EV Fundamentals: Vehicle Control Unit
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Vehicle Control Unit
08.26
| 00:00 | In this section of the course, we're going to be looking at the auxiliary devices that an EV system needs in order to function. |
| 00:05 | That is, components that are separate from the core driveline system but are required if we want to get an electric vehicle moving under its own power. |
| 00:13 | Let's start with the vehicle control unit, which from here on out we'll refer to as the VCU. |
| 00:18 | This should be thought of as the brain of the entire system, similar to an ECU found in an internal combustion vehicle. |
| 00:24 | Like its ICE counterpart, VCUs are found both as factory fitted and as aftermarket components. |
| 00:29 | The VCU performs a number of crucial functions, and while some VCUs are relatively simple, others can be extremely complex. |
| 00:36 | VCUs , which control multiple motors, for example, will inherently need more checks and measures than a simple single motor setup with an external BMS. |
| 00:44 | Most of the analog sensors we have will also connect to the VCU. |
| 00:49 | This means that things like accelerator pedal, brake switch, wheel speed sensors, temperature sensors, and so on will be connected directly to the VCU. |
| 00:57 | So, let's start by discussing the functions that a VCU needs to perform in order for the vehicle to be able to operate. |
| 01:03 | And remember, you can always pause at any point and jump back to the relevant sections of the course if anything discussed here seems a little hazy. |
| 01:10 | First and foremost, the VCU needs to communicate and understand the status of the battery. |
| 01:15 | So, if the vehicle is using an external BMS, this communication is typically accomplished with a few simple CAN messages back and forth between the VCU and BMS. |
| 01:24 | The VCU will ask to bring the high voltage system online, and then the battery will respond by closing the contactors if it's safe to do so, and indicate back the status of the battery. |
| 01:34 | If the battery isn't capable of coming online for whatever reason, the BMS will communicate that back to the VCU. |
| 01:39 | For example, if the car is currently charging, the BMS will report to the VCU that a cable is connected, and the VCU will then disable drive systems ensuring that the car can't drive out of the garage with 20 feet of live cable attached. |
| 01:51 | If a fault ever occurs with the BMS, the VCU will be notified so that it can cut torque to the motors, allowing the BMS to shut down and open the contactors in a low load state. |
| 02:00 | This is because it's much easier on the contactors to open and close when there isn't a lot of current flowing. |
| 02:06 | Switching contactors at hundreds of amps is a good way to wear them out, and you can also weld them shut. |
| 02:11 | It's worth noting that in some cases the VCU also performs the tasks of a BMS, and will directly control the contactors, measure the battery current, isolation, and cell voltages, as well as command balancing of individual cell groups. |
| 02:23 | This is something that doesn't really exist in the OEM world, but some EV conversions are done this way as it's one less controller to buy. |
| 02:31 | Of course, this only works if you have BMS submodules that can communicate directly with the VCU, and have a VCU that is capable of performing these duties. |
| 02:39 | Beyond that, the VCU also controls the inverter by sending it a torque request. |
| 02:43 | Obviously, before the inverter applies power to the motor and starts spinning things, a number of safety checks need to be performed. |
| 02:50 | Unlike an internal combustion engine, where it's pretty clear that it's on and running, an electric power train is pretty silent, right until it's not. |
| 02:57 | This can be incredibly dangerous. |
| 02:59 | Just imagine someone accidentally stepping on the accelerator pedal of a live vehicle as they're getting into the driver's seat. |
| 03:05 | One of the jobs of the VCU is ensuring that this can't happen. |
| 03:08 | This is typically done through the use of a brake switch to get out of neutral, or what can be thought of neutral at least. |
| 03:15 | The system will also generally include a timeout feature that ensures the vehicle will switch back to neutral or idle if the car hasn't moved for a certain period of time. |
| 03:22 | When it comes to torque requests, these systems can be very simple, or they can be incredibly complicated by factoring in things like all four wheel speeds to reduce regen if a certain axle is locking, lateral acceleration to fine-tune regen and torque while cornering, or multiple maps and drive modes that adjust the torque output for different driving conditions. |
| 03:42 | Additionally, the VCU will also implement limits and strategies in order to deal with overheating. |
| 03:47 | This actually isn't too different from an ignition trim map on a standalone ECU for air and coolant temperature, where we can dial out as much power as we want as temperatures approach critical levels. |
| 03:57 | The VCU also needs to communicate with all the other devices that make up the system. |
| 04:02 | This monitoring and control is usually done via CAN and includes components like the DC-DC converter, the onboard charger, both of , which we'll be looking at in the next couple of modules, as well as high voltage heater, air conditioner, and other parts of the car as well. |
| 04:16 | Additionally, the VCU will also often be in charge of the thermal system, which in an EV can be quite complicated. |
| 04:22 | We'll be covering the thermal system in its own section later in this course, so for now, just understand that the VCU may directly control or simply command separate pumps and valve controllers, as well as monitor the temperature of various different thermal systems. |
| 04:35 | It could be as simple as a single coolant loop, or there could be multiple coolant and oil systems on more complicated vehicles. |
| 04:42 | While running, the VCU will monitor the entire vehicle and take appropriate actions if anything goes wrong, including overheating, sensor faults, isolation breakdowns, or other errors reported by an external module like the BMS. |
| 04:56 | This can be done by limiting power, warning the driver with a display prompt, or shutting the vehicle down entirely. |
| 05:01 | A well-executed VCU will provide smooth, precise, and predictable performance and give an OEM vehicle experience, just like a high-quality stand-alone VCU would provide. |
| 05:11 | Entry-level aftermarket VCUs, on the other hand, will get the car down the road, but don't expect much more than that. |
| 05:16 | Shopping for an aftermarket VCU can be quite difficult, and because the industry is so young, there aren't a lot of options currently out there. |
| 05:23 | This will change in the near future though, and there may be plenty of VCU offerings on the market by the time you're watching this course. |
| 05:29 | With that in mind, let's run through a few things to look for in a VCU. |
| 05:33 | First of all, we need to select something that's in line with our needs. |
| 05:37 | If we're building a simple vehicle, often the inverter can perform the required functions on its own, and that may be all that's needed. |
| 05:43 | If we're building a race vehicle, on the other hand, and want precise control over a number of different systems, then we'll need a VCU that firstly has compatibility with the components we're planning to use, or that compatibility could be added, and we understand the costs and lead times involved with that customization. |
| 05:59 | Keep in mind that many VCUs are designed for only one specific set of components, and don't offer much flexibility outside of that ecosystem, so research and talking to others with experience is key. |
| 06:09 | Following that, we'll want to understand the programmability of the VCU to ensure it has all of the features we deem necessary. |
| 06:16 | Some offer almost no programmability at all, it's simply a case of connecting everything up, and effectively getting an on-off switch and a pedal , which commands zero to full torque. |
| 06:25 | Others offer effectively a blank sheet of paper with , which custom software can be developed to the limits of your imagination. |
| 06:31 | Working with input devices like keypads, having the kind of control strategies we're looking for, being able to communicate with the devices you're planning to integrate, and having data logging capabilities are all things to confirm and think about before finalizing your selection. |
| 06:44 | The VCU is the hub of the vehicle, and the main point of interaction, so we want to be sure whatever we buy is something that we'll be comfortable and happy working with in the future. |
| 06:53 | Lastly, and possibly least importantly, is confirming the I-O count of the VCU and ensuring that it physically has the capability to drive the items we need. |
| 07:00 | This usually isn't a huge issue though, as CAN-based expander devices can usually handle any additional needs, and things like power distribution modules can be used to drive high current loads, or a digital display or logger can be used to add more sensors and inputs if needed. |
| 07:17 | It's always a good idea to consider and identify some potential VCU solutions early on in our project, as that may drive decisions for further components down the road. |
| 07:25 | It is all too common for people to select components that are not supported, and then be surprised when a lot of custom software is required to add support. |
| 07:32 | There's a lot to cover when it comes to VCUs, so let's do a quick recap before moving on. |
| 07:37 | These units can be thought of as the brain of an electric vehicle system, and can vary hugely in their complexity depending on what it is they need to control. |
| 07:45 | The VCU essentially manages the entire system, monitoring and communicating with the battery, controlling the inverters, keeping voltage and temperatures in a safe range, while also managing other auxiliary devices like the DC-DC converter, onboard charger, and more. |
| 07:59 | Aftermarket VCU options for EV swap vehicles and race cars are currently limited, but there are still a few good products out there, with more coming onto the market in the near future. |
| 08:08 | When selecting a VCU for your project, it's important to first understand your vehicle's requirements and the components you intend on using. |
| 08:16 | This way, you can be sure you're getting a VCU that's going to meet your needs and be able to communicate with the chosen equipment without expensive customization. |
