Sensorless Motor Control Simplifies Washer Drives

Jun 1, 2006 12:00 PM
By Aengus Murray, Director iMOTION Products, Energy Savings Product Group, International Rectifier,

Implementation Issues

Designers must face several practical considerations when implementing the control algorithm in fixed-point hardware. The most important decision is the selection of control-variable scaling, which must allow a good dynamic signal range while avoiding calculation overflow. The resolution of the digital-to-analog interface circuits could define the control-variable scaling, but this does not ensure numerical stability of the control-loop functions. Fixed control-variable scaling simplifies the design of the control-loop functions with the minor additional complexity of the feedback-gain scaling.

Since velocity is a user input, its scaling requires the highest resolution. To define the velocity scaling for a 16-bit platform, the maximum operating drive speed is set to equal +16383(˜2¹⁴), which allows 1 bit of headroom in case of overshoot.

The velocity value that the rotor-angle estimator calculates is the change in rotor-angle count in every control-loop cycle. The scaling of this value depends on the loop sample rate, the number of motor poles and the maximum rotor-angle count. The feedback-gain scaling is then the ratio of 16384 to the angle-estimator output at the maximum operating speed.

Similarly, the current-variable scaling is defined by setting the name-plate-rated motor current equal to +4095 (˜2¹²) counts. This allows sufficient headroom for the current controller to operate the motor in short-term overload. The gain of the current-feedback circuits depends on the current-sense resistor values, the buffer amplifiers' gains, and the ADC's input range and resolution. The current-sense resistor is also useful for protection.

The value selected should trip the power inverter before the motor current reaches its demagnetization limit. The buffer amplifier's gain defines the operating range of current-control loop since the ADC will saturate when the input signal is out of range. The resolution of the vector-rotation block, which requires all inputs to be in the 11-bit-plus sign range, sets the voltage scaling in the case of this hardware implementation.

The actual voltage scaling is a function of the maximum voltage count and the dc-bus voltage. However, the dc-bus voltage can change with either input ac voltage or the load, and so a bus-voltage measurement contributes to the scaling calculation.

Control Algorithm Tuning

The final stage in the control algorithm design is in the calculation of the feedback-gain parameters. The process usually involves an element of trial and error, but there are design tools that provide close-to-optimum values. Pole-zero placement is a well-known technique that works well with systems such as the current-control loop, whose parameters are given to easy measurement.

The system model of the current controller in Fig. 6 illustrates the technique. Gain elements, which the selection of voltage and current scaling define, model the PWM-inverter and current-feedback circuits. A continuous-time-domain model simplifies the figure, but a sampled-data model only requires some additional scaling of the integrator-gain parameter.

As shown in Fig. 6, the PI-controller gains are calculated in three stages. First, to reduce the order of the loop, select the ratio of the PI compensator gains so that its zero cancels the pole in the motor-winding model. Next, select the integral gain so that the closed-loop bandwidth matches the target specifications. Finally, scale the integral-gain parameter by half of the sample period to calculate the gain in the digital implementation.

The calculation of gain parameters for the current loop is straightforward, but the next section describes some of the tools for tuning the velocity loop when driving the type of complex load found in washing-machine applications.

Simplifying Drive Commissioning

The washing-machine design platform includes a digital IC with the control algorithm already embedded (Fig. 7). The MCE implements the algorithm in hardware based on parameters and variables that the shared-memory area stores. One of the available design tools is a spreadsheet that calculates the controller constants that match the motor parameters and system specifications.

The second tool is MCEDesigner, which transmits the controller constants from a PC to an interface program running on the embedded 8051 microcontroller. The 8051 copies the parameters to the block of shared memory before the MCE starts running the motor.

Another important function of the 8051 interface software is to capture trace data from the MCE, which gives the user access to all the control-system variables. MCEDesigner also generates speed profiles that allow users to evaluate drive performance in the washer application. The combination of an embedded motor control algorithm and easy-to-use evaluation tools simplifies the evaluation of direct-drive motors for washer applications.

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