Speed Reference
The target value that tells the driver at what speed it should run; the source can be a pot, an analogue input, a joystick or a fixed software parameter. The driver converts this value into a duty ratio and thereby sets the average armature voltage applied to the motor. In open loop the reference defines not the actual speed but the applied voltage.
The reference is the start of the driver's internal chain: the analogue input is read, scaled, passed through the ramp and converted into a duty ratio. Every stage in this chain adds its own share to the result.
- Scaling: The whole reference does not have to map onto the whole output. In one application, making 70% of maximum speed the upper limit is the simplest way to protect the mechanics.
- Lower limit: At a very low reference the motor does not turn but still draws current and heats up. The driver generally treats anything below a certain value as zero.
- Relation to the ramp: The reference sets the target, the ramp sets how long it takes to reach that target. The two are not interchangeable.
In open loop, even if you keep the reference constant the speed drops when the load increases; the reference is not a speed guarantee but a voltage command. IN5 on the KS250, and the E input and the SPEED pot on the PT500, provide this reference.
The context in which this term is used: DC Motor Driver Selection Guide