One question that usually arises when an AGV is intended to be manually moved for a distance is how to safely turn it off and what will be the impact of the regenerative energy to the system. Questions also arise related to if the battery should be disconnected from the circuit, if the controller needs to be turned off, if emergency stop should be triggered etc. This article will try to provide sufficient information about the operation of the controller, so one candetermine the method to follow.

Vmot and pwr ctrl

The MCU, MOSFET drivers and all controller peripherals can be powered either from Vmot or pwr ctrl. The two inputs are connected internally by using two diodes in an OR logic. So essentially the voltage that has the higher potential will pass and power the DC/DC converter which sequnetially will power all the other peripherals.

The internal DC/DC converter can be powered by Vmot or Pwr ctrl and the motor is powered only from Vmot, since the diode stucture does not allow the current flow between the two inputs.

Connection cases

Below it will be explaied what will happen if the motor is forced to rotate and  Vmot and pwt ctrl are connected according to the below scenarios:

  • Vmot and pwr ctrl are floating
  • Vmot is floating and pwr ctrl is connected to ground
  • Vmot is connected to the battery and pwr ctrl is connected to ground

1. Vmor and pwr ctrl are floating

Once the motor start to rotate, a renegerative voltage will be developed that will pass through the MOSFETs body diode, the diode of the Vmot and will power the DC/DC converter. The controller will wake up and depending on the mode used, it will try to short the motor phases or provide a voltage to the motor in order to brake it. Because no voltage from the controller can be provided to the motor (since the Vmot is not connected to battery), the behavior of the controller will be unstable and it will possibly start resetting as the input capacitors will drain, the time that the controller is trying to supply voltage back to the motor. 

The regenerative voltage of the motor will be proportional to its speed. When the motor runs at its rated speed, the regenerative voltage cannot exceed the motor's rated voltage, but when the motor is forced to overspeed, the regenerative voltage will be increased proportionaly. If the regenerative vltage exceeds 60 V the controller will be damaged (Mosfets, DC/DC converter and the MCU can be affected).

2.  Vmot is floating and pwr ctrl is connected to ground

In this case, again the regenerative voltage of the motor will pass through the MOSFET body diode and through the Vmot diode and reach the DC/DC converter's input. Having connected the Pwr ctr to ground though, will disable the DC/DC converted and its ouptut will be zero. This will happen due to a direct connection from Pwr ctrl to converters "enable" input, that is not shown on the diagram. The MCU will never turn on and the motor can be safely be rotated up to its maximum speed.

Again here if the motor rotates at higher speed, the regenerative voltage will exceed 60 V and the DC/DC converter and other components related to its operation will be damaged.

3. Vmot is connected to the battery and pwr ctrl is connected to ground

In that case the controller will be turned off, since the pwr ctr will be grounded but the power stage will have a potential equal to the battery voltage. This voltage will not pass to the motor, since the MCU is turned off the the MOSFETs are not switching, but it will bias the body diode of the top MOSFETs. Once the regenerative voltage will exceed the battery voltage plus the diode voltage drop, the battery will start charging. This will be noticable because the motor will start presenting a resistance to its movement. It is possible that this braking behavior will prevent the motor from going to speeds higher than its maximum. But if the external force on the motor increase or if the battery is fully charged and cannot absorb the regenerative energy, the bus voltage will increase with a potential damage to both controller and battery. 

STO and Emergency stop Operation 

There has been also the question if STO and Emergency stop can prevent the controller damage from regeneration. The effect of software Emergency stop and STO is the same, with the difference that STO is a hardware implementation. In both cases the MOSFETs of the power stage will be turned off. This will allow the motor to roate without the controller providing any voltage to it, but still the same limitations and restrictions that apply to the previous cases are applicable here.

Preventing the MOSFETs from switching when there is regen is important because it prevents the boost converter effect that multiplies the regenerating voltage. By doing that, the voltage of DC bus will not exceed the motor regenerating voltage, a thing tha usually happens when controlling the motor with PWM and braking.