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Electrical drive systems and LEE stock drives |
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 | Stepper Drive Description
All Lee LPV pumps require stepper drive circuitry to make the pump motor function. The stepper drive delivers electrical power to the motor in response to low-level signals from the control system. The drive summary at the end of this section describes the recommended types. The stepper motors used on Lee LPV pumps are torque producing devices. Produced by the interaction of magnetic fields, this torque is converted to linear motion through the screw and nut in the armature. The driving force behind the stator field is the magneto-motive force, or MMF, which is proportional to current and the number of turns in the winding. This is often referred to as the number of amp-turns. The drive circuit must serve as a provider of drive current. The applied voltage is only significant as a means of supplying current.
Input signals to the stepper drive usually consist of step pulses and a direction signal. One step pulse is required for every step the motor is to take. This is true regardless of stepping mode. Most Lee LPV pumps are made with 7.5° motors, so 48 pulses are required for one complete revolution. Since most pumps are 500 steps full stroke, one revolution usually corresponds to slightly less than 10% of full stroke volume. Other combinations of step angle and steps for full stroke will yield different values.
While many varieties of control systems can be constructed, the following block diagram illustrates a typical relationship of pump and controller:
The logic section of the stepper drive is often called a translator. Its function is to take the step and direction signals and turn them into control signals for the output transistors. The basic translator functions are common to most drive types, although the translator is necessarily more complex in the case of microstepping. Different types of motor windings require different types of translators.
There are four basic drive types, each with its relative strengths and weaknesses. They are: Voltage drive, L/R drive, Current drive, and Bilevel drive. All of them can be used with bipolar or unipolar motors. A brief overview of each type follows:
Voltage Drive
Voltage drives are considered the simplest and least expensive of the design types. It basically consists of supplying a voltage equal to the rated voltage of the motor. The rise time of the current, and hence the motor response's controlled by the inductance of the motor. These drives are the least expensive, but have the poorest dynamic performance of any of the drive types.
L/R Drive
The L/R drive is similar to the voltage drive, above, but adds a resistor in series with the windings. This necessitates a higher supply voltage, to preserve the current requirement in the motor. The advantage of the L/R design is that the inductive rise time of the current is improved. This allows the current, and hence the motor response to be faster than the voltage drive. While still inexpensive to construct, this type of drive wastes power across the dropping resistor. In larger motors this loss can become very significant.
Current (Chopper) Drive
The current drive uses a supply voltage that is greater than the motor's rated voltage. This type incorporates a current-sensing feedback loop to control and maintain the current in an energized motor phase. Initially, the phase transistor turns on and the current rises according to the inductive time constant of the windings. Since the supply voltage is elevated, it is considerably faster than the voltage drive. If the transistor were left on, the current would quickly exceed the rated current of the motor. However, when the winding current gets to approximately 105% of the rated current, the feedback loop turns the winding off, preventing significant overshoot. When the winding current decays down to approximately 95% of the rated current, the feedback circuit turns the phase back on, and the current begins to rise again. This process is repeated at high frequency (20 kHz.) for the duration of the step. The translator turns the phase off at the end of the step(s). Turning on and off, or "chopping" of the current is a very effective means of controlling current to the motors. This drive possesses excellent high speed performance when high voltages are used.
A slight disadvantage of this type of drive is the relatively complex circuitry. However, there are many sources that can supply complete drive modules that can be incorporated into larger systems, minimizing the circuit design task. We recommend this drive type for use with Lee LPV Pumps.
Bilevel Drive
The bilevel is a combination of the L/R and current drive types. It is called bilevel because two supply voltages are used. When a phase is initially turned on, the current rises rapidly in response to the higher supply voltage. A short time later, when the current is at the desired level, the higher voltage is turned off, and the lower voltage is invoked, to conserve power. This is analogous to the "spike and hold" drive of solenoids. The bilevel drive has the advantage of rapid current rise, but the added expense of a dual voltage supply and some switching circuitry.
Of the four types, we recommend the Current Drive as the most appropriate for Lee LPV pumps. We can direct you to several sources for modular drives, or provide advice on component-level circuit design.
Software Control
When operating above 200 pps, we recommend starting and stopping the pump with acceleration and deceleration ramps. The profile shown below works well in most cases. High back pressure loads or high temperatures may require different ramp profiles. The case below shows 0-200 pps with no ramp, and 200-500 pps taking place over 20 steps.
Some controllers offer an additional set of profiles, sometimes called "S" curves. These are limitless in form, but are generally very useful in operating a stepper pump. Some experimentation may be required to obtain the optimum "S" shape. Lee Company Sales or Application Engineers can provide further information on this subject.
LEE stock drives
There are many acceptable ways to drive the Lee LPV Pump. We have experience with and recommend using drive modules by Intellegent Motion Systems of Marlborough, CT. or Semix of Fremont, CA USA.
Consult Lee Applications Engineering for assistance if required.
Link to Intellegent Motion Systems
Link to Semix
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For more information, please send us your application information via our ASK LEE service. Or contact our application engineers in the USA at 1-800-LEE-PLUG or our representatives around the world.
Copyright 01/2006 by The Lee Company, USA
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