The AMX 201/202 power unit is a device designed to control both the position of the servomotor (AMX 201) as well as the current supplied to the dynamometer (AMX 202). It is encased in a standard 19'' panel. The front view of the device is shown in the picture on the left. This device can also be made in a configuration that includes only one of these controllers (AMX201 or AMX202).

The device can operate in the following two modes:
• Internal Assignment Mode, where the operator sets the load and throttle position values manually, OR
• External Assignment Mode (for example, the external assignment voltage 0-10V, or setting the voltage from microprocessor circuits, or a computer).

The system is provided with built-in security features (described in more detail below) that control the operation of the examined engine. The power unit can be divided into the following three independent functional blocks:
• the servomotor controller block,
• the dynamometer current controller block,
• the protection block.

The throttle controller is a position servomechanism of high dynamics and adjustment accuracy. Dynamic parameters result from minimizing the M.I. (moment of inertia) of the movable parts of the servo motor, especially by using an appropriate driving motor, such as the AMX 203 actuator motor (with a printed rotor) characterized by negligible mechanical inertia and very low rotor inductance. As a result, it takes about 30-60 milliseconds to set the motor into motion. Since the torque generated by the motor is proportional to the consumed current there had to be provided power supply from a source of sufficient efficiency. For this purpose, a specialized three-phase power supply has been developed which is capable of providing temporary current of high volume without the need to use any capacitor batteries. The motor rotary movement in the clearance free toothed gear is changed to reciprocating motion with adjustable pitch. The current position of the actuator is determined using a potentiometer of high linearity and resolution, directly connected to the motor shaft. The controller can set the position in accordance with the adjustment of the inner assignment potentiometer (10-turn, scaled potentiometer) or an external source (the 0-10V signal). The feedback signal of the current position averaging 0-10V is entered into the PID controller. It is used by the electronic unit to accurately fine-tune the current rotor position so that it will conform to the assigned value.

The controller of current flowing through the dynamometer generates current whose value ranges from 0 to 10A. This value can be set in three different ways:
• using a multi-turn precision potentiometer mounted on the front panel,
• voltage supplied to the external assignment connector (0 - 10V),
• filling the square signal supplied at the PWM entry.

The operation mode is set by using the switch located on the front panel of the device. If the current value is assigned using the precision potentiometer or external voltage, it activates the inner controller of the dynamometer's current. The voltage signal is entered into comparator, which compares the average current value with the assigned value and controls the operation of the generator that locks the IGBT transistor. In the PWM signal mode, the IGBT transistor basis is directly controlled by the PWM signal (through optoisolation). The system features protection against exceeding the maximum current value. The dynamometer coil is isolated galvanically from the installation with the isolating transformer. The input unit is also isolated galvanically in all the operating modes from the parts with powerful current. The regulation accuracy is 0.5%.

A standard AMX 201/202 power unit features protection systems protecting against the following conditions:
• exceeding the engine's maximum RPM value;
• exceeding the allowable temperature of the engine's coolant;
• abnormal oil pressure in the engine;
• thermal excess or minimal water flow in the dynamometer.

Each protection system is assigned a specific reaction in response to a condition causing a malfunction. Triggered reactions can be as follows:
• the engine is set to its minimum RPM value (idle) by resetting its actuator to the minimum position. ;
• the engine load is removed (the current flow through the dynamometer is cut off);
• the engine ignition is cut off.

The triggering of protection systems is indicated to the device operator by the indicator lights mounted in the front panel, and the information is relayed to the controlling panel, and through it, to the controlling computer.