Details of the equivalent modification to the NCC series controllers are on another page.

Contents - this page

• Introduction
• When do I need fast ramps
• Modification Details
• Notes
• Afterword

Introduction

The Pro-120 series acceleration and deceleration ramps are adjustable down to as fast as about 330mS. It is possible to reduce this timing but any modifications should be done with extreme caution.

The ramps are there partially to control the mechanical response of any vehicle (particular passenger carrying vehicles) but also to protect the controller. Reducing the ramps (in particular, the deceleration ramp) too far will cause the relays to switch while motor current is still flowing.

Relays contacts switching while current is flowing cause an arc. Arcs quickly destroy relay contacts. Arcs are a known cause of otherwise apparently random controller failures. So decreasing the ramp rates excessively will compromise controller reliability.

Furthermore, motors subjected to sudden reversal of direction and high currents can behave erratically. In particular, commutators can arc and furthr compromise reliability.

It is for these reasons that we have not earlier released the following details of this modification.

Warning:

Be warned also that replacing the relays is not simple: the board is thick copper, through hole plated, and it requires a lot of heat to remove the the relays. All 5 contacts must be simultaneously heated and it is very easy to overheat the board and cause damage, or to underheat the board and damage the through hole plating. Such damage is, essentially, unrepairable.

When do I need fast ramps

Generally, the only time that the standard ramps are not fast enough is when the whole controller is being used inside a feedback loop, as, for instance, in a robot where a gyro is being used for stabilization.

Note that is is possible to include only the output section inside a feedback loop - as occurs when using a tacho generator. In this case, the ramping circuit is before the feedback system so does not require altering. However - this connection does not control reversing so is not suitable for use with a Gyro.

Modification Details

The components which can be changed are indicated in the diagram below,

Notes

• C 1, normally 4µ7, is the main ramp capacitor. Ramp time is directly proportional to this capacitor. Decreasing it to 0.47µ will reduce minimum ramp time to around 33mS, faster than any mechanical system is ever likely to respond. However, if it is reduced to this value, other time constants can start to have a dominant effect.
• C 2, normally 1µ0. In some applications (industrial, with long wiring) can have trouble with large values of induced mains hum. If excessive, this can saturate the input buffer, causing a resultant d.c. offset. This 1µ0 is to decouple hum, so (unless your wiring is bad) you should not need it.
• C 3 affects the timing during reverse. As you go through reverse, the two relays can be heard giving a double click as one relay de-energises and the other energises. Removing this capacitor causes the two relays to change simultaneously. This will affect relay contact burn, but it is a moot point whether it will improve (by reducing the relay operate time and sharing the arcing) or worsen it (by causing more arcing).

Afterword

This modification is essentially the same as that for the NCC series. However: it is far more dangerous as the Pro-120 gives 150% of the NCC's current. Arc energy and heating are both proportional to the square of the current, so the resultant reliability compromise looks like a reduction by 225%. This may explain our reluctance to authorise this modification.

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