e019022.PDF

GENERAL INTEREST

I 2 C Servo Interface

control eight servos using the I2C bus

Design by C. Fuchs

Model servos have improved greatly in the past few years and now offer

greater torque and speed, so much so that they are increasingly being used

for other applications. The circuit described here controls up to eight servos

using the well-established I 2 C bus. Additional cards enable expansion up to

64 servos. Software running under Win95/98 demonstrates the features of

the circuit and allows simple programming on a PC using Visual Basic.

Generating an accurate Pulse Width Modu-

lated (PWM) signal necessary to control a

model servo is not an easy job for any com-

puter particularly so if its running a multi-

tasking operating system like Windows.

Needless to say the problem gets worse

when we add more servos and need to gen-

erate more control waveforms. In this situa-

tion the application of a little hardware often

works wonders. The solution presented here

uses a simple card driving eight servos con-

trolled from an I 2 C bus on a parallel PC inter-

face like the one presented in the March 1999

PC Topics Supplement of Elektor Electronics.

The card allows eight servos to be controlled

independently via commands sent over the

I 2 C bus. Address selector switches on the

card enable eight of these cards to be con-

nected in parallel to the same I 2 C bus thereby

providing a maximum of 64 servos to be con-

trolled from the same I 2 C bus.

other end (usually 90 degrees of

movement). The control is propor-

tional so that intermediate pulse

widths will move the arm to the cor-

responding intermediate position.

The control voltage for each of the

eight monoﬂops is produced by an 8-

channel DAC type TDA8444, which

interfaces to an I 2 C PC port and can

be controlled using software rou-

tines running in Visual basic on the

PC.

High torque/speed servos can

generate appreciable levels of elec-

trical interference and despite care-

ful PCB layout and supply decou-

pling this has led to occasional mal-

function of the TDA8444. If you plan

to use many servos it is advisable to

power the servos independently

from an external supply and use the

+5 V derived from the I 2 C connector

for the circuitry on this interface card

only.

Circuit overview

An outline of the servo controller circuit is

shown in Figure 1. The NE555 timer (IC6) is

configured as an astable multivibrator, gen-

erating a pulse every 20 ms to trigger the

eight monostable multivibrators (monoﬂops)

formed by the four 556 dual timers (IC2-IC5).

The pulse width output signal from these ICs

will control the movement of each servo con-

nected to that channel. A 1-ms wide pulse

will move the servo arm to one end of its

travel while a 2-ms pulse will move it to the

Circuit details

IC1 is the main component of the cir-

cuit diagram shown in Figure 2 it is

an 8-channel Digital to Analogue

Elektor Electronics

9/2001

GENERAL INTEREST

Converter (DAC). The voltage level

of the eight analogue outputs can be

adjusted with a 6-bit resolution, giv-

ing 2 6 or 64 steps. Information to the

DAC is supplied from the clock and

data lines (SCL and SDA) of the I 2 C

interface over the parallel wired mini

DIN connectors K1 and K2. These

connectors also allow the interface

card and servos to be powered from

the I 2 C bus. Details of the command

structure are given later.

The address of the DAC on the I 2 C

bus can be set up on DIP switch S1.

This corresponds to bits 2, 3 and 4 of

the DAC address

(0100_A2_A1_A0_0). The address

switch allows up to eight interface

cards to be controlled over the same

I 2 C bus.

The VMAX (Pin 2) input of the

DAC can be used to reduce its out-

put voltage range if it is connecting

to lower voltage circuitry. In this

IC1

U +

Logic

I 2 C

SDA

SCL

D/A

2 off

mini-DIN socket

6-way

Control

IC2.A

IC2.B

1 / 2

IC3.A

1 / 2

IC3.B

1 / 2

IC4.A

1 / 2

IC4.B

1 / 2

IC5.A

1 / 2

IC5.B

1 / 2

Trigger

NE556

20ms

IC6

NE555

1ms

U +

Power

2ms

010006 - 12

servo motor connections

Figure 1. Servo interface block diagram.

application we are using 5 V

throughout so VMAX can be left

open-circuit. The output voltage of

each channel (pins 9-16) of the DAC

are connected via resistors R3-R10 to

the control inputs (CV) of the dual

timer IC’s. The voltage level will be

approximately in the range of 0.26 V to 3.12 V.

The NE556 dual timers are connected in

the classic monoﬂop conﬁguration. The tim-

ing components R = 18.7 kΩ, C = 100 nF

together with a control voltage input of 0.26 V

on the 6.04 kΩ series resistor will give an out-

put pulse width of 1.0 ms. Increasing the con-

+5V

IC2

IC3

IC4

IC5

DIS

IC2, IC3, IC4, IC5 = 556N

IC6

OUT

555CP

+5V

THR

R11

R12

R13

R14

C10

1µ

33n

63V

IC2.A

IC3.A

IC4.A

IC5.A

DIS

THR

OUT

DIS

THR

OUT

DIS

THR

OUT

DIS

THR

OUT

+5V

100n

6k04

100n

6k04

100n

6k04

100n

6k04

SCL

SDA

C16

C11

100n

470µ

16V

+5V

IC1

R19

4x 10k

+5V

VMAX

DAC0

DAC1

DAC2

DAC3

DAC4

DAC5

DAC6

DAC7

R15

R16

R17

R18

SDA

SCL

2345

IC2.B

IC3.B

IC4.B

IC5.B

DIS

THR

OUT

DIS

THR

OUT

DIS

THR

OUT

DIS

THR

OUT

100n

6k04

100n

6k04

100n

6k04

100n

R10

6k04

TDA8444

654

K11

C12

C13

C14

C15

470µ

K10

010006 - 11

Figure 2. Servo interface circuit diagram.

9/2001

Elektor Electronics

GENERAL INTEREST

trol voltage up to its maximum of 3.12 V will

produce a 2.0 ms pulse. These two pulse

widths represent the maximum and minimum

travel of the servo and should produce a 90

degress travel of the actuating arm. In prac-

tice, component tolerances may introduce an

offset or reduce the travel so it may be nec-

essary to alter component values to achieve

exactly 90 degrees if this is important for your

application. The 1 ms to 2 ms pulse width

signal to each servo must be repeated

approximately every 20 ms. A free running

astable multivibrator formed by IC6 provides

this signal by generating a short 1 ms pulse

every 20 ms to simultaneously trigger each of

the eight monoflops. The output pulses are

then taken to the three pin output connectors

on the PCB along with the supply rails to

drive the servos.

Different servo manufacturers in the past

have always tended to use their own pin des-

ignations for the servo connectors and if you

have some of these older servos the pin con-

ﬁgurations shown in Figure 3 should help. Be

careful here, an incorrectly wired servo will

be an expensive mistake. Thankfully, the sit-

uation in the UK is improving and most mod-

ern servos share a common pin conﬁguration

and are interchangeable. As mentioned ear-

lier, servos can generate a high level of elec-

trical noise so the PCB for this circuit

is laid out with wide supply rails to

the servo connectors and large sup-

ply decoupling capacitors to reduce

this interference.

Graupner/JR

Futuba

Construction

Multiplex

Robbe

The PCB layout is shown in Figure 4.

Building the card should begin by ﬁt-

ting and soldering the more robust

components such as the wire links (9

in all), the IC sockets, connectors

and pin strips to the PCB. Next, fit

the passive components. Don’t forget

that the resistor array R19 must be

ﬁtted the correct way round. Before

the IC’s are ﬁtted into their sockets,

it is a good idea to connect the card

to ensure that the +5 V supply is

available on all the power supply

pins of the IC’s. Only if everything

looks OK can the IC’s be fitted and

the card connected to the I 2 C inter-

face. If many servos are connected it

is important to use an external +5 V

power supply and not rely on the I 2 C

bus supply.

After power up, check that the

outputs of the eight DAC channels

Microprop

Simprop

990030 - 13

Figure 3. Pinouts of some popular

servos.

are at 0 V (0.26 V maximum). Con-

nect an oscilloscope to the output of

IC6 (pin 3) and look for the short

1 ms pulse repeated every 20 ms.

When a servo is plugged into the

card, its output arm should drive to

an end position. If all functions are

satisfactory then the circuit up to the

TDA8444 should be OK.

010006-1

R19

C16

C11

C10

K11

C12

C13

C14

K10

Figure 4. Simple layout using a single sided PCB (board not available ready-made).

Elektor Electronics

9/2001

GENERAL INTEREST

is read with X = 0 to 7 corresponding to scroll

bars 1 to 8. The value of the setting is read (in

the range 0 to 63) and assigned to the vari-

able A3.

VB-variable

Start-

condition

Acknow-

ledge

chip sub-adresses 0 - 7

VB-variable

CN3

CN2

CN1

CN0

Acknow-

ledge

DacValue = VScrollX.Value

Instruction bits (F)

converter number

The maximum value MAX of the vertical

scroll bars are set at 63 so there is no need to

convert this value. The three least signiﬁcant

bits of A2 contain the code for the DAC num-

ber (0 to 7) which corresponds to that scroll

bar. Bit 4 is always set to a 0 and all of the

higher four bits are always set to a 1 (= 240

= Instruction F: sending data without an

address will always write to the same DAC).

The variable A1 contains bits 2,3 and 4 which

defines the chip sub-address. The upper 4

bits contain the ﬁxed chip address 0100 (64 d ).

The three variables A1, A2 and A3 shown

in Figure 5 represent the data telegram sent

over the I 2 C bus to the TDA8444 every time a

scroll bar is moved. The routines required for

this are provided in iic.dll . These func-

tions are defined under GLOBAL. Sending

information over the I 2 C bus begins with

ErrorMsg = StartCon( LPT)

VB-variable

Acknow-

ledge

Stop-

condition

databits for DAC

010006- 13

Figure 5. Format for programming the TDA8444.

The Software

The demonstration software

i2cservo.exe is a 16-bit program

and only runs under Win95/98 and

not under NT. The ﬁle iic.dll con-

tains software to interface to the PC

hardware and must be placed in the

same folder as the application or in

the system folder in Windows. Visual

Basic 3 was used to write the soft-

ware. Options in the menu list allow

selection of the parallel port LPT1

(378hex) or LPT2 (278hex) and also

the subaddress which is decoded on

the card by DIP switch S1. A 7 will

talk to an interface board which has

the all three switches connected to

+5 V while a 0 will talk to one which

has all three connected to ground.

The 8 scroll bars correspond to the

eight DAC output channels and

therefore the servos. Clicking and

dragging on one with the mouse will

cause the corresponding servo to

move. This simple executable

demonstration programme written in

Visual Basic 3 together with the

source code and the PCB layout ﬁles

are available as a free download

from the Elektor Electronics website.

This program is a good starting

point for developers and is relatively

easy to modify and expand.

When the program ﬁrst starts the

default chip sub-address is set to 7

(all of the address pins are switched

to +5 V)

To send the I 2 C-Start condition, and then

ErrorMsg=Transmit(LPT,A1..A3)

is used to send the three bytes. After send-

ing the I 2 C bus will be released again by

using the stop command

ErrorMsg = StopCon(LPT)

COMPONENTS LIST

Any errors detected are assigned to ﬁve error

variables Er1 to Er5 and displayed in the

lower status window. See the box ‘Error

codes’ for a list of the possible error mes-

sages. For further information on the

TDA8444 a data sheet can be downloaded

from the Philips Semiconductors website at

http://www.semiconductors.com/acrobat/data

sheets/TDA8444_3.pdf

ChipAdress = Val(Text1.Text)

Resistors:

R1 = 100 Ω

R2 = 27k

and the port used is set to LPT2.

Ω

R3-R10 = 6k Ω 04, 1%

R11-R18 = 18k

Lpt = Lpt2

7, 1%

R19 = 10k Ω 4-way SIL array

Ω

LPT1 and LPT2 are defined in

Modul1.bas and have the values

&H378 and &H278 respectively. Both

or the settings (7 and LPT2) can of

course be redeﬁned.

Capacitors:

C1 = 33nF

C2-C9,C16 = 100nF

C10 = 1µF 16V radial

C11-C15 = 470µF 16V radial

(010006-1)

Error codes

(SubText1_Change,

SubParallel1/2_Click).

Semiconductors:

IC1 = TDA8444

IC2-IC5 = NE556

IC6 = NE555

Error 0000

No failure detected

Next comes the check to determine

if the I 2 C interface is connected.

The I 2 C bus is not free.

Error 0001

Error 0002

No acknowledge received.

Error 0003

Unable to generate the stop

condition.

ErrorMsg=TestConnected(LPT)

Miscellaneous:

K1,K2 = 6-way mini-DIN socket

K3-K10 = 3-way pinheader

K11 = 2-way PCB terminal block,

lead pitch 5mm

Disk, project software, order code

010006-11

Voltage failure on the I 2 C

bus.

Error 0004

The corresponding failure message

(if any) will be output. During opera-

tion each movement of a scroll bar

The I 2 C Interface is not

connected.

Error 0005

Error 0006

An undeﬁned error has

occurred.

SubVScrollX_Change()

9/2001

Elektor Electronics

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: