-1023 is set by default).

The I²C address of a servomotor can be changed by writing the desired address to the variable saddr in RHDtest and then writing command 0x89 to the variable scmd in RHDtest. When this is done, the servomotor MCU restarts and communication will be lost. To regain communication, the servomotor I²C address needs to be changed in rhdconfig.xml (the ID field) to the new value and the RHD restarted. If the I²C address is forgotten, the servomotor needs to be reprogrammed (the default address is 5 (0x05).

Controller Characteristics

Tests have been conducted with regards to determining the precision of the servomotor depending on the PD-controller gains. The table below shows the results of different step responses. The servomotor is tested while standing on its own without any load. The steady-state position error e_ss, and error e (the step size), are in A/D units.

P = 100, e = 600
D = 0	D = 50	D = 100	D = 150
ess = 12	ess = 15	ess = 15	ess = 15

From these results it is seen that the D-gain has a very limited impact on the controller performance.

In order to avoid overshooting too much, small steps have been performed with large P-gains. The idea is to obtain a precise movement for a large step by making multiple small ones. The results can be seen in the table below. The steady-state position error e_ss, and error e (the step size), are in A/D units (approximate).

P = 400, D = 0
e = 25	e = 50	e = 75	e = 100	e = 200	e = 500
ess = 7	ess = 2	ess = 6	ess = 5	ess = 6	ess = 9

P = 400, D = 50
e = 25	e = 50	e = 75	e = 100	e = 200	e = 500
ess = 7	ess = 2	ess = 7	ess = 8	ess = 7	ess = 8

P = 400, D = 100
e = 25	e = 50	e = 75	e = 100	e = 200	e = 500
ess = 8	ess = 1	ess = 6	ess = 3	ess = 5	ess = 9

These results are approximate as the steady state error varies a little between steps. From these results it can be seen that with a P-gain of 400 and a step-size of 50, good results are obtained (i.e. a small steady-state error). Again it is seen that the effect of the D-gain is negligible.

It is noticed that steady-state errors tend to be larger when stepping on the high end of the potentiometer (i.e. steps 750-775, 800-850, 700-900 etc.). It should also be noticed that steady-state errors tend to be larger when moving from a higher position to a lower one (i.e. a step from 900 to 700 yields a larger steady-state error than moving from 700 to 900).

Further Reading

For more information, the reader is encouraged to see:

• "Haptic Sensor Based Navigation for autonomous Legged Robot", Master's Thesis, DTU, Mark Paw Hansen, March 2015

• "Control of 6-legged Inspection Robot for Iron Constructions", Bachelor's Thesis, DTU, Jan L. Svendsen, December 2013

• "Control and Design of Magnetic Foot for Inspection Robot", Bachelor's Thesis, DTU, Jesper A. Jørgensen, August 2014

• The OpenServo website.

• The short guide to using the Beaglebone Black BBB_guide_101v3.pdf