Controlling Motors using PSLab Device

PSLab device is capable of building up a complete science lab almost anywhere. While the privilege is mostly taken by high school students and teachers to perform scientific experiments, electronic hobbyists can greatly be influenced from the device. One of the usages is to test and debug sensors and other electronic components before actually using them in their projects. In this blog it will be explained how hobbyist motors are made functional with the use of the PSLab device.

There are four types of motors generally used by hobbyists in their DIY(Do-It-Yourself) projects. They are;

  • DC Gear Motor
  • DC Brushless Motor
  • Servo Motor
  • Stepper Motor

DC motors do not require much of a control as their internal structure is simply a magnet and a shaft which was made rotatable around the magnetic field. The following image from slideshare illustrates the cross section of a motor. These motors require high currents and PSLab device as it is powered from a USB port from a PC or a mobile phone, cannot provide such high current. Hence these type of motors are not recommended to use with the device as there is a very high probability it might burn something.

In the current context, we are concerned about stepper motors and servo motors. They cannot be powered up using direct currents to them. Inside these motors, the structure is different and they require a set of controlled signals to function. The following diagram from electronics-tutorials illustrates the feedback loop inside a servo motor. A servo motor is functional using a PWM wave. Depending on the duty cycle, the rotational angle will be determined. PSLab device is capable of generating four different square waves at any duty cycle varying from 0% to 100%. This gives us freedom to acquire any angle we desire from a servo motor. The experiment “Servo Motors” implement the following method where it accepts four angles.

public void servo4(double angle1, double angle2, double angle3, double angle4)

The experiment supports control of four different servo motors at independant angles. Most of the servos available in the market support only 180 degree rotation where some servos can rotate indefinitely. In such a case, the servo will rotate one cycle and reach its initial position.

The last type of motor is stepper motor. As the name says it, this motor can produce steps. Inside of the motor, there are four coils and and five wires coming out of the motor body connecting these coils. The illustration from Wikipedia shows how four steps are acquired by powering up the respective coil in order. This powering up process needs to be controlled and hard to do manually. Using PSLab device experiment “Stepper Motor”, a user can acquire any number of steps just by entering the step value in the text box. The implementation consists of a set of method calls;

scienceLab.stepForward(steps, 100);

scienceLab.stepBackward(steps, 100);

A delay of 100 milliseconds is provided so that there is enough time to produce a step. Otherwise the shaft will not experience enough resultant force to move and will remain in the same position.

These two experiments are possible with PSLab because the amount of current drawn is quite small which can be delivered through a general USB port. It is worth mentioning that as industry grade servo and stepper motors may draw high current as they were built to interact with heavy loads, they are not suitable for this type of experiments.

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Create a Distance Sensor using PSLab

PSLab device is a small lab which supports a ton of features. Among its many features, integrating a distance measuring sensor like HC SR04 sonar sensor into it is one of them. This blog post will bring out the basic concepts behind a sonar sensor available in the current market, how it measures distance and how it is implemented in the PSLab device.

A sonar sensor uses a sound wave with a very high frequency. These waves are called ultrasonic waves. They cannot be heard by the naked ear. Human ear can only hear frequencies from 20 Hz up to 20 kHz. Generally HC SR04 sensors use a wave with frequency as high as 40 kHz so this makes sense. The basic principal behind the sensor is the reflectance property of sound. Time is calculated from the transmission time up to the time receiving the reflected sound wave. Then using general moment equation S = ut; with the use of speed of sound, the distance can be measured.

The figure shows a HC SR04 ultrasound sensor. They are quiet famous in the electronic field; especially among hobbyists in making simple robots and DIY projects. They can be easily configured to measure distance from the sensor up to 400 cm with a measuring angle of 15 degrees. This angular measurement comes into action with the fact that sound travels through a medium in a spherical nature. This sensor will not give accurate measurements when used for scenarios like measuring distance to very thin objects as they reflect sound poorly or there will not be any reflectance at all.

There are four pins in the HC SR04 sonar sensor. Corner pins in the two sides are for powering up the Sonar sensor. The two pins named ECHO and TRIG pins are the important pins in this context. When the TRIG pin (Trigger for short) is excited with a set of 8 square pulses at a rate of 40 kHz, the ECHO pin will reach to logic HIGH state which is the supply voltage (+5 V). When the transmitted sound wave is reflected back to the sensor, this high state of the ECHO pin will shift to logic LOW state. If a timer is turned on when the ECHO pin goes to logic HIGH state, we can measure how long it was taken for the sound beam to return to the sensor by turning off the timer when the ECHO pin goes to logic LOW state.

Having described the general implementation of a sonar sensor; a similar implementation is available in PSLab device. As mentioned earlier, TRIG pin requires a triggering pulse of 8 set of square waves at 40 kHz. This is achieved in PSLab using SQR pulse generating pins. The time is measured from the transmitting point until the receiving point to evaluate the distance. The real distance to the obstacle in front of the sensor can be calculated using following steps;

  1. Measure total round trip time of the sound beam. Take it as t
  2. Calculate the time taken for the beam to travel from sensor to the obstacle. It will be t/2
  3. Use motion equation S = ut to calculate the actual distance taking u = speed of sound in air. Substituting the time value calculated in step 2 to t, S will produce the distance

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