The Lee Company’s Variable Volume Pumps are driven using a bipolar stepper motor. The stepper motor is a brushless DC electric motor characterized by the discrete number of steps in a rotation. This feature allows a motor to be held at a certain step without the need for any feedback, resulting in a very precisely controlled pump. Our pumps utilize two different styles of bipolar stepper motors; a Hybrid NEMA Type 17 and a “Stacked Can” ? 1.5" motor. The NEMA 17 is characterized by high torque and precision, whereas the stacked can motor offers a lower profile and lighter weight.
Stepper motors can be driven in a variety of ways, so it is important to consider the operating current and voltage requirements when selecting a motor controller. While a stepper motor is limited by a certain number of discrete steps per revolution, a microstepping enabled controller allows the motor to move only fractions of a step. A microstep is generally defined as splitting a step into multiple component parts. This is accomplished by very carefully controlling the current in each stepper motor phase, which can effectively produce multiple partial steps within a single step thereby increasing the dispense resolution.
The trade-off to microstepping is that the increments typically decrease by a power of two (e.x. 1/2, 1/4, 1/8, etc.), because dividing a step into smaller parts requires both phases to be energized at once (at different levels). This results in a reduction of the output torque which may limit the discharge pressure of the pump. In practical applications, a maximum of 1/16 microsteps per step should be implemented, but the allowable microstep increment is determined by the stepper motor controller.
Most Lee Variable Volume Pumps include a home sensor, also known as an end of stroke sensor, which is used to indicate when the pump has reached a full dispense position (piston is fully extended). The home sensor is an optical-electrical device which relies on an LED emitter (in the infrared spectrum), and a sensor. Placed within a line of sight of one another, once that line of sight is broken the sensor switches from a HIGH to a LOW state.
To interface with the home sensor, the controller requires an input as well as an output to power the components. Because the home sensor emitter is an LED, which is a current-driven device, an integrated resistor is included to limit current. Some stepper controllers include an integrated resistor which may need to be removed or modified to function properly. The Lee Company has the capability to provide a pump design with the resistor modified or removed if necessary.
Some Lee Variable Volume Pumps include an encoder, which is an optical electronic device that provides a pulse output as the motor rotates. The encoder output is not intended as an indication of fluid dispense, but rather as verification that the stepper motor has completed the requested number of steps. The standard encoder used on our variable volume pumps is 200 steps per revolution, but encoders with 400 steps per revolution can be supplied as a custom design.