In this entry – about some nuances of PWM.
For what is the PWM used?
PWM (Pulse Width Modulation) is used for “analog” management of different actuators. With “analog” this time I mean: smooth, not on/off type. Using PWM, the valves, electromotors, heating elements, bulbs, etc. are managed. The PWM regulates the voltage (accordingly – also current and power), which is supplied to the actuator (load).
Why use PWM to manage the actuators? Why they can not be managed by, for example, DAC (Digital to Analog converter)? They can, but when managed by PWM, the hub has a better efficiency. So – smaller losses. Smaller losses mean saving the energy of the battery (if the engine is not turned on), and also less heating of the management units. Both aspects are a large problem for modern cars. The amount of current, consumed by different consumers of the car’s systems, reaches several tenths of A. Even for DME/DDE, the forced cooling is used – each %, which is spared from losses, is very valuable. For example, activating the actuator by 50 %, using DC method (DAC) heat losses will be around 50 %, but using PWM – the losses are only several %.
How do PWM works?
In the case of using PWM, the electronic switch periodically (in case of the car – typically: several hundred times during a second) is switched on and off. If the switch is activated, the current flows thru actuator; at the moments, when the switch is disabled, the current doesn’t flow thru load. Changing the proportion between “turned on” and “turned off”, it’s possible to change the energy, supplied to the actuator. So – the actuator is swiftly switched in position “on/off”, but as it has a mechanical (thermal) inertia – it doesn’t switch on and off swiftly, but is in “analog” position for all the time, which is defined by proportion of length of “on” and “off” cycles.
What does the PWM indication mean?
What does means, for example, PWM = 67 %? 67 % means, that 67 % of the time the actuator is supplied with the energy (the switch is activated/”on” position). The other part of the time (100 – 67 = 33 %) energy is not supplied to the actuator. As usually the actuators are managed from the onboard voltage of the car, we can simplify the description. 67 % PWM means, that the actuator is supplied with an onboard voltage for 67 % of the time. Or – the average voltage on actuators pins is around 8.0 V if the onboard voltage is 12.0 V. Or 9.4 V if the onboard voltage is 14.0 V.
Control of PWM output and connection
The car electronic units usually control the connection of the actuator (and the performance of their management switches). For this control to be possible, the min and max value of PWM is not 0 % and 100 %, but slightly narrower regulation range, for example, from 0.5 % to 99.5 %. A slight narrowing of the range of PWM don’t worsen the management of the actuator but in “pauses” allows the management unit to make sure, that its power switch works correctly and the actuator is connected (the voltage at the output changes from 0 to Ubat and back).
Stabilization of the position, using Ubat
At the example above, we clearly see – if the onboard voltage of the car changes, by fixed PWM – the average voltage, supplied to the actuator, also changes. Accordingly – the power (energy), supplied to the actuator, also changes. For this reason, the car’s electronic units measure the onboard voltage and recalculate required PWM – take into account the changes of onboard voltage. As higher the onboard voltage, as lower PWM – with appropriate formulas, the changes of the voltage are compensated.
The connection of the actuators
The actuators can be connected in several ways:
a) one of the actuators pins is connected to the Ground (chassis of the car); with PWM, the plus (+) pin is managed;
b) one of the actuators pins is connected to Ubat (15/30 line); with PWM, the minus (-) pin is managed;
c) both pins of the actuator are managed by PWM. In this case the change of polarity of current, flowing via an actuator, is possible (so-called Bridge connection).
First two solutions are used in simple cases – when it’s not necessary to change the “direction” of the current. For example: bulbs, solenoid valves. What exactly (a or b) solution is used – depends on the developer. Technically cheaper is the b option, but more convenient in use – an a option. To understand, which connection is used – see the electrical schematics of the exact hub. The third solution is used in cases when the “direction” of the current should be changed. Typically such need appears if the electromotor is used. It has to turn in both directions, and such a possibility is when using the Bridge connection.
Why I put an accent to these 3 different connections? Depending on the connection, the way, how the voltage supplied to the actuator has to be measured, differs:
- in a case: the voltage has to be measured against Ground (car’s chassis);
- in b case: the voltage has to be measured against the car onboard voltage (line 15/30);
- in c case: the voltage has to be measured directly on pins of the actuator.
If measuring in an incorrect way (for example, in case of b connection – against the Ground), you will get completely wrong (“inverted” ) indications of the voltage!
How does the PWM signal look?
So does the PWM signal of around 90 % for a type, or around 10 % PWM signal for b type connection (the Ground of oscilloscope connected the chassis of the car) looks. Here – a good example how (depending on the connection of the actuator), the PWM value changes!
a) the actuator connected to the Ground; the plus (+) output is managed via PWM
Here – all bulbs with one pin are connected to the Ground (31-st line, wires: brown color); plus (+) pins go to the FRM module, where the brightness of bulb glowing is changed by PWM.
b) the actuator is connected to Ubat (line 15/30), the minus (-) output is managed by PWM
As we see, the plus wire of the exhaust flap via Fuse is connected to Ubat. In the schematics, the lower pin (minus connection) is managed via PWM. Pay attention – the appropriate management switch of DME is connected to the 31-st line: confirmation, that exactly the (-) connection is managed via PWM.
c) both connection of actuator are managed by PWM
Here, the electromotor of the EGR valve (pins 3 and 4 – both are managed by PWM).