Interfacing PIC Microcontrollers 21 pot

The transistor is selected for sufficient collector current handling, and the base resistor to give plenty of base current, which is calculated from the required coil current: Coil curre

since the PIC cannot provide enough current When the coil is activated, the con-tacts change over, completing the load circuit, which operates a lamp Other high power loads such as heaters and motors can also be interfaced in this way, as long

as simple on–off, but infrequent, switching is needed The transistor is selected for sufficient collector current handling, and the base resistor to give plenty of base current, which is calculated from the required coil current:

Coil current  40 mA  collector current ) Base current  40 mA/100  400
A ) Base resistor  (50.6)
400
106 17.6 k ; 10 k

The relay has a diode connected across the coil; this is a sensible precau-tion for all DC inductive loads (anything with a coil such as a motor or

Figure 8.4 Power outputs interfaces

solenoid) When the coil is switched off, a large reverse voltage may be gen-erated as the magnetic field collapses (this is the way the spark is gengen-erated

in a car ignition) The diode protects the transistor from the back EMF by forward conduction In normal operation, the diode is reverse biased and has no effect

The relay is selected for the load current and voltage requirements, and the interface designed to provide the necessary coil operating current However, it

is slow, consumes a fairly large amount of power (40 mA
5 V  200 mW), and is relatively unreliable

The relay provides low on resistance and high off resistance However, it wastes a relatively large amount of power in the coil, is slow and unreliable due

to wear on the contacts An alternative is the solid-state relay, which is typi-cally designed to switch AC loads from digital outputs with a solid-state de-vice It contains TTL buffering, isolation and triac (see below) drive in one package, with high reliability and switching speed

Triac Interface

A relay can be used to control a DC or an AC load, as it operates as a me-chanical switch However, it has significant disadvantages, as outlined above

A solid-state switch, such as a transistor, is inherently more reliable, since it has no moving parts; but the transistor can only handle current in one direc-tion, so is unsuitable for AC loads The thyristor is an alternative type of solid-state switch; it has a latching mode of operation such that when switched on,

it stays on, until the current falls to zero It can therefore be pulse operated, and used to rectify AC current By switching on at different points in the AC cycle, the average current can be controlled, allowing the power to the load to be var-ied However, it only passes current in one direction, providing DC power only The triac is basically two thyristors connected back to back, with a common gate (trigger) input, allowing current flow in both directions The full AC wave can then be utilised, with switching at the same point in the positive and neg-ative half cycles of the current A microcontroller can be used to carry out this function; the AC signal is monitored through its cycle, and the thyristor switched on at the required point in the cycle using a timer

In Figure 8.4, a simple MCU triac interface is shown An opto-coupler is used to isolate the control system from the high voltage load circuit This con-tains an LED and phototransistor, which conducts when the light from the LED falls on its base There is therefore no electrical connection between the two devices, and it will isolate output circuits operating at high voltage When the MCU output is high, the opto-switch is on, and the voltage at terminal 1 of the triac is applied to the gate, turning the triac on when the voltage passes through zero When the switch is off, the triac does not come on

This example is manually controlled, but the output power could be controlled

by monitoring the AC voltage via a feedback voltage divider and sampling it at

an analogue input An MCU timer would then be employed to control the delay between the zero crossing point in the cycle and the trigger point, where the triac is switched on each half cycle A block diagram for this system is shown

in Figure 8.5 (c)

(a)

(b)

(c)

Current

Power delivered to load

MCU

TRIAC CONTROL

Sample instantaneous voltage

Trigger

Load

AC Power supply

∼

Trigger

Time

Current

Power delivered to load

Figure 8.5 Thyristor and triac control: (a) thyristor; (b) triac; (c) MCU control

Oscillator Interface

If an output is required at a set frequency, it can be generated in a variety of ways A software loop can set the output, delay, clear the output, delay and re-peat However, this will prevent the processor from carrying out other useful tasks in the meantime Using a hardware timer and interrupts is one option; but

if these MCU resources are required for other tasks, the oscillator function can

be delegated to external hardware, so that the MCU simply switches an output

to enable the oscillator

A simple low-frequency oscillator can be implemented using a 555 Timer chip; the same chip can also be used for generating timed pulses and delays In Figure 8.4, it drives a loudspeaker via a bipolar transistor Input R on the chip enables the oscillator, and C2 controls the frequency

This is an illustration of a very important design principle A given interface can

be implemented principally in hardware or software The software implementa-tion will use more MCU resources in terms of both the available peripheral inter-faces, processor time, and programming effort The hardware approach saves on these resources, but involves additional cost, both in hardware design effort and components for each system produced Software, on the other hand, once written, has a negligible reproduction cost

Motor Interfacing

As discussed above, the basic function of a motor is to convert electrical input cur-rent into output mechanical power (torque) All use electromagnetic coils to pro-vide this conversion, and need current switches or amplifiers to operate them from an MCU

A simple method of controlling AC motors is to use a relay as switch Another is to use a triac to control the current, as outlined above, but in prac-tice there are some tricky issues associated with controlling inductive loads with thyristors and triacs which require reference to specialist texts Three-phase motors require, in simple terms, each Three-phase to be controlled by a sepa-rate device, but simultaneously, that is, three relays or triacs opesepa-rated by the same controller

Three typical small motor interfaces are shown in Figure 8.6, a DC motor, a

DC servo and a stepper motor The motors can be operated in turn by pressing the select button Operating parameters (speed, position, direction) can then be changed via the additional push buttons The control program outline is given

in Figure 8.7, and the source code in Program 8.1 The operation of each in-terface will be explained in turn

PWM Speed Control

The DC motor is controlled from the PWM output of the PIC MCU (see Chapter 6), via a power FET VN66 This has an operating current of about 1 A maximum, giving a maximum motor input rating of 12 W at the operating voltage of 12 V The motor characteristics can be set in the simulation, so a minimum motor resistance of about 10  would be suitable, as the FET itself has a forward resistance of about 1 

The VN66 is a convenient device to use as it operates at TTL level gate volt-ages; that is, 0 V switches it off, 5 V switches it on (threshold about 1 V) It has a very high input impedance, so reliability is improved by adding shunt resistance to the gate, to improve the noise immunity The diode across the motor is required to cut off the back EMF from the inductive load

When the system is started and the DC motor selected, a default PWM output is generated with 50% mark/space ratio The MSR (mark space ratio)

Figure 8.6 Motor interfaces schematic

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

; Project: Interfacing PICs

; Source File Name: MOTORS.ASM

; Devised by: MPB

; Status: Working

;

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;

; Demonstrates DC, SERVO & STEPPER MOTOR control

; Select motor and direction using push button inputs

; DC Motor PWM speed control - working

; DC Servo position control - rollover not fixed

; Stepper direction control - working

;

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

PROCESSOR 16F877

; Clock = XT 4MHz, standard fuse settings CONFIG 0x3731

; LABEL EQUATES ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

INCLUDE "P16F877A.INC"

; standard register labels

; -

; User register labels

; -

Count1 EQU 20 ; delay counter Count2 EQU 21 ; delay counter Target EQU 22 ; servo target position

; -

; PROGRAM BEGINS

; -

ORG 0 ; Default start address NOP ; required for ICD mode

; -

; Port & PWM setup

init NOP BANKSEL TRISB ; Select control registers CLRF TRISC ; Output for dc motors CLRF TRISD ; Output for stepper MOVLW B'00000010' ; Analogue input setup code

; PortA = analogue inputs

; Vref = Vdd MOVWF ADCON1 ; Port E = digital inputs MOVLW D'249' ; PWM = 4kHz

MOVWF PR2 ; TMR2 preload value

BANKSEL PORTB ; Select output registers CLRF PORTC ; Outputs off CLRF PORTD ; Outputs off MOVLW B'01000001' ; Analogue input setup code MOVWF ADCON0 ; f/8, RA0, done, enable MOVLW D'128' ; intial servo position MOVWF Target

; -

; MAIN LOOP

; -

but0 BTFSC PORTE,0 ; wait for select button GOTO but0

MOVLW B'00001100' ; Select PWM mode MOVWF CCP1CON ;

MOVLW D'128' ; PWM = 50%

MOVWF CCPR1L ;

but1 BTFSS PORTE,0 ; wait for button release GOTO but1

CALL motor ; check for speed change BTFSC PORTE,0 ; wait for select button GOTO but1

MOVLW B'00000000' ; deselect PWM mode MOVWF CCP1CON ;

CLRF PORTC ; switch off outputs

but2 BTFSS PORTE,0 ; wait for button release GOTO but2

CALL servo ; move servo cw or ccw BTFSC PORTE,0 ; wait for select button GOTO but2

CLRF PORTC ; switch off servo

but3 BTFSS PORTE,0 ; wait for button release GOTO but3

CALL step ; output one step cycle BTFSC PORTE,0 ; wait for select button GOTO but3

CLRF PORTD ; disable stepper outputs

GOTO but0 ; start again

Program 8.1 Motor control program

; -

; SUBROUTINES

; -

; Change dc motor speed by one step and wait 1ms

; to debounce and control rate of change motor BSF PORTC,1 ; switch on motor LED BTFSS PORTE,1 ; inc speed?

INCF CCPR1L ; yes MOVLW D'248' ; max speed?

SUBWF CCPR1L,W BTFSS STATUS,Z GOTO lower ; no DECF CCPR1L ; yes - dec speed

lower BTFSS PORTE,2 ; dec speed?

DECFSZ CCPR1L ; yes - min speed? GOTO done ; no INCF CCPR1L ; yes - inc speed done CALL onems ; 1ms debounce RETURN

; Move servo 10 bits cw or ccw

servo BSF PORTC,4 ; switch on servo LED BSF PORTC,7 ; enable drive chip BTFSC PORTE,1 ; move forward?

GOTO rev ; no wait1 BTFSS PORTE,1 ; yes- wait for button GOTO wait1 ; release MOVLW D'10' ; add 10

ADDWF Target ; to servo target position BSF PORTC,5 ; move BCF PORTC,6 ; forward getfor CALL getADC ; get position BSF STATUS,C ; set carry flag MOVF Target,W ; load position SUBWF ADRESH ; compare with target BTFSS STATUS,C ; far enough?

GOTO getfor ; no - repeat BCF PORTC,5 ; yes - stop MOVLW D'250' ; wait 250ms

CALL xms ; before next step rev BTFSC PORTE,2 ; move reverse?

wait2 BTFSS PORTE,2 ; yes- wait for button GOTO wait2 ; release MOVLW D'10' ; yes - sub 10 from SUBWF Target ; servo target position BCF PORTC,5 ; move

BSF PORTC,6 ; reverse getrev CALL getADC ; get position BSF STATUS,C ; set carry flag MOVF Target,W ; load position SUBWF ADRESH ; compare with target BTFSC STATUS,C ; far enough?

GOTO getrev ; no - repeat BCF PORTC,6 ; yes - stop MOVLW D'250' ; wait 250ms

CALL xms ; before next step RETURN

; Output one cycle of stepper clock step BSF PORTD,0 ; switch on stepper LED BSF PORTD,1 ; enable stepper drive BTFSS PORTE,1 ; test cw button BSF PORTD,2 ; select clockwise BTFSS PORTE,2 ; test ccw button BCF PORTD,2 ; select counter-clockwise BSF PORTD,3 ; clock high

MOVLW D'25' ; load delay time CALL xms

BCF PORTD,3 ; clock low MOVLW D'25' ; load delay time CALL xms

RETURN

; Stepper software delay xms MOVWF Count2 ; receive x ms in W down2 CALL onems

DECFSZ Count2 GOTO down2 RETURN

onems MOVLW D'249' ; delay one millisec MOVWF Count1

down1 NOP DECFSZ Count1 GOTO down1 RETURN

; Read ADC input and store getADC BSF ADCON0,GO ; start ADC

wait BTFSC ADCON0,GO ; and wait for finish GOTO wait

MOVF ADRESH,W ; store result, high 8 bits RETURN

; - END ; of source code

Program 8.1 Continued

can then be increased and decreased using the up/down buttons Note that the software has to check each time the MSR is modified for the maximum (FF)

or minimum (00) value, to prevent rollover and rollunder of the PWM value

DC Motor Position Control

DC motors cannot be positioned accurately without some kind of feedback; in applications such as printers and robot arms, the DC motors have feedback de-vices, which allow the controller to monitor the motor shaft position, speed or acceleration

In digital control systems, this is usually achieved by using a slotted wheel and opto-sensor attached to the motor shaft This may often be followed by a gearbox in the drive chain, for example, in robot arm where the output range

of movement is less that 360° The controller counts the pulses from the wheel

to determine how far the output has moved; also, the pulse frequency can be converted to speed In a printer, the linear position of the print head is moni-tored by a graduated strip attached to the traverse mechanism The accuracy of the system can be further improved by interpolation; this means the reference strip has a sinusoidal pattern so that each cycle can be subdivided by a contin-uous variation in the sensor signal

The system block diagram shown in Figure 8.7 represents a general purpose position or speed controller The motor has a slotted or perforated wheel attached Say there are 100 slots, then there will be 200 edges, giving a reso-lution of 360/200 1.8° The motor is driven via a current amplifier with a PWM signal; the speed can then be controlled, and ramped up and down to prevent the motor from overshooting the target position The MCU may act as

a slave device, receiving a position or speed command from a master con-troller, carrying it out, and then signalling completion of the operation

An alternative speed control system could use a tachogenerator to measure the speed This is a small DC generator that outputs a voltage or current in proportion

to the speed of the shaft, operating in the inverse mode to a DC motor The analogue tacho signal can then be used to control the speed Analogue position control is even simpler, in principle A pot is attached to the motor shaft, and pro-vides a voltage, which represents the position An all analogue position controller can be implemented with op-amps, which will position the output according to an analogue input signal from a pot, DAC or amplifier The main problem is that the pot only has a range of about 300°, and may not allow continuous rotation

A servo motor is one that incorporates a position feedback element In Figure 8.6, the DC servo has a built-in pot, which provides a voltage representing the position, between 5 and 0 V The motor is driven from an L6202 full bridge driver This is an IC, which provides drive to the motor in either direction under digital control A block diagram of the chip is shown in Figure 8.8

Test DC motor PWM speed, DC position step servo and stepper motor direction with push button inputs, using P16F877 (4MHz)

Main

Initialise

Port A = Analogue inputs, servo pot = RA0 Port C = Outputs, DC motors

Port D = Outputs, stepper motor Port E = Digital inputs, push buttons: Select, Up, Down PWM rate = 4kHz

Servo target value = 128 Wait for ‘Select’ button REPEAT

Select PWM mode, 50% MSR REPEAT

CALL Motor UNTIL ‘Select’ button pressed again REPEAT

CALL Servo UNTIL ‘Select’ button pressed again REPEAT

CALL Step UNTIL ‘Select’ button pressed again ALWAYS

Subroutines

Motor

IF ‘Up’ button pressed Increment speed unless maximum

IF ‘Down’ button pressed Decrement speed unless minimum RETURN

Servo

IF ‘Up’ button pressed Add 10 to target position Move forward, until target position reached

IF ‘Down’ button pressed Subtract 10 from target position Move reverse, until target position reached RETURN

Step

IF ‘Up’ button pressed Select forward mode

IF ‘Down’ button pressed Select reverse mode Output one drive pulse RETURN

Figure 8.7 Motor test program outline

The bridge circuit contains four power FETs connected such that when two are switched on together, current flows through the load When the other pair

is on, the current in the load is reversed In a motor, the direction of rotation is reversed The FETs are represented as simple switches They are controlled from a simple logic circuit (see the L6202 data sheet), as summarised in the function table Forward and reverse are selected by setting the IN1 and IN2 in-puts to opposite logic states

The chip operates from the motor supply voltage (12 V) and the digital logic supply is derived from it, so no separate 5 V supply is needed A current sensing resistor can be inserted in the 0 V connection, so that the current flow in either direction can be monitored for control purposes Bootstrap capacitors must be fitted as shown in Figure 8.6 to ensure reliable switching of the bridge FETs Although the FETs are protected internally with diodes, a series CR snubber network is connected across the output terminals

to further protect the driver chip from current switching transients

The test program allows the user to move the servo in steps The required position is represented by an 8-bit number, which is initially set to the mid-value of 128 If the ‘up’ button is pressed, the mid-value is increased by 10, and the servo started in the forward direction The actual position is monitored from the servo pot voltage read in via AD0 When the input value matches the tar-get value, the drive is stopped The servo is moved in the reverse direction in the same way

Stepper Motor Control

The third subcircuit in Figure 8.6 is the stepper motor interface This also uses dedicated hardware, because a current driver chip would be needed in any case, and the stepper controller incorporates sequencing logic, which reduces the software burden The stepper motor has a set of windings distributed around the stator, and a passive rotor, with fixed or induced magnetic poles Incremental movement of the rotor is achieved by activating the windings in a suitable sequence

PWM

MCU

Motor Drive Interface Position

/ Speed

Slotted Wheel Pulses from opto-sensor

Action achieved

Figure 8.8 Digital position control system