Preventing Wheel Lock Scenarios - Sevcon Gen4 Applications Reference Manual

2 Curve applied within 0.4% of target
3 Curve applied within 0.8% of target
4 Curve applied within 1.6% of target
5 Curve applied within 3.2% of target
This table continues, with each increment in 2933
applied.

Preventing Wheel Lock Scenarios

For certain vehicle types, particularly on-highway vehicles or electric motorcycles, the possibility
of wheel locking during drive must be considered.
During braking, the controller will maintain a speed limit to ensure the vehicle does not over
speed if entering braking whilst travelling downhill. If proportional speed limit is set then the
speed limit will follow actual speed toward zero whenever actual speed is dropping rapidly,
usually due to some external influence such as application of mechanical brakes.
If the brakes are applied too harshly, then there is possibility to lock the drive wheels. In these
circumstances, the normal reaction of the driver is to release the brake to allow the wheels to
rotate again. The correct operation of the controller in this scenario is to allow the wheels to
continue to rotate, and not impose a speed limit.
The maximum rate at which the speed limit can increase or decrease is specified in object 291E
. By limiting the rate at which the speed limit can decrease, we can ensure that if the brakes are
h
released after they had locked the drive wheels, the controller's speed limit will allow them to
rotate again. The operation of this is shown in Figure 32 below.
Figure 32 - Example of behaviour of speed limit when drive wheels are locked
WARNING: It is important to consider the behaviour of the vehicle under all drive
conditions, including when traction is lost due to locking of the drive wheels. When
testing a vehicle, check that the vehicle behaves in a safe manner when performing harsh
braking on low-friction surfaces such as gravel.
Doc. # 177/52701
Rev. 3.3
doubling the range over which the curve is
h
Actual Speed
Speed Limit
Vehicle wheels lock, but rather than
stepping down speed limit, it ramps
down at the maximum speed limit
deceleration rate as specified in
object 291E. When wheels recover,
the motor control will allow them to
rotate again.
Configuration
6-29