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Stepper motor drive

    Influence on the accuracy of the peristaltic pump The bioreactor’s requirements for feeding materials, acid and alkali in the process determine the equipment’s high requirements for the control accuracy of the peristaltic pump, and there are many ways for the stepper motor to control the peristaltic pump, which are used in bioreactors There are usually three types of Pulse, Analog and RS-485 communication. In order to study the intuitive effects of these three control methods on the control accuracy of the peristaltic pump, BIOREACTEK™ specially conducted a comparative experiment.


    First of all, we must first understand what these three driving methods are? 

    Pulse quantity: It is a digital quantity whose value always alternates between 0 (low level) and 1 (high level). The number of times the Pulses alternate per second is called the frequency. The Pulse amount is mainly used for position control, speed control, torque control, etc. of stepper motors and servo motors.

    Analog quantity: It is a continuous signal quantity such as voltage and current. An Analog signal is a signal whose amplitude changes continuously with time. After sampling and quantization, it is a digital quantity. Most of the Analog quantities are non-electricity quantities, while PLCs can only handle digital quantities and quantities of electricity, so to realize the conversion between them, a sensor is required to convert the Analog quantities into digital quantities of electricity. If this power is not standard, it is also necessary to convert the non-standard power into a standard electrical signal through a transmitter. In addition, an Analog input unit (AD) is required to convert these standard electrical signals into digital signals.


    RS-485: It is a serial data interface standard. In order to expand the communication capability of the application range, multi-point and two-way communication capability is added, that is, it allows up to 32 receivers to be connected on a balanced bus, and also increases the transmitter’s The drive capability and communication collision protection features extend the common-mode range of the bus through differential transmission.


    # Experimental test #


    01 Experimental Design

    Test 3 kinds of peristaltic pumps with different drive modes in the two cases of “pumping the same volume of liquid at different speeds” and “pumping different liquid volumes at the same speed”, and judge the step by the difference between the liquid volume and the set value The effect of the motor drive method on the accuracy of the peristaltic pump when it is different from the calibration parameters.


    02 Experimental methods and steps

    (1) Experimental materials

    Pulse volume peristaltic pump (calibrated before test) Analog volume peristaltic pump (calibrated before test) RS-485 communication peristaltic pump (calibrated before test) 16 gauge silicone tube graduated cylinder 1 (range: 20mL, accuracy: 0.5mL ) measuring cylinder 2 (volume: 50mL, accuracy: 1mL) beaker of pure water


    (2) Experimental steps

    • Fasten the No. 16 silicone tube into the center of the peristaltic pump roller, and fasten the cover to fix the peristaltic pump;
    • Place both the liquid inlet and outlet ends of the silicone tube in a beaker filled with pure water;
    • Set the manual speed to discharge the air in the silicone tube. When the pure water pump reaches the liquid outlet port of the silicone tube, that is, the position of the red line in the figure below, stop the pumping;

    • Place the liquid outlet end in a measuring cylinder with a suitable range, and be careful not to stick to the wall to prevent the liquid hanging on the wall from affecting the experimental results;
    • After setting the speed of the peristaltic pump and the volume of the target pumped liquid on the equipment, run the peristaltic pump and record the running time;



    • After the pumping is over, observe and record the actual volume of the pumped liquid;
    • Repeat the above operation to test and record the operating data of all peristaltic pumps.


    03 Notes

    Before the experiment, the peristaltic pump uniformly uses a 16-gauge silicone tube to pump out a volume of 20 mL of the pumped liquid at a speed of 50 rpm for calibration. During the course of the experiment, no further calibrations can be performed, regardless of the difference in the pumped fluid volume. When reading the volume of the pumped liquid, place the graduated cylinder on a flat table so that the line of sight is level with the lowest part of the liquid concave surface in the graduated cylinder.


    04 Experimental data analysis

    (Stepping Motor Drive Accuracy Test)


    (Schematic diagram of accuracy test scatter)


    As can be seen from the data in the table, once the speed and pump output are changed, the peristaltic pumps controlled by different driving methods will have different results. Among the three drive modes, the RS-485 communication mode has the best control accuracy, followed by the Pulse, and the Analog quantity has the largest error.

    Advantages of RS-485 Communication

    In addition to giving the peristaltic pump high control accuracy, the RS-485 communication method also has the following advantages:

    • The system runs stably – the concentrator and the master station are connected safely and reliably by a dedicated communication bus. Unless the device interface hardware is damaged, or the bus line is disconnected, the bus meter reading system will always maintain a good communication effect and success rate of reading.
    • High communication rate – due to the use of a dedicated wired communication line connection, there is no other signal on the line except for communication signals. And the attenuation after the external interference signal is coupled to the line is very large, so the concentrator can communicate with the master station at a higher rate.

    Anti-interference – The RS-485 bus channel is a dedicated communication channel. The shielding added to the communication cable can effectively ensure the communication effect, so it has strong anti-spatial interference performance.