Engee documentation

H-Bridge

H-bridge motor driver.

h bridge

Description

The H-Bridge block is an H-bridge motor driver. The unit has the following two modelling mode options:

  • PWM - in this mode, the output of the H-Bridge block is a controlled voltage that depends on the input signal at the PWM (PWM) port. If the value of the input signal exceeds the Enable threshold voltage parameter, the H-Bridge output voltage has a value equal to the Output voltage amplitude parameter. If the input signal is less than the Enable threshold voltage, the unit maintains the load circuit using one of the following three Freewheeling mode options:

    • Via one semiconductor switch and one freewheeling diode.

    • Via two freewheeling diodes.

    • `Via two semiconductor switches and one freewheeling diode.

    The first and third options are sometimes referred to as synchronous operation.

    The signal on the REV port determines the polarity of the output. If the REV port signal is less than the Reverse threshold voltage, the output is positive polarity, otherwise it is negative polarity.

  • Averaged - in this mode two variants of load current characteristics are possible:

    • Smoothed

    • `Unsmoothed or discontinuous.

    In the Smoothed variant, it is assumed that the current is nearly continuous due to load inductance. In this case, the output of the H-Bridge block is as follows:

    where:

    • - is the value of the Output voltage amplitude parameter, V.

    • - is the value of PWM port voltage, V.

    • - value of PWM signal amplitude, V.

    • - value of output current, A.

    • - parameter Bridge on resistance, Ohm.

    The current will be smooth if the PWM frequency is sufficiently high. Synchronous operation, in which free oscillation is carried through the bridge arm back to the power supply, also helps smooth the current. For cases where the current is non-smoothed or possibly discontinuous (i.e. it reverses to zero between PWM cycles), use the `Unsmoothed or discontinuous' option. For this option, the values Load total series resistance, Load total series inductance and PWM frequency must also be specified. During simulation, the unit uses these values to calculate a more accurate value for the H-bridge output voltage that achieves the same average current as the PWM simulation.

To speed up the simulation when controlling the H-Bridge block with the Controlled PWM Voltage block, set the Simulation mode parameter to Averaged. The Simulation mode parameter of the Controlled PWM Voltage unit must also be set to `Averaged'. In this case, the average value of the required PWM voltage is applied to the motor. The accuracy of the simulation results in `Averaged' depends on the assumption you make about the load current. If you specify that the current is non-smoothed or intermittent, the accuracy also depends on how demonstrative the load resistance and inductance values are. This mode also makes some simplifying assumptions about the basic equations for the case where the current is intermittent. For typical motor and bridge parameters, the accuracy should be within a few per cent. To check the accuracy of the `Averaged' mode, run the simulation in PWM mode and compare the results with those obtained in `Averaged' mode.

Braking mode is enabled when the voltage applied to the BRK port exceeds the Braking threshold voltage. Regardless of the PWM or averaging mode, in braking mode the H-bridge is modelled by a series combination of two resistors and , where:

  • - is the resistance of one arm of the bridge, i.e. half the value of the Total bridge on resistance parameter.

  • - resistance of one bridge arm in parallel with diode resistance, i.e. , where is diode resistance.

Assumptions and limitations

  • When linearising the model, set the Simulation mode parameter to Averaged' and ensure that the operating point is correctly specified. The H-Bridge block can only be linearised for duty cycles that are greater than zero and less than the amplitude of the PWM signal. If linearisation is required at zero duty cycle and if the controller always uses regenerative braking, set Regenerative braking to `Always enabled (suitable for linearisation).

  • In `Averaged' mode and when `Unsmoothed or Discontinuous' is selected, the load inductance and load resistance values must be provided for the load current characterisation. For a DC motor, the resistance is the armature resistance and the inductance is the sum of the armature inductance and the series smoothing choke (if any). For a universal motor, the total resistance is equal to the sum of the armature and field winding resistances, and the total inductance is equal to the sum of the armature and field inductances with the addition of the series smoothing inductance. For a shunt motor, Engee recommends the construction of a Thevenin equivalent circuit to determine the appropriate values.

Ports

Non-directional

+ - positive load connection port
electricity

Electrical port associated with the positive load contact.

- - negative load connection port
electricity

The electrical port associated with the negative load contact.

PWM - pulse width modulated signal (PWM signal)
electricity

The electrical voltage port associated with the pulse width modulation signal. The voltage is defined with respect to the REF port.

REF is the reference voltage port
electricity

The electrical port associated with the reference zero voltage.

REV - REV
electricity

An electrical port associated with a voltage that controls the polarity change of the H-Bridge block output. The voltage is defined relative to the REF port.

BRK - BRK
`electricity'

An electrical port associated with a voltage that controls the closing torque of the H-Bridge block output. The voltage is defined relative to the REF port.

Parameters

Modelling mode and load assumptions

Simulation mode - Simulation mode
PWM (By default) | `Averaged `

Select one of the following options for the output voltage type:

  • PWM - The output voltage is a pulse width modulated signal. This is the By default option.

  • Averaged - the output voltage is a constant value whose value is equal to the average value of the PWM signal.

Freewheeling mode - free-wheeling mode
Via one semiconductor switch and one freewheeling diode (by default) | Via two freewheeling diodes | Via two semiconductor switches and one freewheeling diode.

Select one of the following options for the type of H-bridge scattering pattern:

  • Via one semiconductor switch and one freewheeling diode - in this mode the unit controls the load by keeping one arm of the bridge on the top side permanently on and using a PWM signal to modulate the corresponding arm of the bridge on the bottom side. This means that when the bridge is switched off, the block uses only one of the shunt diodes to complete the dissipation circuit. This option is used by default.

  • Via two freewheeling diodes - in this mode, all arms of the bridge are switched off in the off state. This means that the unit dissipates the load current through the power supply using two freewheeling diodes.

  • Via two semiconductor switches and one freewheeling diode - in this mode, the unit controls the load by keeping one arm of the bridge on the top side permanently on and using a PWM signal to switch between switching on the corresponding arm of the bridge on the bottom side and the opposite arm of the bridge on the top side. This means that a diode switched in parallel to the bridge arm and another bridge arm in series is used to complete the dissipation circuit when the bridge is switched off.

Dependencies

This parameter is only used when PWM is selected for the Simulation mode parameter, or when Averaged is selected for the Simulation mode parameter and Unsmoothed or discontinuous is selected for the Load current characteristics parameter.

Regenerative braking - regenerative braking
Depends on REV flag and current sign (by default) | Always enabled (suitable for lianerisation)

Select one of the following options for the H-bridge scattering pattern type:

  • Depends on REV flag and current sign - this option is used by default.

  • Always enabled (suitable for lianerisation) - this option can be used when the controller always sets the REV flag to enable regenerative braking. This is useful if it is necessary to linearise the model for control purposes, to ensure that the H-bridge is linearized near zero duty cycle.

Dependencies

This parameter is only used when Averaged is selected for the Simulation mode parameter.

Load current characteristics - load current characteristics
Smoothed (by default) | Unsmoothed or discontinuous.

Select one of the following options for the load current type:

  • Smoothed - it is assumed that the current is almost continuous due to the load inductance. This option is used by default.

  • Unsmoothed or discontinuous - this option is used for cases where the current is non-smoothed or possibly discontinuous (i.e. it reverses to zero between PWM cycles). The parameters Load total series resistance, Load total series inductance and PWM frequency must also be set for this option. During simulation, the unit uses these values to calculate a more accurate value for the H-bridge output voltage that achieves the same average current as the PWM simulation.

Dependencies

This parameter is only used when Averaged is selected for the Simulation mode parameter.

Load total series resistance - total series resistance of the load
`10 ohms (by default)'.

The total series resistance of the load seen by the H-bridge.

Dependencies

This parameter is only displayed when Averaged is selected for Simulation mode and Unsmoothed or discontinuous for Load current characteristics.

Load total series inductance - total series inductance of the load
1e-5 Gn (by default).

The total series inductance of the load sensed by the H-bridge. In addition to the motor inductance, any series inductance added outside the motor for current equalisation must be included.

Dependencies

This parameter is only displayed when Averaged is selected for Simulation mode and Unsmoothed or discontinuous for Load current characteristics.

PWM frequency - PWM frequency
`10000 Hz (By default)

The PWM frequency at which the H-bridge is controlled. For consistency, this value must match the PWM frequency set by the Controlled PWM Voltage block controlling the H-bridge block.

Dependencies

This parameter is only displayed when Averaged is selected for Simulation mode and Unsmoothed or discontinuous for Load current characteristics.

Freewheeling diode off-state conductance - shunt diode off-state conductance
1e-6cm (by default).

Conductance of the rewheeling diode off-state conductance.

Dependencies

This parameter is only used when Averaged is selected for Simulation mode and Depends on REV flag and current sign for Regenerative braking.

Input thresholds

Enable threshold voltage - Enable threshold voltage
`2.5 V (by default)

The threshold above which the voltage on the PWM port must rise to switch on the output of the H-Bridge unit.

Dependencies

This parameter is used when Simulation Mode and Simulation Mode & Load Assumptions are set to `PWM'.

PWM signal amplitude - PWM signal amplitude
5.0 V (By default).

PWM input signal amplitude.

Dependencies

The H-Bridge unit only uses this parameter if the Simulation mode parameter on the Simulation Mode & Load Assumptions tab is set to Averaged.

Reverse threshold voltage - reverse threshold voltage
2.5 V (by default).

When the REV port voltage exceeds this threshold, the output polarity becomes negative.

Braking threshold voltage - Braking threshold voltage
`2.5 V (by default).

When the BRK port voltage exceeds this threshold, the output pins of the H-Bridge unit are shorted through the next series of devices:

  • One arm of the bridge.

  • One arm of the bridge is in parallel with a conductive free-running diode.

Bridge parameters

Output voltage amplitude - output voltage amplitude
`12 V (by default)

Voltage amplitude at the output ports of the H-Bridge unit when the output is enabled.

Total bridge on resistance - total bridge resistance
`0.1 ohms (by default)

The total effective resistance of the two semiconductor collectors connecting the load to the two power rails when the PWM port voltage exceeds the value of the Enable threshold voltage parameter in the Input Thresholds parameter group.

Freewheeling diode on resistance - resistance of the shunt diode
`0.05 ohm (by default).

The total resistance of the shunt diodes that dissipate the current flowing through the motor when the PWM port voltage is less than the value of the Enable threshold voltage parameter in the Input Thresholds parameter group.

Examples