Mohan Rokkampage 11/29/2019

Mohan Rokkampage 11/29/2019

Mohan RokkamPage 11/29/2019

Introduction

The goal of our project is to design and build an automated shopping cart that follows a shopper around. Ultrasonic waves are used due to the slower speed of sound compared to that of light or RF waves. This allows for greater resolution in determining the direction of an ultrasonic wave. The shopper is required to wear a wrist band with an ultrasonic beacon that directs and controls the shopping cart. The shopping cart will have directional ultrasonic receiver transducers that will be able to identify the direction of the beaconing device and follow the beacon. It also has a set of Infrared obstacle sensors that allow it to avoid obstacles. The wrist band beacon also allows the shopper the control the cart to a certain extent. The shopper can order it to stop, order it to approach the shopper, or order it to assume its normal operation of following the shopper. These commands are transmitted to the cart by incorporating a Morse code kind of signal onto the ultrasonic beacon. Using the code also allows for usage of multiple shopping carts simultaneously.

As the beacon will auto-correct the motion of the cart, plain DC geared motors can be used to run the cart. As the cart will need quite a bit of torque to keep moving when loaded, 9V DC motors will be used. These will be powered by 9V batteries. This voltage can be stepped down to be used by other components of the design. An Atmel micro-controller is used to receive the various sensor inputs and direct the motors.

Theory of Operation

The project consists of two major parts, namely, the main board on the shopping cart and the wrist-worn beacon device. The main board consists of circuitry to receive data from the various sensors, analyze them, and direct further action. The ultrasonic receiver circuitry receives the signal and amplifies it. The Atmel ATMega32L receives this amplified voltage input from the ultrasonic receivers and also input from the Infrared obstacle sensors. Based on this information, the micro-controller controls the motors through its PWM ports, adjusting the duty cycle to cause the cart to move in the desired direction.

The wrist-worn beacon device has an ultrasonic transmitter controlled by a timer to output a 40kilohertz signal. This is further controlled by a PLD state machine clocked by a second timer, to be able to allow the beacon to transmit different kinds of coded signals for the different operations of the cart. The major components of the project are listed below in greater detail.

Atmel ATMega32L micro-controller :

The micro-controller is run at 1MHz and is run off a 3.3V power rail [1]. At this speed, it draws 1.1mA of current (25oC) [1] which is relatively low enough. The micro-controller has to receive the 40kilohertz wave into the A/D converter and convert it, which takes an average of 13 ADC clock cycles [1].The reference voltage for this is supplied by the supply voltage to the chip after a fall over a small resistance (as the reference voltage has to be lower than the chip supply voltage) [1]. It also has to be able to analyze the difference in the strengths of the signals input from the three ultrasonic receivers, and determine the code encoded in the received signal. Also, the system is battery powered and needs to be as power efficient as possible. The combination of 1 Megahertz operation and using of a low power chip running off the lower 3.3V power rail allows for a good combination of computing power as well as power efficiency.

The micro-controller controls the motors via the two PWM ports. The ports control the DC motors via a H11A1 optical isolator to ensure that the micro-controller is protected from any power surges from the high power motor. The two infrared sensors are connected to the two external interrupt ports of the micro-controller. This allows for the micro-controller to not only stop at obstacles but also possibly navigate around obstacles based on the sensor generating the interrupt. Also, the ATMega32L supports on-chip programming and debugging via the JTAG ports. These ports are connected to a separate header to allow for programming them without need for a 12V external voltage.

Maxim Max1744 step down DC-DC converters:

The Maxim max1744 step down DC-DC converter has an internal feedback network that allows a pin selectable output voltage of 3.3 or 5 volts [2]. It has a maximum power output rating of over 50 watts [2] that is much more than the required amount. It also has an efficiency greater than 90% [2]. The circuit design requires three power rails. The first is a 9V power rail for powering the DC motors in order to get sufficient torque out of them. The second is a 5V rail to power the infrared sensors. The third is a 3.3V rail to power the micro-controller. The 9V supply rail can be supplied by a 9 volt battery on the board. This needs to be stepped down to 3.3 and 5 volts for the remaining components on the board. The max1744 chip can do this efficiently and can also handle any of the power requirements. It will also be able to handle any changes in the power supply as it has a wide input supply voltage of up to 36V [2].

Sharp GP2D15 IR sensor:

The Sharp GP2D15 IR sensor provides a logical high if it detects an object closer than 24 cm and a logical low otherwise [3]. This can be used as an interrupt to the micro-controller. Two of these sensors will be placed at the two front facing corners of the cart. If only one of the two sensors triggers an interrupt, the cart can take action and move away or around the obstacle. These infrared sensors are operated on a VCC of 5V @ 25oC.

Polaroid “L” Series ultrasonic transmitters and receivers:

The Polaroid “L” series ultrasonic transmitters and receivers operate at a center frequency of 40 KHz +/- 1 KHz [4]. They have a bandwidth of around 2.5-3 KHz where they provide the best results. This frequency of 40 KHz is ideal with respect to power and range requirements. Initial testing revealed an approximate range of around 4-5 feet. The small beam angle of around 72o allows for better directional sensitivity [4]. These transmitters and receivers are also pretty tolerant with respect to temperature. They require a voltage of around 9V to provide sufficient range.

Jameco geared DC motor:

The Jameco geared DC motor (part # 161373) has a pretty wide input voltage range of 4.5 – 12 volts [5]. It has a no load speed of 150 rpm, a gearing ratio of 30:1 and a stall torque of 3Kg-cm [5]. With these specs, it should be able to draw the cart at sufficient speed without too much of a problem. It also has a no load current draw of 69mA and a maximum current draw of 185mA [5]. This is a pretty large current draw but is small for a motor of this size.

IC555 Timer Chip:

The IC555 timer chip is a robust timer chip. It has a maximum speed of around 1 MHZ and can be adjusted for much lower speeds, keeping track of as low as a few seconds [6]. In our circuit, it is used as a timer to supply a 40 KHz clock to the ultrasonic transmitter. It also is used to supply a much slower clocking signal to the 22V10 PLD with a time period of around 2ms. This allows the PLD to send out coded messages on the ultrasonic signal to determine the action to be performed by the cart.

Cypress Semiconductor PALCE22V10 PLD:

The PALCE22V10 is a programmable low power highly reliable PLD that can operate as a state machine at frequencies as high as 110 MHz [7]. In our product, the PALCE22V10 is used as to maintain one of three states to determine the operation of the cart. It is clocked by a 555 timer chip and controls the power to the ultrasonic transmitter so as to be able to send coded ultrasonic signals. The state diagram is as shown below:

References

[1] Atmel ATMega32L datasheet:

[2] Maxim High Voltage Step Down DC-DC controller (Max 1744) datasheet:

[3] Sharp GP2D15 infrared sensor datasheet:

[4] Polaroid “L” series ultrasonic transmitters and receivers datasheet:

[5] Jameco Geared DC motor datasheet:

[6] The 555 timer chip datasheet:

[7] Cypress Semiconductor PALCE22V10 datasheet: