PDM Installation & Configuration: Logical Functions
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Logical Functions
08.03
| 00:00 | - One of the most basic functions of a PMU is to replace the relays in an automotive electrical system. |
| 00:06 | Relays are used in our electrical systems to allow us to switch on and off high load circuits. |
| 00:12 | As in those that draw a lot of current with a separate low current signal. |
| 00:17 | A good example of this is a common fuel pump circuit. |
| 00:20 | The vehicle ECU needs to control when the fuel pump is turned on and off but it cannot directly supply the high level of current the fuel pump requires to operate. |
| 00:30 | Instead, it hands off the switching of this higher current auto relay which it then controls with its low current output. |
| 00:37 | A relay is really nothing more than an electrically controlled mechanical switch. |
| 00:42 | PMUs replace relays by performing this same switching with solid state electronic components called MOSFETs. |
| 00:49 | MOSFETs are conceptually very similar to relays but they have no moving components and are much smaller. |
| 00:55 | This lets the manufacturers mount lots of MOSFETs inside a single enclosure, giving us PMUs with lots of switchable outputs all independently controlled. |
| 01:04 | So instead of an individual relay being required for each high current switching application, we now have a single integrated module, the PMU doing the switching for the entire system. |
| 01:16 | PMUs take things a step further too. |
| 01:19 | Instead of giving us just a bunch of MOSFETs inside a single enclosure that we still have to manually turn on and off, they put a micro controller in there that lets us configure the power switching in really useful ways. |
| 01:31 | The micro controller collects information from various sources like digital inputs, analog inputs and CAN bus communications. |
| 01:38 | Then performs logic operations based on this information combined with our configuration to determine if a MOSFET which is an output, should be switched on or not. |
| 01:49 | This is a really important point as this breaking of that direct hardwired connection between the inputs and the outputs is where all the flexibility of a PMU comes from. |
| 01:58 | We're now able to have the PMU follow a complex decision path to determine if an output should be switched on or not. |
| 02:05 | That might sound a little confusing but let's take a look at a quick example and hopefully things will become a little clearer. |
| 02:11 | Suppose we have a fuel system set up with two fuel pumps. |
| 02:15 | One of which is the primary pump and the other being used for back up purposes. |
| 02:19 | With a PMU, we can configure the logic around how these fuel pumps operate. |
| 02:24 | Most commonly the ECU in the vehicle decides when fuel pressure should be at the fuel rail, so we can use this signal from the ECU to determine if the primary fuel pump should be switched on or not. |
| 02:37 | The PMU can also be sent a fuel pressure signal and we'll be internally monitoring the current flowing to that primary fuel pump as we've discussed in the current faults course section. |
| 02:49 | We can then configure the PMU to switch on the secondary fuel pump in the case that there is a fault with the primary fuel pump electrical circuit which it will detect, or the fuel pressure falls below a certain value. |
| 03:03 | This means we now have a system with good redundancy. |
| 03:06 | In the event of a primary fuel pump failure or a particularly high load event that demands more fuel than the primary pump can supply, the secondary fuel pump will step in to help out. |
| 03:15 | We can even configure the PMU to send out a warning signal that this has occurred, letting the driver and pit crew know that there could be something in the fuel system which needs to be investigated. |
| 03:26 | I've used some specific language in this example, the words if and or. |
| 03:31 | These are what are known as logical operators and are at the heart of how we configure our PMUs. |
| 03:37 | We'll have a look at the basic logical operators next to get you familiar with them but you shouldn't be intimidated, you'll already have a good understanding of how these logical operators work as they function mostly the same way as they do in our everyday language. |
| 03:51 | The first operator we'll look at is the if operator. |
| 03:54 | This is where we configure the PMU to do something if another condition is met. |
| 03:59 | In the case of the example we're looking at we're configuring the PMU to turn on the fuel pump system in the case that the ECU is commanding it. |
| 04:07 | It is as simple as that, if this input condition is met, then perform that required output action. |
| 04:14 | The rest of the logical operators build upon the if operator, adding in clauses to alter the overall behaviour. |
| 04:20 | The first of these is the not operator, using this has the affect of inverting the result of an if operator. |
| 04:27 | Instead of the PMU performing a function if an input condition is met, it will now perform that function only if that input condition is not met. |
| 04:36 | For example, if I have a PMU controlling a vehicle's headlights and daytime running lights, I would program the PMU to switch on the daytime running lights when the headlights are not switched on. |
| 04:47 | So if the headlights are not on, switch on the daytime running lights. |
| 04:53 | We can also have operators that base their output on multiple input conditions. |
| 04:57 | Let's take a look at them now. |
| 04:59 | The first is the and operator. |
| 05:01 | Using this operator lets us command the PMU to do something only when multiple input conditions are all met. |
| 05:07 | If condition one and condition two and condition three are all met, then perform the required output action. |
| 05:15 | An and operator can have just two inputs or as many as you'd like. |
| 05:19 | But it will only result in the output actually happening when all of those input conditions are satisfied. |
| 05:26 | Next up is the or operator. |
| 05:28 | This is similar to the and operator except that the output action will happen if any of those input conditions are met. |
| 05:34 | If input condition one or input condition two or condition three are met, then perform that required output action. |
| 05:42 | Conditions one, two and three could all be met or any combination of them, as long as one of them is met, the PMU will perform that output action. |
| 05:51 | The XOR operator is the least intuitive of them all because it's not one that we use in our daily language. |
| 05:58 | XOR is writen XOR and is short for exclusive or. |
| 06:02 | This is just like an or operator but now only one of those input conditions can be met for the PMU to perform the output action. |
| 06:10 | Condition one or condition two or condition three must be met but only one of them at a time. |
| 06:17 | If we have a look back at our fuel pump example, we can break it down into chains of these operators. |
| 06:22 | The first of these is a simple if operator based on the ECU command. |
| 06:26 | If the ECU is commanding it, turn on the primary fuel pump. |
| 06:31 | The second is another if operator but combined with an and operator and an or operator. |
| 06:38 | If the ECU is commanding the primary fuel pump to be switched on and there is a fault with the primary fuel pump, or the fuel pressure is below a certain value, turn on the secondary fuel pump. |
| 06:49 | These logical operators are at the heart of how PMUs operate but you may find different manufacturers approach the configuration of them in different ways. |
| 06:57 | Some will have you directly using these operators to configure the PMU, others will give you a graphical interface where you configure the PMU by connecting up logic blocks and others will give you pull down menus to configure conditions that need to be met for output actions to occur. |
| 07:12 | However the PMU approaches this configuration process, it's important to have a good grasp on these logical operators as it'll help you understand how to achieve more complex operations. |
| 07:23 | In this module, we've looked at how PMUs replace conventional relays in an automotive electrical system by instead using solid state switching devices called MOSFETs. |
| 07:32 | The PMU also includes a micro controller which lets us configure them with software to perform output actions like switching on an output in response to various input conditions. |
| 07:43 | These configurations are generally based on chains of logical operators, being the if, not, and, or and XOR operators. |
| 07:52 | By chaining together these relatively simple operators, we can program the PDU to perform quite complex functions. |
