Blue Sparq, Inc. was contracted to develop the electronics for a laparoscopic suction pump. The driver electronics needed to
- Implement a six position capacitive touch slider and a power button.
- Drive a DC motor
- Read an infra-red sensor
The device operated in the following manner:
- The power button and the capacitive touch slider will be used to choose a D.C. motor speed level.
- The IR sensor will be used to ensure that the liquid is actually flowing.
- If there is no flow, the motor will be turned off.
Brushed DC motors are widely used in applications ranging from toys to push-button adjustable car seats. Brushed DC (BDC) motors are inexpensive, easy to drive, and are readily available in all sizes and shapes.
BDC motors are driven in a variety of ways. In some cases the motor only needs to spin in one direction. The circuits used for driving a BDC in such cases are shown below:
Simple Low Side "Chopper" Brush DC Motor Driver Schematic
Simple High Side Brush DC Motor Driver Schematic
The first is a low-side drive and the second is a high-side drive. The advantage to using the low-side drive is that a FET driver is not typically needed. A FET driver is needed to:
- Bring the TTL signal driving a MOSFET to the potential level of the supply voltage,
- Provide enough current to drive the MOSFET
- Provide level shifting in half-bridge applications
Bidirectional control of a BDC motor requires a circuit called an H-bridge. The H-bridge, is able to move current in either direction through the motor winding. A picture of this schematic is shown below:
Full H-Bridge Brush DC Motor Driver Schematic
In the above circuit when Q1 is on, Q2 is off, Q3 is off and Q4 is on, the BDC will spin in one direction and when Q1 is off, Q2 is on, Q3 is on and Q4 is off, the BDC will spin in the reverse direction. For the BDC to coast, all transistors need to be off. Finally when Q1 is off, Q2 is on and Q3 is off and Q4 is on, the motor is in the brake state.
The speed of a BDC motor is proportional to the voltage applied to the motor. When using digital control, a pulse-width modulated (PWM) signal is used to generate an average voltage. The motor winding acts as a low pass filter so a PWM waveform of sufficient frequency will generate a stable current in the motor winding.
Though the speed of a BDC motor is generally proportional to duty cycle, no motor is ideal. Heat, commutator wear and load all affect the speed of a motor. In systems where precise speed control is required, it is a good idea to include some sort of feedback mechanism in the system.
In this project, the client wanted bidirectional control of the BDC. So, a full bridge circuit was chosen. The client also wanted speed control. Hence, a PWM output was generated from our touch controller to control the motor speed.
Having a DC motor within an enclosure places a lot of stress on the enclosure. As the mechanical enclosure design got close to its final form, it is important to test its strength. We developed a simple Windows forms application in Visual C++ to test the enclosure’s durability. The provided an idea of the weaker spots of the mechanical design and allowed the customer to strengthen the enclosure.
We have a deep background in robotics and we are no strangers to motor drives and control systems. We can design a discrete motor control system as well as use off the shelf ones depending on your project needs. Call us today to see how we can help you.