In order to get maximum performance from a stepper motor, careful consideration should be given as to what type of driver will be used. A well designed driver will ensure that the maximum torque is generated by the motor at the speed that is required.
The drive circuit that switches the direction of current supplied to the stepper motor coils as well as the excitation sequence of coils is called the "driver." The driver controls motor current switching in accordance with the speed and number of pulse signals input from the controller. There are two basic drive modes: Bipolar and Unipolar. The drive mode is determined by the direction of motor current, excitation mode and power supply mode.
Current Direction and Excitation Mode
The coils of a New Pentagon (Bipolar) stepper motor receive the current in two directions: the direction for generating a south (S) polarity and the other for generating a north (N) polarity. This drive mode is called the "bipolar drive mode," because the current flows in two directions. The drive mode in which the current flows only in one direction is called the "unipolar drive mode." Fig. 1 shows a basic connection diagram of the unipolar drive mode, while Fig. 2 gives a basic connection diagram of the bipolar drive mode.
Fig. 1 Basic connection diagram of the unipolar drive mode Fig. 2 Basic connection diagram of the bipolar drive mode
The unipolar drive mode has the advantages of producing useable torque out to higher speeds and making the output circuit simpler, requiring only two transistors for each motor coil phase. However, the coil utilization rate is lower than in the bipolar mode and the torque generation characteristics in the low-speed range are also slightly lower.
In the bipolar drive mode, four transistors are used for each motor coil phase. However, this mode generates higher output torque at lower speed.
The star unipolar drive type (Fig. 3) is an example of an older unipolar driver design. This design is simple in that there are only 5 transistors used. However, this drive design has low coil utilization factor and does not offer the ability to take advantage of the high resolution characteristics of a New Pentagon (Bipolar) stepper motor.
Figure 4 is an example of an older standard bipolar driver design. This design effectively utilizes the 5 coils in the motor and can take advantage of the high resolution capabilities. However, since the motor has 10 lead wires and this design requires 20 transistors, the design is more complicated and more expensive.
ORIENTAL MOTOR has developed the New Pentagon bipolar drive type that offers better operation characteristics than the older designs in terms of speed, torque, smoothness and step accuracy. A New Pentagon (Bipolar) stepper motor system using the New Pentagon bipolar driver uses a motor with only five motor leads and a driver using only 10 transistors, thereby achieving a smaller, less expensive driver design and simpler motor wiring.
Fig. 5 shows a connection diagram of the bipolar drive mode with new pentagon connection.
The New Pentagon connection is a unique connection method for New Pentagon (Bipolar) stepper motors, in which four phases are excited at a time with one phase always effectively turned OFF. In this mode, the step angle becomes 0.72°. This mode effectively suppresses vibration due to motor revolution and thereby achieves stable operation. Fig. 6 shows the excitation sequence in this mode.
In the, 4/New Pentagon (Bipolar) excitation mode, 4-phase and New Pentagon (Bipolar) excitation are alternated. The step angle becomes 0.36°, meaning that one revolution can be divided into 1,000 steps. Fig. 7 shows the excitation sequence in this mode.
Power Supply Mode
Generally, two methods are used for supplying the power to operate stepper motors. They are the "constant-voltage drive mode" and "constant-current drive mode." The most commonly used power supply mode is the "constant-current drive mode," which can be easily implemented using constant-current drive ICs that are widely available in the market.
Constant-voltage drive mode
In the constant-voltage drive mode, a constant voltage is supplied to the stepper motor coils from a constant-voltage power supply. The driver circuit has a simple structure. However, the output torque will decrease when the stepper motor is operated at high speed, because the motor current decreases as the motor coil impedance increases. One way to increase the output torque at high speed is to reduce the electrical time constant of the motor with respect to the driver by connecting a resistor RO in series with the motor coils, as shown in Fig. 8. Since the motor coil consists of a series circuit containing resistor R and inductance L, the electrical time constant of the motor is expressed as L/R. In other words, the electrical time constant can be reduced by way of increasing the value of resistor R, which can in turn be achieved by connecting a larger external resistor RO in series. In this case, however, the power-supply voltage must also be increased to compensate for the current drop resulting from connecting the external resistor.
When external resistor R0 = 3R (three times the motor coil resistance) is connected, the time constant becomes L/4R. Although the output torque at high speed increases as the value of the external resistor increases, the power consumed by the external resistor also increases and consequently more heat is generated. As a result, the efficiency will drop. Fig. 9 shows the current rise waveform and speed vs. torque characteristic when the motor is driven with an additional external resistor (L/4R).
Fig. 9 Example of characteristic improvement by driving with an additional external resistor
Constant-current drive mode
The electrical time constant refers to the transient response time of the current flowing into the motor coils when the power-supply voltage is input to the motor, and is expressed by the equation below:
The current at time T is 63.2% of the final value. Since the current waveform rises faster as the electrical time constant decreases, it can be deduced that a smaller electrical time constant enables quicker electrical response.
In the constant-current drive mode, a constant current is supplied to the stepper motor coils from a constant-current power supply. Normally in this mode, a voltage higher than the rated voltage of the stepper motor is supplied to the motor coils via a voltage chopping circuit in order to maintain a constant current in all speed ranges. Since the coils receive a high voltage, the current rises quickly and, therefore, the characteristics at high speed are significantly better than in the constant-voltage drive mode. Fig. 10 shows the basic constant-current drive circuit, as well as the current waveform applied to the coils when switching transistor Tr2 is turned ON and OFF.
Fig. 10 Basic constant-current drive circuit and corresponding coil current waveform
In the constant-current drive circuit, the coil current is measured as voltage via the current detection resistor and the measured voltage is compared against the reference voltage. Switching transistor Tr2 will then be turned ON and OFF at an appropriate pulse width according to the results of comparison circuit. The longer the ON time of Tr2, the higher the current becomes. A shorter ON time results in a smaller current.
Power Supply Voltage of the Driver
The driver receives its power-supply voltage input as alternating current (100 or 200 VAC) or direct current (24 VDC) (both input voltage driver types use the constant-current drive mode).The stepper motor characteristics at high speed vary significantly depending on whether an AC or DC input driver is used. An AC input driver converts 115 VAC to a direct current and supplies approximately 162 VDC to the motor, while a DC input driver supplies 24 VDC directly to the motor. This difference in input voltage affects the rise of the current flowing into the motor coils. AC input drivers generate a higher applied voltage and are therefore capable of generating the rated current in the motor coils even when the motor is operated at high speed. Fig. 11 compares the characteristics of AC and DC input drivers.
AC input drivers offer excellent torque characteristics in all speed ranges and are, therefore, suitable for applications requiring high-speed positioning. On the other hand, DC input drivers are smaller in size, so they are well suited for space-saving applications where the positioning equipment is small.