View source for 5. Joint PD Control Without Hand Library ← 5. Joint PD Control Without Hand Library

In the past 4 tutorials, we learned how to control the Allegro Hand using the included grasping and motions library. In this tutorials, we will take a step back and control the hand at a lower level. We will go over basic filtering of the encoder data and PD position control of the joints.

For this lesson, we must roll back our controller code to the point before we implemented any code to move the hand joints. It may be a better idea to start a new project with the following code. Remember, you configure the project according to the DLL project properties from tutorial 1. Your code should reflect the code developed in the three steps of tutorial 2:

*[[2._Allegro_Hand_Joint_Position_Control#Allegro_Hand_Controller_Basis | 2.1 Allegro Hand Controller Basis]]
*[[2._Allegro_Hand_Joint_Position_Control#2.2 | 2.2 Hand and Joint Initialization]]
*[[2._Allegro_Hand_Joint_Position_Control#Updating_the_Controller | 2.3 Updating the Controller]]


<div class="toccolours mw-collapsible mw-collapsed">
'''myAHController.cpp'''
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<syntaxhighlight lang="cpp">

#include "myAHController.h"
#include "myAHControllerCmd.h"
 
 
myAHController::myAHController(rDC rdc) 
:rControlAlgorithmEx(rdc)
, _jdof(0)                              // everything is NULL to start
, _hand(NULL)
, _is_left_hand(false)
, _demo_mode(true)		  // will be used when
, _demo_start_time(0)	          // we make our own motion
{
}
 
myAHController::~myAHController()
{
	if (_hand)		// if there is already a hand,
		delete _hand;	// delete it before creating a new one
}

void myAHController::init(int mode)
{
 
	// this property is read from the myAHController XDL file
	// to determine if the hand we are using will be the right or the left
	const TCHAR* prop = NULL;
	prop = getProperty(_T("whichHand"));
	// if using a right hand
	if (prop && _tcsicmp(prop, _T("right")) == 0)
	{
		_is_left_hand = false;		        // used to tell left from right
		_hand = bhCreateRightHand();	// create the right hand
	}
	// if using a left hand
	else
	{
		_is_left_hand = true;
		_hand = bhCreateLeftHand();	// create the left hand
	}
	assert(_hand);				// if hand was not created, abort
	_hand->SetTimeInterval(0.003);	// control period is set (333Hz)
 
	_jdof = JDOF;				// 16 DOF
 
	_arrangeJointDevices();		// finds all hand motors and encoders
 
	_q.resize(_jdof);			        // array (16) holds current joint positions
	_qdot.resize(_jdof);			// array (16) holds current joint velocities	
	_torque.resize(_jdof);			// array (16) holds current joint torques
 
	_q.zero();				        // all positions, vel and torque set to zero
	_qdot.zero();
	_torque.zero();
 
	_demo_q_des.resize(_jdof);		// desired joint positions
	_demo_q_des.zero();			// used to create motion sequences
 
	memset(_x, 0, sizeof(_x[0])*4);	// sets x, y and z position to (0,0,0)
	memset(_y, 0, sizeof(_y[0])*4);	// for all four fingers
	memset(_z, 0, sizeof(_z[0])*4);
 
}
 
void myAHController::_arrangeJointDevices()
{
	// _jdof is 16.
	for (int i=0; i<_jdof; i++)
	{
		// oversited array intialized for storing the device name string
		TCHAR devname[32];
 
		// find all 16 motors on hand (motor1, motor2, ..., motor16)
		_stprintf(devname, _T("motor%d"), i + 1);	// device name built and stored
		_motor[i] = findDevice(devname);		// device located
 
		// find all 16 encoders on hand (enc1, enc2, ..., enc16)
		_stprintf(devname, _T("enc%d"), i + 1);
		_enc[i] = findDevice(devname);
	}
 
}
 
void myAHController::update(const rTime& t)
{
	_cur_time = t;			// controller is updated every control period
	rControlAlgorithm::update(t);
}
 
void myAHController::_readDevices()
{
	// all 16 encoder pos. values are read and stored in _q[]
	float val;
	for (int i=0; i<JDOF; i++)
	{
		if (_enc[i] != INVALID_RID)
		{
			readDeviceValue(_enc[i], &val, 4);
			_q[i] = (float)val;
		}
	}
}
 
void myAHController::_writeDevices()
{
	// all 16 motor _torque[] values are written to the motor device
	float val;
	for (int i=0; i<JDOF; i++)
	{
		val = (float)_torque[i];
		if (_motor[i] != INVALID_RID)
		{
			writeDeviceValue(_motor[i], &val, 4);
		}
	}
}
 
void myAHController::_compute(const double& t)
{
	// Computes control inputs
}
 
int myAHController::command(const short& cmd, const int& arg)
{
	// Handles user-defined commands according to cmd.
	// Further information can be retrieved from the second argument.
 
	// The variable cmd will be received from Allegro Application Studio
	// and will be used to envoke hand actions
	return 0;
}
 
rControlAlgorithm* CreateControlAlgorithm(rDC& rdc)
{
	 return new myAHController(rdc);
}

</syntaxhighlight>
</div>
</div>
<br>
<div class="toccolours mw-collapsible mw-collapsed">
'''myAHController.h'''
<div class="mw-collapsible-content">
<syntaxhighlight lang="cpp">

#ifndef __MY_AH_CONTROLLER_H__
#define __MY_AH_CONTROLLER_H__
 
#include <list>
#include "rControlAlgorithm/rControlAlgorithm.h"
#include "BHand/BHand.h"
 
#define JDOF	16
 
// myAHController inherited from algorithm interface class
class REXPORT myAHController : public rControlAlgorithmEx
{
public:
	myAHController(rDC rdc);
	~myAHController();
 
	virtual void init(int mode = 0);
	virtual void update(const rTime& t);
	virtual int  command(const short& cmd, const int& arg = 0);
 
private:
	virtual void _readDevices();
	virtual void _writeDevices();
 
	virtual void _compute(const rTime& t);
 
	void _arrangeJointDevices();
 
 
private:
 
	// algorithm variables go here
	rTime		_cur_time;	   // current time in controller
 
	BHand*		_hand;		   // Allegro Hand
	bool		_is_left_hand;	   // bool, left?
 
	rID		_motor[16];	   // motor array
	rID		_enc[16];	   // encoder array
 
	dVector		_q;		   // joint current position
	dVector		_qdot;		   // joint current velocity
	dVector		_torque;	   // joint torque
 
	int		_jdof;		   // degrees of freedom
 
	double		_x[4];		   // location of each (4) fingertips
	double		_y[4];		   // in x, y, and z
	double		_z[4];
 
	int		_demo_mode;	   // bool, used later as flag to envoke user control
	rTime		_demo_start_time;  // time at which the demo mode starts
	dVector		_demo_q_des;	   // desired position for position controller
 
};
 
#endif

</syntaxhighlight>
</div>
</div>
<br>

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'''myAHControllerCmd.h'''
<div class="mw-collapsible-content">
<syntaxhighlight lang="cpp">

#ifndef __MY_AH_CONTROLLER_CMD_H__
#define __MY_AH_CONTROLLER_CMD_H__
 
#include "rCommand/rCmdManipulator.h"
 
// These commands will be fed into command()
// and can be used to envoke certain actions
// by the robot. Allegro Application Studio
// will use these to interface with the
// cotroller plug-in.
#define BH_NONE		(RCMD_USER + 0)
#define BH_HOME		(RCMD_GO_HOME)
// #define BH_ONE		(RCMD_USER + 1)
 
#endif

</syntaxhighlight>
</div>
</div>
<br>

==Check the code==
Let's quickly compile the code above to make sure we copied everything correctly. It should compile with no errors.

Next, open up AHAS via your virtual Allegro Hand shortcut. None of the buttons should work anymore but you should see the command prompt indication that all motors and encoders have been found. These are all we need!

==Reading and Writing==
We have two functions, ''_readDevices()'' and ''_writeDevices()'', that are called every control period. For the Allegro Hand controller, this means that the encoder values are accessed and motor commands are calculated and written at 333Hz, or once every 0.003 seconds.


When ''_readDevices()'' accessed the encoder data, it saves it to the array ''_q[]" which has 16 floating point entries (once for each joint). The joint positions in ''_q'' are in radians.

Similarly, when writing a torque command to the motors, "_writeDevices()" accesses the 16-long floating-point array ''_torque[]'. The torque values are in the unit Newton-meters.


Alike ''_readDevices()'' and "_writeDevices()", the function '_compute()'' is also called every control iteration. This is where we will implement are controller code, or the computation of motor torque based on joint positions.


'''Remember:''' The 16 joint torque commands and joint positions are stored in float arrays with 16 entries each and indexed as follows:

{|style=" border-collapse: separate; border-spacing: 15; border-width: 2px; border-style: solid; border-color: #CCCCCC; padding:  0px; text-align: center; width: 200px"
|-style="background: #D0D0D0"
|'''Finger'''
|'''Joint'''
|'''Index'''
|-style="background: #F0F0F0"
|1||1||0
|-
|1||2||1
|-
|1||3||2
|-
|1||4||3
|-style="background: #F0F0F0"
|2||1||4
|-
|2||2||5
|-
|2||3||6
|-
|2||4||7
|-style="background: #F0F0F0"
|3||1||8
|-
|3||2||9
|-
|3||3||10
|-
|3||4||11
|-style="background: #F0F0F0"
|4||1||12
|-
|4||2||13
|-
|4||3||14
|-
|4||4||15
|}


Let's add a bit of code to apply a small torque to all four (4) joints on the index finger and subsequently read the joint position of each and print it to the command window. 

===myAHController.h===
<syntaxhighlight lang="cpp">

#ifndef __MY_AH_CONTROLLER_H__
#define __MY_AH_CONTROLLER_H__
 
#include <list>
#include "rControlAlgorithm/rControlAlgorithm.h"
#include "BHand/BHand.h"
 
#define JDOF	16
 
// myAHController inherited from algorithm interface class
class REXPORT myAHController : public rControlAlgorithmEx
{
public:
	myAHController(rDC rdc);
	~myAHController();
 
	virtual void init(int mode = 0);
	virtual void update(const rTime& t);
	virtual int  command(const short& cmd, const int& arg = 0);
 
private:
	virtual void _readDevices();
	virtual void _writeDevices();
 
	virtual void _compute(const rTime& t);
 
	void _arrangeJointDevices();
 
 
private:
 
	// algorithm variables go here
	rTime		_cur_time;	   // current time in controller
 
	BHand*		_hand;		   // Allegro Hand
	bool		_is_left_hand;	   // bool, left?
 
	rID		_motor[16];	   // motor array
	rID		_enc[16];	   // encoder array
 
	dVector		_q;		   // joint current position
	dVector		_qdot;		   // joint current velocity
	dVector		_torque;	   // joint torque
 
	int		_jdof;		   // degrees of freedom
 
	double		_x[4];		   // location of each (4) fingertips
	double		_y[4];		   // in x, y, and z
	double		_z[4];
 
	int		_demo_mode;	   // bool, used later as flag to envoke user control
	rTime		_demo_start_time;  // time at which the demo mode starts
	dVector		_demo_q_des;	   // desired position for position controller
	FILE* _fp; 
};
 
#endif

</syntaxhighlight>

<br>

===myAHController.cpp===
<syntaxhighlight lang="cpp">

myAHController::myAHController(rDC rdc) 
:rControlAlgorithmEx(rdc)
, _jdof(0)                              // everything is NULL to start
, _hand(NULL)
, _is_left_hand(false)
, _demo_mode(true)		  // will be used when
, _demo_start_time(0)	          // we make our own motion
, _fp(NULL)
{
	fopen_s(&fp, "C:\\myAHController.txt", "w");
}
 
myAHController::~myAHController()
{
	if (_hand)		// if there is already a hand,
		delete _hand;	// delete it before creating a new one

	if (_fp)
		fclose(_fp);
}

...

void myAHController::_compute(const double& t)
{
	// Computes control inputs
        // Joints 0, 1, 2 and 3 are commanded at 0.1 N.m
	for (int i=0; i<4; i++)
	{
		_torque[i] = 0.1;
	}
 
        // Joint positions 0, 1, 2 and 3 are printed
	for (int i=0; i<4; i++)
	{
		fprintf(_fp, "%i: %f\t",i,_q[i]);
	}
	fprintf(_fp, "\n"); // next line
}
</syntaxhighlight>

Compile and execute the shortcut in the Desktop.<br>
It is necessary to execute the shortcut with Administrator Privilege.<br>
You can see the file that has joint positions in C:\myAHController.txt file.<br>
We can now design any controller with a position input and a torque output.

==PD Control==

One of the simplest and most widely used control algorithms, Proportional-Derivative (PD) control can be easily implemented to control the position of a robotic joint.

To use PD control, we need to calculate the error between the joint's set, or desired, position and the joint's current position. For each joint, this error value and a gain value will comprise the proportional controller. Along with the current error, we will also calculate the error's rate of change from time-step to time-step. This will be calculated using the difference between the current and previous error values divided by the time-step.
[[Category:Programming Guides]]