A unique type of motor useful for moving things in small increments is stepper motor. A stepper motor is a digital motor because it is moved in discrete steps; as it traverses through 360o We are using a permanent magnet type in this example. The stepper motor find extensive applications in printers, robotics etc.. The size is 360o/(Ns.Nr). Ns is the number of static pole, Nr is the number of pairs of rotor poles. Generally step size of a stepper motor depends upon Nr. These stable positions can be obtained by simply energizing the windings on anyone of the stator poles with a D.C. There are 3 different schemes available for “stepping a stepper motor”.
These are:
a) wave scheme.
b) Two phase scheme.
c) Half stepping or mixed scheme
Two – phase scheme:
Two adjacent stator windings are energized. There are two magnetic fields active in quadrature and more if the motor pole faces can be in direct alignment of the motor poles is of course possible.
Typical equilibrium conditions of the rotor are when the windings on two successive stator poles are excited. In step a) A1 and B1 are energized. The pole face S2 with B1 (N). the north pole N3 of rotor finds itself in the neutral zone between A1 (N) and B1 (N). S1 and S2 of the rotor positions themselves symmetrically with the two stator north pole.
Next when B1 and B2 are energized S2 tends to align with B1 (N) and S3 with A2 (N). Align under equilibrium conditions only partial alignment is possible and N1 finds itself in neutral regions, midway between B1 (N) and A2 (N) in slip (b). In slip (c), A2 (N) and B2 (N) are ON, S3 AND S1 tends to align with A2 (N) and B2 (N) respectively with N2 in the neutral zone. Step (b) is the case when A1 and B2 are ON. Two phase drives produces more torque than wave drives. Switching sequence for the 2 phase scheme is given:
ANTICLOCKWISE CLOCKWISE
Step | A1 | A2 | B1 | B2 | **** **** **** **** **** | Step | A1 | A2 | B1 | B2 |
1 | 1 | 0 | 0 | 1 | 1 | 1 | 0 | 1 | 0 | |
2 | 0 | 1 | 0 | 1 | 2 | 0 | 1 | 1 | 0 | |
3 | 0 | 1 | 1 | 0 | 3 | 0 | 1 | 0 | 1 | |
4 | 1 | 0 | 1 | 0 | 4 | 1 | 0 | 0 | 1 |
Hardware Descriptions:
Among the Vi range of add-on cards, the VLIM 013A supports the stepper motor interface. This board can be plugged into 8255 PPI connector in any kit using 26 core cable. It consists of 3 sections. They are:
(i) Address decoding
(ii) Stepper motor driving circuitry.
(iii) Operating voltage selection.
(i) Address Decoding:
20H – PORTA
26H – CONTROL REGISTER
(ii) Driving Circuitry:
Stepper motor requires large logic signals of relatively high power. In this board, silicon Darlington pair transistors are used to supply that required power. The driving pulses are generated under software by the interface circuit. The inputs of interface circuit are TTL pulses generated under software control using a microprocessor trainer kit. The Darlington pair transistors drive the stepper motor as they withstand high current. A 220Ω resistor and 1N4148 diode are connected between the power supply and the collector for supporting fly-back current.
PROCEDURE:
- Set up the interface and stepper motor properly.
- Determine the scheme, whether it is half-stepping, 2- phase or wave scheme.
- Connect the buses properly.
- Execute the program and verify the output.
(a) For the first program, different speeds are achieved by changing delay time.
Sample Programs
(To run a stepper motor at different speeds)
MOV AL, 80H
OUT 0026H, AL
L3: MOV SI,1050H
MOV BL, 04H
L2: MOV AL,[SI]
OUT 0020H, AL
CALL L1
INC SI
DEC BL
JNZ L2
JMP L3
L1: MOV CX, 0A03
L4: DEC CX
JNZ L4
RET
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