Bi-directional variable speed drive
includes regeneration

Designed for use in a self-balancing scooter, this variable speed pulse-width modulated (PWM) drive is bi-directional and includes regenerative braking to improve overall efficiency.

With PWM drives, the key to efficient regeneration is to make sure the pulse repetition period is at least an order of magnitude shorter than the effective time constant of the DC motor armature winding. For most motors, this means a pulse repetition frequency of 2kHz or more, although to be inaudible, above 20kHz is preferable.

BITS

Many "analog" drives use a sawtooth wave generator and a comparator circuit to generate the required PWM signal but often the pulse frequency is limited to only a few 100Hz. This design uses the internal PWM hardware module in a PICAXE-08M micro and the pwmout software command. This ‘digital’ approach resulted in a stable drive capable of up to 32kHz.

Software running in the PICAXE generates additional FWD and REV signals, which are then logically combined with the PWM signal to produce the appropriate gate signals (G1-G4) for four power Mosfets Q1-Q4 arranged in a conventional H-bridge.

It is very important with H-bridge configurations to ensure that both devices on one side of the bridge are NEVER switched on simultaneously, otherwise they will be destroyed. This is achieved by having "dead time" and is provided by flipflops comprising IC2c&d and IC5c&d. The RC circuits at pins 10 & 12 of the Schmitt trigger NAND gates ensure that, for example, the gate drive pulse for Q1 is removed 100ns before the drive pulse is applied to the gate of Q2, and vice versa.

By using both P-channel & N-channel power Mosfets, the need for the usual supplementary power supply to drive the gates of the high-side Mosfets is avoided.

The TLP251 photocoupler devices not only provide the necessary low-impedance drive for the Mosfet gates very simply, they also provide optical isolation from the potentially noisy 24V, high-current circuitry.

In the software, an 8-bit speed command is sent to the PICAXE as two bytes of data via a serial input at pin P4. Each byte contains four bits of address data and four bits of the speed command data. This approach allows up to eight drives to be controlled from one RS232 serial port. The baud rate is 2400bps (8N1 frame).

The 8-bit speed command expected by the PICAXE software is a ‘2s-complement’ binary number in the range +127 to -128. Full forward speed corresponds to +127 and full reverse speed corresponds to -127, with stationary corresponding to zero.

A speed command of -128 disables the drive by clearing both the FWD & REV signals, which results in all four Mosfets being switched off. This feature could be useful on those occasions when it is preferred that the motor coast to a stop rather than coming to a "controlled" stop with regenerative braking.

This circuit is very energy efficient. Apart from being regenerative, the switchmode operation ensures that power dissipation in the Mosfets is very low. When driving a 250W (12A) motor, and with minimal heatsinks, the Mosfets remain cool to the touch. Since each Mosfet is rated at above 40A, the drive should be capable of controlling motors up to 1kW with appropriately sized heatsinks and wiring.

Note that a suitable fuse or other short-circuit protection will be needed for the 24V supply line.

The TLP251 devices are available from RS Components (Cat. 2610104), and the IRF4905 & IFRZ44 Mosfets are available from Oatley Electronics for about $6 a pair. The PICAXE-08M can be obtained from MicroZed for under $6, giving a total cost for all the semiconductors of around $50.

The software will be available on our website for download.