ELECTRICAL - EDGE



ELECTRICAL

The following sections

The subsystem decomposition of our design concepts will be presented as follows in the subsequent pages of this document:

1) Wiring and Connectors/PC 104 Enclosure

2) Power

3) Communication/User Interface

4) Coordinate Storage/Software

5) Accessories:

2) Power Distribution Board:

Overview (Features Summary)

From an electrical standpoint, one of the main jobs of the robotic platform for both the 10kg and 100kg teams is the distribution of power from the platform’s battery sources to its many subsystems. These subsystems include the motor control module, motor module, data acquisition module, small board computer, and user accessories. Each subsystem has unique power needs and therefore a robust power distribution system must be developed. The power specification of the power distribution board, and thereby the power needs for the platform, are summarized in table 2.1 and 2.2 for the RP10 and RP100 teams, respectively. The tables also specify which subsystems require a regulated (+/-5%) voltage input to operate correctly.

|Subsystem |Voltage |Current |Total Power |Regulated Source |

| |(V) |(A) |(W) |Needed? |

|Motor Control |12 |1.0 |12 |No |

|Motor Module |12 |2.0 |24 |No |

| |24 |4.0 |96 |No |

|Small Board Computer |12 |0.5 |6 |Yes |

| |5 |1.0 |5 |Yes |

|DAQ |12 |1.5 |18 |No |

|Accessories |5 |2.0 |10 |Yes |

| |12 |2.0 |24 |Yes |

Table E2.1: Platform Power Needs RP10, two motor module configuration

|Subsystem |Voltage |Current |Total Power |Regulated Source |

| |(V) |(A) |(W) |Needed? |

|Motor Control |12 |1.0 |12 |No |

|Motor Module |12 |2.0 |24 |No |

| |24 |28 |672 |No |

|Small Board Computer |12 |0.5 |6 |Yes |

| |5 |1.0 |5 |Yes |

|DAQ |12 |1.5 |18 |No |

|Accessories |5 |2.0 |10 |Yes |

| |12 |2.0 |24 |Yes |

Table E2.2: Platform Power Needs RP100, two motor module configuration

The platform is to have two sources of power; a 12V battery source for the electrical systems and a 24V batter source for the motor modules. Based on work done by the motor module teams (P07201 and P07202), a 12V sealed lead acid battery from B.B. Battery Co. (part number BP28-12) was chosen as the power source for the platform. Two of these batteries are to be stacked in series to provide the 24V needed to power the drive motors while one battery will be used to provide 12V to the electronics. Figure 2.1 shows a top level view of the platform’s power distribution needs.

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Figure E2.1: Top level platform power needs

Preliminary Power Board Design

For those modules requiring an unregulated input voltage, their supply voltage can be taken directly off the battery, as shown in figure 2.1. For the sources requiring a regulated input to function correctly, their supply voltage cannot be taken directly off the battery. This is because battery voltage is actually a function of current draw and remaining battery charge, as shown in figure 2.2.

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Figure E2.2: Battery voltage as a function of current draw and battery charge.

From figure 2.2, at full charge, the battery voltage is actually slightly greater than 12V, depending on load current. As the battery becomes depleted, its voltage drops to a value below 12V. In order to provide the regulated input voltage required by the small board computer and user accessories modules, a DC-DC regulator must therefore be employed. For those sources requiring a regulated 5V rail, a simple Buck DC-DC regulator can be used. The Buck regulator will take 12V in from the battery and output a regulated 5V. Based on work done by team P07202, a TPS5420 DC-DC regulator was chosen for this application. This regulator has many integrated features, including built-in power MOSFETS, and therefore requires only an external power inductor and a few passive components to operate. The application circuit for this part is shown in Figure 2.3.

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Figure E.2.3: TPS5420 Application Circuit

For those subsystems requiring a regulated 12V rail from the 12V battery source, a Buck DC-DC regulator alone cannot be used. This is because a buck regulator takes an input voltage VIN and provides a regulated output VOUT based on the formula.

(1)

In equation (1), D refers to the duty cycle of the switching MOSFET’s. From the TPS5420 datasheet, D can be a maximum of .87. This brings the maximum regulated output to approximately .87*12V or 10.4V. From equation (1), a Buck DC-DC regulator is limited to providing a regulated output below its input voltage.

Another type of DC-DC converter, called a Boost Regulator, is able to provide a regulated VOUT using the formula

(2)

Again, D is the duty cycle of the switching MOSFETs. Since D is once again limited to less then 1, a boost regulator cannot provide a regulated output voltage equal to its input voltage. From equation (2), a Boost regulator is able to provide a regulated output voltage greater then its input voltage.

From the preceding discussion, neither a Buck nor Boost regulator by themselves can be used to provide a regulated 12Vout from a 12V battery source. In researching this problem, two solutions emerged.

RP100 Solution

The first probable solution is to utilize as much of the completed work by team P07202, which implemented the TPS5420 DC-DC regulator for the required 5V output. In this manner, much of the ambiguity and risks of the power construction can be minimized, as the regulator circuit constructed by the P07202 team has already undergone exhaustive simulation and design review.

The solution involves breaking the regulation needed for the 12VOUT rail into two stages. The first stage utilizes the existing TPS5420 DC-DC regulator for the required 5V output created by the P07202 team. The second stage implements a Boost DC-DC regulator to boost the battery voltage up to 12V using the 5V regulated voltage from the TPS5420. After researching Boost converters from the major power management suppliers, the chosen Boost DC-DC regulator is the National Semiconductor LM3478 High Efficiency Low-Side N-Channel Controller. The application schematic of the LM3478 is shown in Figure 2.4.

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Figure E2.4: LM3478 Application Schematic

The utilization of the LM3478 regulator enables a simple solution to be implemented to solve the power board problem, although its efficiency level is at approximately 85% during current loads of approximately 1A, as shown in Figure EE2.4a.

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Figure 2.4a: LM3478 Efficiency

RP10 Solution

The second idea is to remove two of the 5V regulators and replace them with one power management IC which is able to take 12Vin and output 12V, alternating between Buck and Boost modes as needed. After searching the major power management IC suppliers, the LTC3780 from Linear Technology meets these needs. This device uses proprietary technology to automatically switch between Buck, Boost, or Buck-Boost modes to provide a regulated output at, below, or above the battery voltage level at efficiencies up to 98%.

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Figure E2.5: LTC3780 Application Schematic

The ability to operate as a Buck, Boost, or Buck-Boost regulator gives this part extraordinary versatility – it will allow the user to program the output voltage of this part anywhere from .8V up to 15V. Through the use of different external MOSFETS, the maximum current draw of the device can be increased or decreased, depending on user needs. The high efficiency of the converter is also a selling point – in a battery operated system like the robotic platform, higher efficiency equates to longer runtime.

Simulation

RP100 Solution

The available method to simulate the operation of the LM3478 regulator is the National Semiconductor WEBENCH Simulator Tool. The simulation program allows the simulation schematic, as seen in Figure EES1a, to be constructed and its operation to supply the required 12VOUT with 2A from the 5VIN to be verified.

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Figure EES1a: WEBENCH Simulation Schematic

Simulation of the LM3478 application schematic began with the startup of the either the RP100 or RP10 platform, with a minimum VIN of 0V, VINmax of 5.1V, a trise of 2ms, and a load of 6Ω to simulate worst case scenario load in which 2A is actually needed at the output.. All the passive components seen on the simulation schematic were chosen by National Semiconductor, and only the input voltage and its range of values as well as the required output voltage of 12VOUT were taken as “inputs” to the calculation of the passive component values. As seen in Figure EES1b, the LM3478 does not suffer from startup current spikes, although its performance does degrade when the worst case scenario of 2A is required at the load. The voltage has a smooth startup, also reaching its required value of 12VOUT after 5ms. During worst case startup, the LM3478 is able to achieve a Vmax of 12.56V, which reveals its limitation of fluctuating within +/-5% during extreme loads. The same range of fluctuation exists for the current output during extreme loads.

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Figure EES1b: LM3478 Startup

In the unlikely event that the input voltage dips slightly below the 5V level to 4.9V, the LM3478 will be able to keep the output voltage at slightly above 11.8V, while VIN fluctuations up to 5.1V will yield an LM3478 output voltage of 12.5V. During these VIN fluctuations however, the current supply at the output, during a worst case load, will be at exactly 2A. This was observed in the transient input simulation of the LM3478 application circuit. The simulation is seen in Figure EES1c.

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Figure ES1c: LM3478 Transient Input Simulation

In the event that the load current shifts or fluctuates between two values, such as 1.5A and 2A, the LM3478 is able to keep the voltages stabilized between an extreme VOUT,min of 11.7V and a VOUT,max of 12.58V. The current values, however, are able to be kept at the constant required values of the load. This transient load simulation is seen in Figure EES1d.

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Figure ES1d: LM3478 Transient Load Simulation

During steady state operation, without the worst case scenario of a current load equivalent to 2A, the LM3478 operation is stable, as seen in Figure EES1e. The VIN value has been set to 5V, which is the expected output value of the TPS5420 DC-DC regulator while the output current has been set to 1A, which is an approximate load current of a random sensor. During steady state operation, the output load current value is maintained at exactly the required current value, while the output voltage is maintained within the +/- 1% range.

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Figure ES1d: LM3478 Steady State Simulation

RP10 Solution

Linear Technologies also provides a way to simulate operation of their ICs. To simulate the LTC3780, they provide a version of SPICE they call SWCad III. Using this simulation program, and the schematic shown in figure 2.5, the part was simulated to verify it would meet our regulation needs.

During simulation, the input voltage was varied between 13V (fully charged battery) to 9V (depleted battery). The output voltage for the part was set at 12V using resistors R7 and R8. A 5.5Ω resistive load was also placed on the device to simulate a worst case load of ~2A. All other components were picked based on Linear Technology’s application circuit. The results of the simulation are shown in figure 2.6.

.

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Figure EE2.6: SWCad III Simulation Schematic

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Figure E2.7: LTC3780 Simulated 12Vout with 9V ................
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