What are some methods to control the speed of a stepper motor?
What are some methods to control the speed of a stepper motor?
Speed Control Methods
- Series Resistance
Stepper motors operate based on input. speed and current are directly proportional to each other
Potentiometers allow for the most flexibility in this setup. In cases where the load will require different torques, the demanded current varies thus the resistance must be flexible. Because stepper motors vary a great deal in capabilities, a single circuit will not suffice. For the purposes of this paper, a general case is considered.
Considerations for this method include the range of current. The resistor or potentiometer must be able to limit the current. Often times the current levels are larger then most normal components are meant to handle. The selected part must have high current tolerances as a result.
2. Gearboxes
Mechanical gears provide another solution for speed regulation. This method requires a stepper motor to connect through an intermediate gearbox. The properties of gear ratios allow for this method.
Gear Ratio= Output gear # teeth / Input gear # teeth. The output speed is given by the equation Output Speed = (Input Speed)/(Gear Ration). It is important to understand gear ratios because it affects torque, as well. Motor Torque x gear ratio = torque at the wheel (Boston Gear Catalog).
3. Voltage Regulation
The final method for stepper motor speed control is through voltage regulation. Many chips exist to achieve this, but the one discussed is the LM 555 Timer. The layout for Astable Operation may be seen below:
By selecting the proper values for RA, RB and C, the output can be manipulated into a square wave. The “high” time is given through the equation Thigh = 0.693(RA + RB)C and “low” time is given by Tlow = 0.693(RB)C. The Duty Cycle is thus given through the equation D = (RA + RB)/ (RA + 2RB). Another important piece of information is the frequency. It is given by 1/( Thigh+ Tlow) or 1.44/( (RA+ RB)C). With this information, the speed can be regulated as needed (DC Speed Control).
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