For dynos requiring the control of a valve, for example Water Brake dynos, you can use an RC servo or Servo motor or a stepper motor, see further down.
Here is how to connect a servo to YourDyno:
For an RC servo, set up the PWM parameters like this:
With these parameters, YourDyno can directly control the servo.
Selecting an RC Servo
RC servos are not just small, weak servos with max 180 degrees movement. You can find very strong servos and servos with many revolutions from min to max. They can be bought from hobby stores.
Here is a servo capable of 110kg/cm(!) at 180 degrees.
All these servos will require a separate power supply, they cannot be powered directly from YourDyno. Make sure Ground is shared with YourDyno.
Make sure to choose a servo that is fast. The faster the servo, the faster the regulation will work, which means it can deal with sudden changes in power and quickly finds new RPM targets.
Servo motors are motors that run to a certain position based on the input. So they work much like an RC servo, but they can be very powerful. They can be linear or rotational. They are controlled using either a built in or separate servo controller. Ion Motion Control has many types of motor controllers. Check out for example the Roboclaw 30A motor controller.
They can all use a PWM as input, so they are controlled directly by YourDyno.
Servo motors come finished with built in encoders or you can use a standard motor and add your own encoder. You will want a potentiometer type encoder (not a quadrature encoder). They are very simple in operation; if fed by 5V they give out a 0V-5V signal based on the absolute rotational position. Make sure to choose one that has at least as many turns as you need to control your valve. Using a potentomenter encoder you achieve absolute position control, which is what you need. A specific output from YourDyno will translate into an absolute position of your valve.
There are many potentiometer encoders to choose from. Some are hollow shaft and go directly onto your motor shaft. Here is a cheap option with 3600 degree (10 rotations) operating range. It can be connected to for example the Roboclaw controller to turn a DC brushed motor into a servo motor: