Details of the equivalent modification to the Pro-120 are on another page.

Contents - this page

• Introduction
• When do I need fast ramps
• Modification Details
• Notes
• Reverse relay timing

Introduction

The ramps on the NCC are adjustable from around 300mS to around 7 second, zero to full speed. For most users, this is adequate, but occasionally an application occurs where even faster response is required. This page explains how the ramps on the NCC series controllers may be changed.

It also describes other factors affecting response times.

This is not intended as invitation to all and sundry to start modifying! If you really do need the faster ramps, please talk to us first!

Any motor and mechanical system does take a finite time to respond. Generally it is sensible to let the controller control this time, so you would not adjust the controller to ramp up faster than the mechanical system can handle.

Also, the current required to accelerate a mass depends on the time taken to accelerate it. The faster the acceleration, the greater the current. A truly instantaneous acceleration would require a truly infinite current! So you need to be aware of the real-life time response of your system and to know exactly what instantaneous current you are demanding of the controller - accelerate too fast and the controller will simply hit its current limit and that will define the acceleration ramp!

Be aware also that, if the deceleration ramp is too fast, the motor will still be running when the relays de-power. This shorts out the motor and - if the motor is still running at any significant speed, causes an arc at the relay contacts. That will shorten the relay's life. So be careful. Do not simply modify as stated here and turn the ramps to zero. Some ramping, especially on the deceleration, is required.

Be warned: relays whose contacts have been damaged from arcing because the controller has been modified will not be replaced under guarantee!

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 drawing shows the motor speed control end of the NCC controller, issue 18 (See NCC series controllers - Issue number history). Four capacitors are marked.

Notes

• C 3, 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 is an unused supply decoupling capacitor position, only mentioned here to clarify the layout.
• C 1, normally 1µ0, was not fitted to early boards but in some applications (industrial, with long wiring) some customers had 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 4 normally 1µ0 is in the current limit to maintain stability. Reduce it to 100n (0.1µ). On all Mark 2 NCCs, it is 0.1µ as standard.

Reverse relay timing

As the relays go through zero, there is a small 'dwell'. This is probably not noticable to the performance but it may be reduced by removing the 'Reverse Dwell Timing' capacitor - item 25 on the page NCC key components

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