GB2502060A

GB2502060A - Hydraulic drive system with regenerative braking for mobile work platform

Info

Publication number: GB2502060A
Application number: GB1208393.7A
Authority: GB
Inventors: Roger Bowden
Original assignee: Nifty Lift Ltd
Current assignee: Nifty Lift Ltd
Priority date: 2012-05-14
Filing date: 2012-05-14
Publication date: 2013-11-20
Also published as: GB201208393D0

Abstract

A drive system for a mobile work platform that includes a variable displacement drive pump 14 coupled to an electric motor 13, where the pump 14 is in a closed hydraulic loop 16 with at least one drive motor 15 driving a wheel of the machine. Energy can be recovered from braking, deceleration and downhill locomotion to charge battery 11 via motor 13 and pump 14. The drive system preferably also includes a combined steer/charge pump 19 and an auxiliary pump 18. Preferably pumps 14, 18, 19 are coupled to a common drive shaft driven by motor 13. The drive system is particularly suitable for use with smaller battery-powered mobile work platforms with working heights less than 15 metres. A drive system comprising a steer/charge pump 19 and an auxiliary pump 18 is also independently claimed.

Description

Drive System for a Mobile Work Platform The present invention relates to a drive system for a mobile (elevating) work platform (MEW?) ; particularly but not exclusively, an hydraulic drive system for use with smaller mobile work platforms that operate on battery power as the main energy source, for example having a working height less than 15m. The drive system of the invention seeks to provide improved efficiency and utilise a regenerative braking capability.

Background to the Invention

Most mobile work platforms have an hydraulic drive system. In smaller platforms in particular it is common to use simple drive systems that use fixed displacement gear pumps because these are much cheaper than more complicated variable displacement pumps. As such, fixed displacement pumps are generally seen to be more commercially viable for smaller mobile work platforms.

However, since fixed displacement pumps do not have flow control and are not exposed to the braking load it is not possible to control, recover or reuse any excess energy that occurs during operation, for example energy that results from heavy braking.

In the case of larger mobile work platforms (for example those with a working height above l4m) a more complicated variable displacement hydraulic pump is generally required. These variable displacement pumps allow the displacement, or amount of fluid pumped per revolution, to be varied by adjusting its swashplate angle while the pump is running. Variable pumps used in transmission systems are also reversible, meaning that they can convert fluid energy into mechanical energy (and, in turn, into electrical energy via a generator to be stored) Particularly, when used in a closed-loop hydraulic system, it is possible to recover and reuse any excess energy that occurs during operation. However, variable displacement pumps are much more expensive than fixed displacement pumps and generally it is not commercially viable to employ a drive system that uses variable displacement pumps in smaller mobile work platforms. Moreover, complicated drive systems that use open ioop hydraulic pumps and motors are generally seen to be inefficient and are not optimised for regenerative braking.

The present invention aims to improve the efficiency of an hydraulic drive system for a mobile work platform in order to make it more viable to use a variable displacement pump system on smaller mobile work platforms.

In one broad aspect of the present invention there is provided a drive system for a mobile work platform including: a battery; a control means; an electric motor; and a variable displacement drive pump, coupled to the electric motor, in a closed hydraulic loop with at least one drive motor that drives a wheel of the mobile work platform, such that the drive pump is able to hydraulically drive the wheels via the drive motor to propel the mobile work platform and/or recover energy from the turning wheels during braking, deceleration or downhill locomotion to charge the battery via the electric motor, acting as a generator.

According to the invention, the closed loop enables hydraulic pressure to either actively drive the wheels of the mobile work platform or retrieve energy from the slowing wheels when decelerating, braking, or driving down hill; both functions being possible via the hydraulically controlled variable displacement drive pump coupled to an electric motor which becomes a generator when in braking mode; where the wheel drive motors load the pump.

The regenerative braking function of the invention supplements battery power making the system more energy efficient (less frequent charging is required from a mains power supply) In a preferred form of the invention the drive pump is mounted on a common drive shaft from the electric motor with a combined steer/charge pump, and an auxiliary or "booms" pump for assisting the steer/charge pump when reguired. The combined steer/charge pump pressurises a steer cylinder for steering control of the work platform and charges the closed loop to maintain pressure therein. The auxiliary pump hydraulically powers auxiliary functions such as "booms" (e.g. raising and lowering the platform) and also, preferably, is able to provide a charge boost to the charge pump for maintaining pressure in the closed loop (e.g. on start up when a boost of pressure is reguired) Both the steer/charge pump and the auxiliary pump are fixed displacement pumps in communication with an hydraulic fluid tank. Preferably, the auxiliary functions hydraulic circuit returns fluid to the tank.

The regeneration principle enabled by the invention can be scaled for larger machines as well, so long as such machines have a closed loop pump and motor generator set up.

In a second broad aspect of the invention there is provided a drive system for a mobile work platform including: a combined steer/charge pump and an auxiliary pump able to provide an additional boost to the charge pump.

Preferably the drive system further includes a drive pump in an hydraulic circuit charged by the charge pump, further assisted when necessary by the auxiliary pump. The auxiliary pump is otherwise used to power auxiliary functions such as booms operation (raising and lowering the work platform) Preferably the drive system includes a battery, a control means and an electric motor. Preferably the electric motor drives the three pumps (drive, steer/charge, auxiliary) via a common drive shaft.

Preferably the drive pump is in a closed hydraulic loop with at least one drive motor that drives a wheel of the mobile work platform, such that the drive pump is able to hydraulically drive the wheels via the drive motor to propel the mobile work platform and/or recover energy from the turning wheels during braking, deceleration or downhill locomotion to charge the battery via the electric motor, which acts as a generator.

The present invention in totality addresses ways to make an hydraulic drive system suitable for smaller work platforms more efficient, which involves keeping system losses to a minimum. Efficiency savings according to broad aspects of the invention have been made by: 1. Reducing the number of pumps in the system to three.

Typically four pumps are used, one each for drive, booms, steer, and charge. In the system of the invention the steer and charge pump is shared because the charging function is able to be boosted when needed by the auxiliary pump.

2. where possible the size of the pumps have been reduced.

Preferably, the steer/charge pump is very small, for example 1.2CC, whereas the auxiliary ("booms") pump is larger than the steer/charge pump (for example 5.0CC) and oan be used as a boost to assist the steer/charge pump when required, for example on start up. Finally the drive pump, by way of example, may be a 12CC pump. It will therefore be evident that, in a preferred form of the invention, the booms pump compared to the steer/charge pump will be a ratio of 2:1 to 8:1, preferably 3:1 to 6:1 but most preferably between 4:1 and 5:1.

As general background, a typical pump configuration on a larger JI4EWP (working height above 15m) would be 50CC drive pump, 8CC booms pump, 4CC steer pump and 12CC charge pump. If the present invention were applied to a larger machine the above pump sizes would be scaled up by approximately double, i.e. 20CC drive pump, 10CC booms pump and 2.5CC steer/charge pump which has apparent savings over existing pump configurations.

3. The system intentionally utilises high efficiency motors, without gearboxes (see item 4. below), to drive the wheels, e.g. radial piston (dual displacement drive) motors. Radial piston motors generally produce a smooth output with high efficiency. The smaller the size of the motor, the less leakage and corresponding losses in the system are lower. Hence In the present application small dual displacement motors are used to provide high speed with lower displacement and low speed, high gradeability with higher displacement.

4. Cearboxes have been removed from the system. Typical efficiency loss through a two stage gearbox is 4%. In addition to improving drive efficiency this also optimizes the energy recovery during braking and down hill driving.

5. Electric pump motor characteristics have been developed where a control device controls the speed of the motor/generator to allow it to operate at very low speeds (high torque) . By running at low speeds the hydraulic drag in the pumps is reduced. As an example, typical pump driven electric motors run from 1800 RPM to 4000 RPM, the motor of the present invention is designed to run from 500 RPM to 2000 RPM. Using the same scaling as above, a large machine may operate at 1000 RPM to 4000 RPM.

6. The charge pump guarantees a minimum level of pressure in the system and compensates for any leaks in the main pump. Charge pressure is most preferably set between 15 and 20 Bar or at least 10 to 25 Bar. This lower value reduces energy waste whereas a standard system uses 25 bar or more for the charge pump.

An essential aspect of the system, compared to other small MEWP drives, is the capability of recovering braking energy.

So-called "regenerative braking" using electric drive motor technology is well known in other industries, for example on fork trucks. Such known systems are ideal for heavy vehicles whose work rhythms are characterised by short, successive start-stop cycles. Regenerative braking using hydraulic drive technology is less known.

Particularly, electric drive technology is often preferred over hydraulic systems due to the inefficiency of hydraulic pumps and motors. However, the associated cost of an electric drive to provide the same performance as a hydraulic drive system is significantly high. The present invention aims to eliminate the need for electric drive technology by improving the efficiency of the hydraulic drive system.

The present invention will be described hereinafter by reference to the accompanying drawings wherein:-Figure 1 illustrates a simplified drive system layout according to the invention.

Fig. 1 shows the main components of a drive system for an NEWP according to the invention; namely a battery U supplying power to (or being charged via) a control device 12, a motor/generator 13 controlled by the control device that in turn powers (or is powered by) a variable displacement drive pump 14. The drive pump 14 hydraulically drives at least one, but preferably two, drive motors 15 connected to wheels (not shown) of the platform, thereby enabling it to be propelled.

It will be apparent that pump 14 and drive motors 15 are located in a closed loop 16 which enables energy to pass in either direction through the pump 14 and motor 13 (to drain or charge stored energy from battery 11) . A flow divider/combiner 17 may be required between the drive motors and the loop 16 dependent on how many drive motors are utilised.

Preferably powered by the same drive shaft off the motor 13 in line with drive pump 14 are an auxiliary pump 18 and a combined steer/charge pump 19. A steer cylinder 20 is hydraulically powered by the steer/charge pump 19, with output hydraulic pressure being directed to a charge flow 21 which charges the closed loop system 16 (accounting for any fluid losses) Hydraulic fluid is supplied to the auxiliary 18 and steer/charge 19 pumps from a tank 22. Fluid is returned to the tank 22 after auxiliary functions 23 (e.g. raising and lowering a work platform) have been engaged.

It will be apparent that the hydraulic circuit from the auxiliary pump 18 en route to the auxiliary functions 23 also has provision to be diverted by a charge boost function 24 that assists the charge flow 21 otherwise provided solely by steer/charge pump 19. This capability is employed in times when pressure drops in the closed loop 16 need to be compensated, e.g. on start-up.

Pressure from the charge pumps is also routed for other functions, e.g. wheel motor brake release, wheel motor displacement change, pump displacement control (via an hydraulic joystick) and for operating the divider/combiner 17; all of which are generally denoted as a "control pressure" 25 in Figure 1. These functions do not use much flow and act as pilot control lines.

As Illustrated, the system includes a control device (means) 12 for managing the transfer of power between the electric motor/generator 13 and the variable displacement hydraulic drive pump 14, and between the battery 11 and the motor/generator 13. The electric "motor" 13 may be used either as a motor or generator, to generate electricity. The motor/generator 13 is electrically connected via the control device 12 to the battery 11 of electric cells.

The control device 12 controls operation of the motor/generator 13, either supplying electrical power to the battery 11 to recharge it when the motor/generator is in generator mode, or supplying electrical power from the battery to the motor/generator when it is in motor mode.

The control device 12 controls the speed of the motor, preferably to a limit of around 2000rpm, although it is capable of much higher speeds, for example 3000rpm. During operation, excess energy may be generated by the mobile work platform causing the speed of motor 13 to increase (thus increasing the output capacity), for example when decelerating, travelling downhill or under heavy braking. In these scenarios the variable displacement drive pump acts as a motor and converts excess fluid energy into mechanical energy, which can be used to drive the motor/generator 13 to generate electricity. The excess energy can then be stored in the battery 11 for later use. The control device 12 preferably controls the delivery of current to the battery and prevents over-charging. It will be apparent that such a system will reduce the freguency of charging the battery 11 from a mains source and extend its operating time.

In alternate embodiments an internal combustion engine can be added to the system to help charge the batteries or to drive the pumps directly.

As previously stated, the regeneration principle can also be extended to larger machines, so long as they have a closed loop pump and motor generator set up.

The purpose of the present invention is to improve overall efficiency of a hydraulic drive system in a mobile elevated work platform. It is expected that the combined effect of the novel pump configuration (sharing functionality between combined pump operations as described) and the regenerative nature of the closed loop driving the wheels could improve efficiency by 40% over known systems leading to a clear operating cost benefit.

Claims

CLAIMS: 1. A drive system for a mobile work platform including: a battery; a control means; an electric motor; and a variable displacement drive pump, coupled to the electric motor, in a closed hydraulic loop with at least one drive motor that drives a wheel of the mobile work platform, such that the drive pump is able to hydraulically drive the wheels via the drive motor to propel the mobile work platform and/or recover energy from the turning wheels during braking, deceleration or downhill locomotion to charge the battery via the electric motor, acting as a generator.
2. The drive system of claim 1 further including a combined steer/charge pump, and an auxiliary or "booms" pump powered by the electric motor.
3. The drive system of claim 2 wherein the steer/charge pump is configured to charge the closed hydraulic loop.
4. The drive system of claim 3 wherein the auxiliary pump is configured to assist or boost the steer/charge pump when needed.
5. The drive system of any of claims claim 2 to 4 wherein the steer/charge pump is in a circuit communicating with a steer cyclinder and/or the circuit is divertible to a charge flow for charging the closed hydraulic loop.
6. The drive system of any of claims 2 to 5 wherein both the steer/charge pump and auxiliary pump are supplied with hydraulic fluid from an hydraulic fluid tank.
7. The drive system of claim 6 wherein the auxiliary pump is in communication with auxiliary functions that, in turn, returns fiuid to the hydraulic fluid tank.
8. The drive system of any of claims 2 to 7 wherein the capacity ratio of the auxiliary pump compared to the steer/charge pump is between 2:1 and 8:1, preferably 3:1 to 6:1 but most preferably between 4:1 and 5:1.
9. The drive system of any of the preceding claims wherein the control means controls the speed of the motor/generator to between 500RPM to 2000RPM.
10. The drive system of any preceding claim wherein the drive motor(s) is a dual displacement radial piston motor, without any gearbox transmission.
11. The drive system of claim 3 wherein the steer/charge pump guarantees a minimum level of pressure in the closed loop between 10 to 25 Bar, most preferably between 15 and 20 Bar.
12. A drive system for a mobiie work platform including: a combined steer/charge pump and an auxiliary pump able to provide an additional boost to the charge pump.
13. The drive system of claim 12 further including a variable displacement drive pump in an hydraulic circuit for driving at least one motor associated with a wheel of the mobile work platform, the circuit being charged by the charge pump, further assisted when necessary by the auxiliary pump.
14. The drive system of claim 12 or 13 wherein the auxiliary pump is otherwise used to power auxiliary functions such as booms operation.
15. The drive system of any of claims 12 to 14 further including a battery, a control means and an electric motor.
16. The drive system of olaim 15 wherein the electric motor drives the three pumps (drive, steer/charge, auxiliary) via a common drive shaft.
17. The drive system of claim 13 wherein the drive pump is in a closed hydraulic loop, such that the drive pump is able to hydraulically drive the wheel via the drive motor to propel the mobile work platform and/or recover energy from the turning wheel during braking, deceleration or downhill locomotion to charge the battery via the electric motor, which acts as a generator.
18. The drive system substantially as herein described with reference to the accompanying drawing.